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Yiming Lin

Hongtai Zhang

China Satellite Network Group

Co., Ltd., China

University of Luxembourg,

Luxembourg

Lianchong Zhang

Aerospace Information Research

Institute,

Chinese Academy of Sciences, China

Xiaopeng Xue

Central South University, China

Space: Science & Technology

( Special Issue )

CONTENTS

(1)

(3)

(17)

(33)

Editorial Entry, Descent, and Landing of China’s Tianwen-1 Mars Mission

Zezhou Sun, Wei Rao

The Tianwen-1 Guidance, Navigation, and Control for Mars Entry, Descent, and Landing

Xiangyu Huang, Maodeng Li, Xiaolei Wang, Jinchang Hu, Yu Zhao, Minwen Guo,

Chao Xu, Wangwang Liu, Yunpeng Wang, Ce Hao, and Lijia Xu

Study on Dynamic Characteristics of Mars Entry Module in Transonic and Supersonic Speeds

Qi Li, Rui Zhao, Sijun Zhang, Wei Rao, and Haogong Wei

Analysis and Verification of Aerodynamic Characteristics of Tianwen-1 Mars Parachute

Mingxing Huang, Wenqiang Wang, and Jian Li

Thermal Environment and Aeroheating Mechanism of Protuberances on Mars Entry Capsule

Wenbo Miao, Qi Li, Junhong Li, Jingyun Zhou, and Xiaoli Cheng

Numerical Simulation of Decompression Process of a Mars Rover in the Launch Phase

Weizhang Wang, Wei Rao, Qi Li, Hao Yan, and Rui Zhao

Ballistic Range Testing Data Analysis of Tianwen-1 Mars Entry Capsule

Haogong Wei, Xin Li, Jie Huang, Qi Li, and Wei Rao

Study on Effect of Aerodynamic Configuration on Aerodynamic Performance of Mars Ascent Vehicles

Qi Li, Wu Yuan, Rui Zhao, and Haogong Wei

(45)

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MAIN TEXT

Launched into orbit on July 23, 2020, China’s first Mars

mission Tianwen-1 has implemented the key missions

of “near-Mars capture and brake,”; “entry, descent, and

landing (EDL),” and “rover leaving the landing platform”

successively in nearly one year. The three major tasks

of “orbiting around Mars,” “Mars surface landing,” and

“exploration and detection” have also been accomplished. At

present, the Zhurong Mars rover is performing exploration

and detection tasks as planned. For Tianwen-1, China’s first

probe landing on an extraterrestrial planet to carry out such

elaborate tasks, EDL is the risk point of the Mars mission

that is mostly difficult to accomplish.

This special issue focuses on various innovations in tasks

during Tianwen-1’s EDL phase and presents a collection

of original works by authors from higher education and

research institutions. Such institutions include Beijing

Institute of Spacecraft System Engineering, Beijing

Institute of Control Engineering, Beijing Institute of Space

Mechanics and Electricity, Beijing Institute of Technology,

China Academy of Aerospace Aerodynamics, and China

Aerodynamics Research and Development Center. The

total seven articles in this special issue cover a broad

spectrum of research topics concerning the first Mars

mission, including EDL control [1], analysis of the dynamic

characteristics of the entry module [2], design of the Mars

exploration parachute and its aerodynamic verification

[3], aeroheating mechanism simulation [4], simulation of

pressure equilibrium process [5], and test data processing [6].

It also involves technologies for aerodynamic characteristic

analysis of the Mars Ascent Vehicle (MAV) for future Mars

Entry, Descent, and Landing of China’s Tianwen-1 Mars Mission

Zezhou Sun, Wei Rao*

Beijing Institute of Spacecraft System Engineering, Beijing 100094, China

*Corresponding author. Email: rauwei@163.com

sample return [7].

Specifically, the team led by Xiangyu Huang and

Maodeng Li contributed a research article [1] on guidance,

navigation, and control (GNC) system for the EDL phase.

Deducing the GNC system design of Tianwen-1 inversely

from mission requirements, their article provided structure

and algorithm for a GNC system that fits those requirements.

The actual flight results of the whole EDL phase was also

presented in the article.

Considering the large blunt-nosed and short body of the

Mars entry capsule in terms of its aerodynamic shape, the

Mars aerodynamics team from China Academy of Space

Technology developed an integrated numerical simulation

method of computational fluid dynamics and rigid body

dynamics (CFD/RBD) on the basis of detached eddy

simulation (DES). this method was applied to study the

dynamic characteristics of Mars entry capsule in free flight

from transonic to supersonic release with one degree of

freedom (1-DOF) at a low angle of attack, from which the

influence of different afterbody shapes on dynamic stability

was discussed [2] .

Mingxing Huang’s team optimized and verified the

parachute design of Tianwen-1 according to the particular

open environment of the landing phase. Drawing on various

tests and data, the research team predicted and analyzed the

aerodynamic characteristic parameters of the parachute in

Mars conditions. In addition, the high-altitude flight tests

of nine parachutes were carried out in order to verify its

aerodynamic characteristics and reliability, serving as an

important reference for the Mars exploration parachute

design [3].

Wenbo Miao’s team investigated the thermal

environment of the interaction region on the heat shield

surface of the Mars lander. The flow characteristics of

interactions from protuberances at different parts of the

heat-shield were studied through numerical simulation.

The heating mechanism of interactions from protuberances

at different parts was also obtained by analyzing flow

velocity, pressure, Mach number, and other characteristic

parameters [4].

Rui Zhao’s team from Beijing Institute of Technology

numerically simulated the decompression processes of the

Mars rover to study the internal-external pressure differences

under a changing ambient pressure on the rocket fairing.

As for numerical calculations, PROFILE was developed

to outlet boundary conditions and to investigate the

influences of ambient pressure setting, time step, and grid

density to improve the accuracy of simulation results [5].

To further explore the transonic and supersonic dynamic

characteristics of the Tianwen-1 lander and rover and verify

the aerodynamic shape and mass property design, the team

also carried out free-flight dynamic simulations with the

free-flight ballistic range test model under test conditions.

The aerodynamic coefficients of the model were obtained

by linear regression. A dynamic derivative model was

constructed under assumed linearization with a low angle of

attack, and the static moment coefficient and the dynamic

derivative were thereby identified [6].

To meet the mission requirements of Mars surface

takeoff and ascent, researchers including Haogong Wei

and Qi Li from the Mars aerodynamics team analyzed the

aerodynamic performance requirements of the MAV. In light

of literature survey and the results of supersonic static CFD

simulation, the team analyzed the influences of the nose and

afterbody shapes of the MAV on the aerodynamic drag and

static stability. On this basis, they proposed a nose shape

with favorable aerodynamic performance and clarified the

subsequent improvement direction of the aerodynamic

layout. Their research provided necessary theoretical and

data support for the aerodynamic model selection of a

MAV [7].

To summarize, this special issue reviewed the exciting

progress in various fields during the EDL of Tianwen-1,

outlined the frontiers of related research worldwide, and

shared the thoughts and practices of Chinese scientists with

fellow researchers around the globe.

Competing interests

The authors declare that there is no conflict of interests

regarding the publication of this article.

Acknowledgments

We would like to further extend our gratitude to all the

authors for their research contribution to this special issue.

We also thank all the reviewers and the editorial team for

their support to this special issue.

References

[1] Xiangyu Huang, Maodeng Li, Xiaolei Wang, Jinchang

Hu, Yu Zhao, Minwen Guo, Chao Xu, Wangwang

Liu, Yunpeng Wang, Ce Hao, Lijia Xu, “The

Tianwen-1 Guidance, Navigation, and Control for

Mars Entry, Descent, and Landing”, Space: Science &

Technology, vol. 2021, Article ID 9846185, 13 pages, 20

21. https://doi.org/10.34133/2021/9846185

[2] Qi Li, Rui Zhao, Sijun Zhang, Wei Rao, Haogong

Wei, “Study on Dynamic Characteristics of Mars Entry

Module in Transonic and Supersonic Speeds”, Space:

Science & Technology, vol. 2022, Article ID 9753286, 1

5 pages, 2022. https://doi.org/10.34133/2022/9753286

[3] Mingxing Huang, Wenqiang Wang, Jian Li, “Analysis

and Verification of Aerodynamic Characteristics

of Tianwen-1 Mars Parachute”, Space: Science &

Technology, vol. 2022, Article ID 9805457, 11 pages, 20

22. https://doi.org/10.34133/2022/9805457

[4] Miao Wenbo, Li Qi, Li Junhong, Zhou Jingyun, Cheng

Xiaoli, “Thermal Environment and Aeroheating Mechanism

of Protuberances on Mars Entry Capsule”, Space: Science &

Technology, vol. 2021, Article ID 9754068, 8 pages, 202

1. https://doi.org/10.34133/2021/9754068

[5] Weizhang Wang, Wei Rao, Qi Li, Hao Yan, Rui

Zhao, “Numerical Simulation of Decompression Process

of a Mars Rover in the Launch Phase”, Space: Science &

Technology, vol. 2022, Article ID 9827483, 12 pages, 20

22. https://doi.org/10.34133/2022/9827483

[6] Haogong Wei, Xin Li, Jie Huang, Qi Li, Wei

Rao, “Ballistic Range Testing Data Analysis of

Tianwen-1 Mars Entry Capsule”, Space: Science &

Technology, vol. 2021, Article ID 9830415, 6 pages, 202

1. https://doi.org/10.34133/2021/9830415

[7] Qi Li, Wu Yuan, Rui Zhao, Haogong Wei, “Study on

Effect of Aerodynamic Configuration on Aerodynamic

Performance of Mars Ascent Vehicles”, Space: Science

& Technology, vol. 2022, Article ID 9790131, 11 pages,

2022. https://doi.org/10.34133/2022/9790131

Research Article

The Tianwen-1 Guidance, Navigation, and Control for Mars

Entry, Descent, and Landing

Xiangyu Huang,1 Maodeng Li ,

1 Xiaolei Wang,2 Jinchang Hu,1 Yu Zhao,2 Minwen Guo,1

Chao Xu,1 Wangwang Liu,2 Yunpeng Wang,2 Ce Hao,2 and Lijia Xu2

Science and Technology on Space Intelligent Control Laboratory, Beijing Institute of Control Engineering, Beijing 100091, China

Beijing Institute of Control Engineering, Beijing 100091, China

Correspondence should be addressed to Maodeng Li; mdeng1985@gmail.com

Received 15 July 2021; Accepted 14 September 2021; Published 16 October 2021

Commons Attribution License (CC BY 4.0).

Tianwen-1, the first mission of China’s planetary exploration program, accomplished its goals of orbiting, landing, and roving on

the Mars. The entry, descent, and landing (EDL) phase directly determines the success of the entire mission, of which the

guidance, navigation, and control (GNC) system is crucial. This paper outlines the Tianwen-1 EDL GNC system design by

introducing the GNC requirements followed by presenting the GNC system architecture and algorithms to meet such

requirements. The actual flight results for the whole EDL phase are also provided in this paper.

1. Introduction

Mars has atmosphere and surface environment similar to

the Earth, making it a prime target for deep space exploration. In order to investigate the Mars surface environment,

it is necessary to perform soft landing missions to place a

lander on the surface. Of the eighteen landing missions that

have been carried out, nine achieved a complete success.

They are Viking-1, Viking-2 [1], Mars Pathfinder [2], Mars

Exploration Rover [3], Phoenix [4], Mars Science Laboratory

(MSL) [5], InSight [6], Mars 2020 [7], and Tianwen-1 [8]. As

the first Chinese Mars landing mission, Tianwen-1 was

launched successfully at 12:41 p.m. Beijing Time (BJT) on

23 July 2020, and delivered its lander to the Mars surface

with a soft touchdown velocity and a stable predefined attitude

at 7:18 a.m. BJT on 15 May 2021. The successful deployment

of the rover on 22 May 2021 completed the mission’s goals

of orbiting, landing, and releasing a rover on the Mars.

The entry, descent, and landing (EDL) phase, which

began at the Mars atmosphere interface and ended with a

surface touchdown, is crucial for a Mars landing mission

and directly determines the success of the entire mission.

The success rate of mars missions is about 50%, and most

failures occur during the EDL phase [9]. The guidance, navigation, and control (GNC) system guarantees the touchdown safety and accuracy, playing an important role in the

EDL phase. Because of the time urgency of the EDL process

and large communication delay between the Mars and the

Earth, the spacecraft must perform autonomous GNC to

provide reliable key event triggers and accurate and reliable

state estimates and to implement accurate and reliable trajectory and attitude controls. Any mistake may lead to a mission failure.

The uncertainties such as Mars environments, parachute

descent motion, and initial state, the complexity of the EDL

process (multistage deceleration, many key events, etc.), and

the limited on-board computational ability bring great challenges to the design of EDL GNC system. To meet these

challenges, the GNC hardware should have a certain degree

of redundancy, and the GNC algorithms should be suitable

for on-board implementation, robust to sensor and actuator

partial failures, and adaptive to uncertainties.

This paper summarizes the Tianwen-1 EDL GNC design

by analyzing the EDL GNC requirements, presenting the

GNC modes and GNC hardware configurations, and finally

describing the EDL GNC algorithms. The flight results that

have validated the GNC design will also be provided.

2. Mission Overview

2.1. Tianwen-1 Spacecraft Configuration. To fulfill the goals

of orbiting, landing, and roving in a single mission [8, 10],

AAAS

Space: Science & Technology

Volume 2021, Article ID 9846185, 13 pages

https://doi.org/10.34133/2021/9846185

Research Article

The Tianwen-1 Guidance, Navigation, and Control for Mars

Entry, Descent, and Landing

Xiangyu Huang,1 Maodeng Li ,

1 Xiaolei Wang,2 Jinchang Hu,1 Yu Zhao,2 Minwen Guo,1

Chao Xu,1 Wangwang Liu,2 Yunpeng Wang,2 Ce Hao,2 and Lijia Xu2

Science and Technology on Space Intelligent Control Laboratory, Beijing Institute of Control Engineering, Beijing 100091, China

Beijing Institute of Control Engineering, Beijing 100091, China

Correspondence should be addressed to Maodeng Li; mdeng1985@gmail.com

Received 15 July 2021; Accepted 14 September 2021; Published 16 October 2021

Commons Attribution License (CC BY 4.0).

Tianwen-1, the first mission of China’s planetary exploration program, accomplished its goals of orbiting, landing, and roving on

the Mars. The entry, descent, and landing (EDL) phase directly determines the success of the entire mission, of which the

guidance, navigation, and control (GNC) system is crucial. This paper outlines the Tianwen-1 EDL GNC system design by

introducing the GNC requirements followed by presenting the GNC system architecture and algorithms to meet such

requirements. The actual flight results for the whole EDL phase are also provided in this paper.

1. Introduction

Mars has atmosphere and surface environment similar to

the Earth, making it a prime target for deep space exploration. In order to investigate the Mars surface environment,

it is necessary to perform soft landing missions to place a

lander on the surface. Of the eighteen landing missions that

have been carried out, nine achieved a complete success.

They are Viking-1, Viking-2 [1], Mars Pathfinder [2], Mars

Exploration Rover [3], Phoenix [4], Mars Science Laboratory

(MSL) [5], InSight [6], Mars 2020 [7], and Tianwen-1 [8]. As

the first Chinese Mars landing mission, Tianwen-1 was

launched successfully at 12:41 p.m. Beijing Time (BJT) on

23 July 2020, and delivered its lander to the Mars surface

with a soft touchdown velocity and a stable predefined attitude

at 7:18 a.m. BJT on 15 May 2021. The successful deployment

of the rover on 22 May 2021 completed the mission’s goals

of orbiting, landing, and releasing a rover on the Mars.

The entry, descent, and landing (EDL) phase, which

began at the Mars atmosphere interface and ended with a

surface touchdown, is crucial for a Mars landing mission

and directly determines the success of the entire mission.

The success rate of mars missions is about 50%, and most

failures occur during the EDL phase [9]. The guidance, navigation, and control (GNC) system guarantees the touchdown safety and accuracy, playing an important role in the

EDL phase. Because of the time urgency of the EDL process

and large communication delay between the Mars and the

Earth, the spacecraft must perform autonomous GNC to

provide reliable key event triggers and accurate and reliable

state estimates and to implement accurate and reliable trajectory and attitude controls. Any mistake may lead to a mission failure.

The uncertainties such as Mars environments, parachute

descent motion, and initial state, the complexity of the EDL

process (multistage deceleration, many key events, etc.), and

the limited on-board computational ability bring great challenges to the design of EDL GNC system. To meet these

challenges, the GNC hardware should have a certain degree

of redundancy, and the GNC algorithms should be suitable

for on-board implementation, robust to sensor and actuator

partial failures, and adaptive to uncertainties.

This paper summarizes the Tianwen-1 EDL GNC design

by analyzing the EDL GNC requirements, presenting the

GNC modes and GNC hardware configurations, and finally

describing the EDL GNC algorithms. The flight results that

have validated the GNC design will also be provided.

2. Mission Overview

2.1. Tianwen-1 Spacecraft Configuration. To fulfill the goals

of orbiting, landing, and roving in a single mission [8, 10],

AAAS

Space: Science & Technology

Volume 2021, Article ID 9846185, 13 pages

https://doi.org/10.34133/2021/9846185

The Tianwen-1 Guidance, Navigation, and Control for Mars

Entry, Descent, and Landing

Xiangyu Huang,1

Maodeng Li,1

Xiaolei Wang,2

Jinchang Hu,1

Yu Zhao,2

Minwen Guo,1

Chao Xu,1

Wangwang Liu,2

Yunpeng Wang,2

Ce Hao,2

and Lijia Xu2

Science and Technology on Space Intelligent Control Laboratory, Beijing Institute of Control Engineering, Beijing 100091, China

Beijing Institute of Control Engineering, Beijing 100091, China

Correspondence should be addressed to Maodeng Li; mdeng1985@gmail.com

Abstract: Tianwen-1, the first mission of China’s planetary exploration program, accomplished its goals of orbiting, landing, and roving

on the Mars. The entry, descent, and landing (EDL) phase directly determines the success of the entire mission, of which the guidance,

navigation, and control (GNC) system is crucial. This paper outlines the Tianwen-1 EDL GNC system design by introducing the GNC

requirements followed by presenting the GNC system architecture and algorithms to meet such requirements. The actual flight results for

the whole EDL phase are also provided in this paper.

the Tianwen-1 spacecraft consists of an orbiter and a descent

module, as shown in Figure 1, where the descent module is

composed of a heatshield, a backshell, and a lander that consists of a landing platform and a rover.

2.2. Mission Profile. Figure 2 illustrates the mission profile of

the Tianwen-1 [8], which is divided into five stages: EarthMars transfer stage, Mars orbit insertion stage, Mars orbit

parking stage, deorbit and landing stage, and scientific

exploration stage. In the former three stages, the orbiter

and the descent module form as a single probe. In the deorbit and landing stage, the descent module is separated from

the orbiter and then will enter the Mars atmosphere a few

hours later, starting its EDL process. With deceleration of

Mars atmosphere, parachute, and the main landing engine

(MLE), the lander lands on the Mars surface softly. And

after a few days, the rover is released from the lander for scientific exploration.

2.3. EDL Sequence Profile. The EDL sequence profile of the

Tianwen-1 consists of atmospheric entry, parachute descent,

and landing phases.

The atmospheric entry phase begins when the vehicle

reaches the atmospheric boundary of Mars (at an altitude

of approximately 125 km) and ends in parachute deployment at a specified value of navigated velocity. During this

phase, Tianwen-1 performs a guided lifting entry at a liftto-drag ratio of 0.13 with a nonzero trim angle-of-attack

(AOA) and then deploys the trim wing at a specified navigated velocity. With the effects of the trim wing, the trim

value of the total AOA is secured suitable for the parachute

deployment.

The atmospheric entry phase is followed by the parachute descent phase, during which the heatshield is first jettisoned at a specified value of navigated velocity, and then

landing radars begin to work such that the vehicle’s altitude

and velocity with respect to the Mars can be measured.

When the vehicle reaches its stable descent velocity and

the navigated altitude and velocity are deemed suitable for

terminal braking within the available propellant budget,

the descent module releases the backshell and ignites the

MLE, implying that the landing phase begins.

The landing phase is also known as the powered descent

phase. In this phase, the lander performs a velocity reduction

via the MLE, executes a backshell evasion maneuver, when

necessary, selects a safe landing site, and finally lands on

the selected landing site safely to achieve the soft landing.

3. Overview of the EDL GNC System

3.1. GNC Requirements. The GNC system requirements for

Tianwen-1 EDL process are as follows:

(1) Key events, such as trim wing deployment, parachute deployment, heatshield jettison, backshell separation, MLE ignition, and touchdown detection,

should be triggered properly

(2) After backshell separation, the lander should not be

collided with the backshell

(3) The actual landing site should be selected on-board

within the preselected landing area

(4) The lander’s touchdown attitude, angular rate, and

vertical and horizontal velocities should meet the

requirements

(5) The fuel consumption of the EDL process needs to

be within a reasonable range.

3.2. GNC Modes. According to the EDL process profile and

GNC requirements, the EDL process is divided into eight

GNC modes: the AOA-trim mode, the lift control mode,

the parachute descent control mode, the powered deceleration mode, the hover and imaging mode, the hazard avoidance maneuver mode, the slow descent mode, and ended

with the no-control mode. The transition of these eight

modes is shown in Figure 3.

In the atmospheric entry segment, the GNC system

operates with the AOA-trim mode initially, in which the

descent module’s attitude is adjusted to a predefined attitude

for guided lift entry. Once the sensed acceleration magnitude

exceeds 0.2 Earth g, the lift control mode is activated to produce proper lift to control the entry trajectory.

With the deployment of parachute, the GNC system

switches to the parachute descent control mode and uses this

mode throughout the whole parachute descent phase. In this

mode, the descent module’s velocity would descent to about

95 m/s at an altitude of about 1.2 km above the Mars surface.

In the landing phase, the GNC system operates with the

powered deceleration mode, hover and imaging mode, hazard avoidance maneuver mode, slow descent mode, and

ends at the no-control mode when the lander has softly

touched down. The powered deceleration mode begins with

the backshell separation and ends before hover. The main

task of this mode is to use the MLE to reduce the lander’s

velocity, to avoid collisions with the detached backshell,

and to image and select a wide safe landing area for coarse

hazard avoidance. In the hover and imaging mode, the

lander maintains a hover state to take 3D images of the landing area and then selects a safe landing site. Once the landing

site is selected, the GNC system switches to the hazard

avoidance maneuver mode to perform hazard avoidance

and descent such that the lander would descent to 20 m altitude above the landing site with a zero horizontal velocity

and a preset value of vertical velocity (about 1.5 m/s). Then,

the GNC system switches to the slow descent mode. In this

mode, the lander slowly descends at a preset speed, eliminates the horizontal speed, and maintains a vertical attitude.

Once the lander’s touchdown is detected, the GNC system

sends a shutdown signal to turn off the MLE and then

switches to the no-control mode, in which no further orbit

control and attitude control are performed any more.

3.3. GNC System Configuration. The EDL GNC system

scheme is illustrated in Figure 4, where sensors, actuators,

and the GNC computer will be described in this subsection,

and the GNC algorithms will be presented in the following

sections.

2 Space: Science & Technology

3.4. Sensors

3.4.1. Star Sensors. A pair of star sensors working from the 8

hours prior to orbiter/descent module separation to 10 seconds before atmospheric entry are equipped for attitude

determination.

3.4.2. Inertial Measurement Units (IMUs). A pair of IMUs

are carried out, each of which consists of three orthogonal

accelerometers and three orthogonal gyroscopes to measure

the specific force and angular rate, respectively. During the

EDL, the IMUs are used for inertial navigation and key event

triggers.

3.4.3. Landing Radars. A microwave radar and a phased

array sensor (PAS) are configured to provide Mars-related

measurements. The microwave radar contains four beams,

each of which works automatically. The PAS contains nine

beams, four of which are selected by the GNC system to provide measurements at every measurement time. Every beam

for the two radars can measure slant-range and groundrelative velocity along its axis simultaneously.

3.4.4. Hazard Avoidance Sensors. Two hazard avoidance sensors are equipped for hazard avoidance and landing site

selection. One is called the optical obstacle avoidance sensor

Tianwen-1 probe = Orbiter

Orbiter

+ Descent module

Descent module

Backshell

Rover

Landing platform

Heatshield

Figure 1: Main components of the Tianwen-1 spacecraft.

Scientific exploration stage

Mars parking stage

Mars capture stage

Earth-mars transfer stage

Deorbitand landing stage

Figure 2: Mission profile of Tianwen-1 [8].

Space: Science & Technology 3

the Tianwen-1 spacecraft consists of an orbiter and a descent

module, as shown in Figure 1, where the descent module is

composed of a heatshield, a backshell, and a lander that consists of a landing platform and a rover.

2.2. Mission Profile. Figure 2 illustrates the mission profile of

the Tianwen-1 [8], which is divided into five stages: EarthMars transfer stage, Mars orbit insertion stage, Mars orbit

parking stage, deorbit and landing stage, and scientific

exploration stage. In the former three stages, the orbiter

and the descent module form as a single probe. In the deorbit and landing stage, the descent module is separated from

the orbiter and then will enter the Mars atmosphere a few

hours later, starting its EDL process. With deceleration of

Mars atmosphere, parachute, and the main landing engine

(MLE), the lander lands on the Mars surface softly. And

after a few days, the rover is released from the lander for scientific exploration.

2.3. EDL Sequence Profile. The EDL sequence profile of the

Tianwen-1 consists of atmospheric entry, parachute descent,

and landing phases.

The atmospheric entry phase begins when the vehicle

reaches the atmospheric boundary of Mars (at an altitude

of approximately 125 km) and ends in parachute deployment at a specified value of navigated velocity. During this

phase, Tianwen-1 performs a guided lifting entry at a liftto-drag ratio of 0.13 with a nonzero trim angle-of-attack

(AOA) and then deploys the trim wing at a specified navigated velocity. With the effects of the trim wing, the trim

value of the total AOA is secured suitable for the parachute

deployment.

The atmospheric entry phase is followed by the parachute descent phase, during which the heatshield is first jettisoned at a specified value of navigated velocity, and then

landing radars begin to work such that the vehicle’s altitude

and velocity with respect to the Mars can be measured.

When the vehicle reaches its stable descent velocity and

the navigated altitude and velocity are deemed suitable for

terminal braking within the available propellant budget,

the descent module releases the backshell and ignites the

MLE, implying that the landing phase begins.

The landing phase is also known as the powered descent

phase. In this phase, the lander performs a velocity reduction

via the MLE, executes a backshell evasion maneuver, when

necessary, selects a safe landing site, and finally lands on

the selected landing site safely to achieve the soft landing.

3. Overview of the EDL GNC System

3.1. GNC Requirements. The GNC system requirements for

Tianwen-1 EDL process are as follows:

(1) Key events, such as trim wing deployment, parachute deployment, heatshield jettison, backshell separation, MLE ignition, and touchdown detection,

should be triggered properly

(2) After backshell separation, the lander should not be

collided with the backshell

(3) The actual landing site should be selected on-board

within the preselected landing area

(4) The lander’s touchdown attitude, angular rate, and

vertical and horizontal velocities should meet the

requirements

(5) The fuel consumption of the EDL process needs to

be within a reasonable range.

3.2. GNC Modes. According to the EDL process profile and

GNC requirements, the EDL process is divided into eight

GNC modes: the AOA-trim mode, the lift control mode,

the parachute descent control mode, the powered deceleration mode, the hover and imaging mode, the hazard avoidance maneuver mode, the slow descent mode, and ended

with the no-control mode. The transition of these eight

modes is shown in Figure 3.

In the atmospheric entry segment, the GNC system

operates with the AOA-trim mode initially, in which the

descent module’s attitude is adjusted to a predefined attitude

for guided lift entry. Once the sensed acceleration magnitude

exceeds 0.2 Earth g, the lift control mode is activated to produce proper lift to control the entry trajectory.

With the deployment of parachute, the GNC system

switches to the parachute descent control mode and uses this

mode throughout the whole parachute descent phase. In this

mode, the descent module’s velocity would descent to about

95 m/s at an altitude of about 1.2 km above the Mars surface.

In the landing phase, the GNC system operates with the

powered deceleration mode, hover and imaging mode, hazard avoidance maneuver mode, slow descent mode, and

ends at the no-control mode when the lander has softly

touched down. The powered deceleration mode begins with

the backshell separation and ends before hover. The main

task of this mode is to use the MLE to reduce the lander’s

velocity, to avoid collisions with the detached backshell,

and to image and select a wide safe landing area for coarse

hazard avoidance. In the hover and imaging mode, the

lander maintains a hover state to take 3D images of the landing area and then selects a safe landing site. Once the landing

site is selected, the GNC system switches to the hazard

avoidance maneuver mode to perform hazard avoidance

and descent such that the lander would descent to 20 m altitude above the landing site with a zero horizontal velocity

and a preset value of vertical velocity (about 1.5 m/s). Then,

the GNC system switches to the slow descent mode. In this

mode, the lander slowly descends at a preset speed, eliminates the horizontal speed, and maintains a vertical attitude.

Once the lander’s touchdown is detected, the GNC system

sends a shutdown signal to turn off the MLE and then

switches to the no-control mode, in which no further orbit

control and attitude control are performed any more.

3.3. GNC System Configuration. The EDL GNC system

scheme is illustrated in Figure 4, where sensors, actuators,

and the GNC computer will be described in this subsection,

and the GNC algorithms will be presented in the following

sections.

2 Space: Science & Technology

3.4. Sensors

3.4.1. Star Sensors. A pair of star sensors working from the 8

hours prior to orbiter/descent module separation to 10 seconds before atmospheric entry are equipped for attitude

determination.

3.4.2. Inertial Measurement Units (IMUs). A pair of IMUs

are carried out, each of which consists of three orthogonal

accelerometers and three orthogonal gyroscopes to measure

the specific force and angular rate, respectively. During the

EDL, the IMUs are used for inertial navigation and key event

triggers.

3.4.3. Landing Radars. A microwave radar and a phased

array sensor (PAS) are configured to provide Mars-related

measurements. The microwave radar contains four beams,

each of which works automatically. The PAS contains nine

beams, four of which are selected by the GNC system to provide measurements at every measurement time. Every beam

for the two radars can measure slant-range and groundrelative velocity along its axis simultaneously.

3.4.4. Hazard Avoidance Sensors. Two hazard avoidance sensors are equipped for hazard avoidance and landing site

selection. One is called the optical obstacle avoidance sensor

Tianwen-1 probe = Orbiter

Orbiter

+ Descent module

Descent module

Backshell

Rover

Landing platform

Heatshield

Figure 1: Main components of the Tianwen-1 spacecraft.

Scientific exploration stage

Mars parking stage

Mars capture stage

Earth-mars transfer stage

Deorbitand landing stage

Figure 2: Mission profile of Tianwen-1 [8].

Space: Science & Technology 3

(OOAS), which consists of only a single optical imaging lens.

The other is called the multifunction obstacle avoidance sensor (MOAS), consisting of an optical imaging lens and a

laser imaging lens. During the powered deceleration mode,

2D images of the landing area are taken by the OOAS and

then a safe area for coarse hazard avoidance is selected. During the hover and imaging mode, 3D images are taken and a

safe landing site for precise hazard avoidance is selected. An

Heatshield

jettison

Backshell

separation

Optical

imaging

(Not in actual proportion)

Parachute

deployment

(1.8 mach)

Trim wing deployment

(2.8 mach)

0 km

AOA-trim

~740 km

~125 km

~60 km

~10 km

~100 m

Slow

descent

Parachute descent

~20 m

control Powered

decelearation

Hover and

imaging Hazard

avoidance

maneuver

No-control

Lift control

Mars surface

Figure 3: Schematic and GNC modes of Tianwen-1 EDL [11].

Navigation

algorithm

Estimated state

Attitude

command

Guidance

law

Attitude

control

IMU & landing

radars

Dynamic &

environment

models

Actuators

(MLE & thrusters)

Estimated attitude

GNC

computer

Landing site selection

Hazard avoidance sensors

Thruster

command

MLE command

Figure 4: The Tianwen-1 EDL GNC system scheme.

4 Space: Science & Technology

additional option for precise hazard avoidance is that the

OOAS can work in conjunction with the optical imaging

mode of the MOAS.

3.5. Actuators. 6 × 25 N thrusters and 20 × 250 N thrusters

are mounted for attitude control, of which 8 × 250 N

thrusters are additionally used for translation control in

the horizontal plane after the hover and imaging phase. In

addition, a MLE for translation control is carried out to produce either a constant thrust with 7500 N or a throttleable

thrust in the range from 1500 N to 5000 N.

3.6. GNC Computer. The GNC computer, referred to as

entry and descent control unit (EDCU), collects and processes data from the sensors, actuators, and On-Board Data

Handling (OBDH) system, performs real-time GNC calculation, and then sends control signals for orbit and attitude

control. During the powered descent phase, the EDCU is

also responsible for hazard recognition and landing site

selection by analyzing the images obtained from the hazard

avoidance sensors.

4. Guidance Algorithm

The Tianwen-1 performed a guided entry, an unguided parachute descent, and a guided powered descent in succession.

The entry guidance is based on the MSL [12, 13] and

ChangE-5 reentry probe [14, 15], and the powered descent

guidance is based on ChangE-3 [16, 17].

4.1. Entry Guidance. For the entry phase, early missions

adopted the unguided ballistic trajectory, leading to a large

landing error ellipse. The MSL was the first mission that flew

the guided lifting entry at Mars. Because of the displacement

of the center of mass of the entry with its axis of symmetry, a

nonzero trim angle-of-attack can be generated to produce a

lift force. By modulating the bank angle to change the direction of the lift vector, the entry trajectory can be controlled

such that the parachute deploy ellipse is minimized. To minimize size of the parachute deploy ellipse, the Tiawen-1 also

adopted active guidance during the entry phase.

Depending on the bank angle command, the entry guidance of the Tianwen-1 is divided into four phases: prebank,

range control, heading alignment, and zero-bank.

In the AOA-trim mode, the prebank guidance is executed by commanding a constant nominal bank angle.

When the AOA-trim mode is switched to the lift control

mode, the range control begins. In this mode, the descent

module flies with its trim angle-of-attack. Based on the estimated drag accelerations, altitude rate, and range errors with

respect to a reference trajectory [18] stored on-board, an

analytic predictor-corrector guidance algorithm calculates

the commanding bank angle and then sends it to the attitude

control system such that the range error can be minimized.

When the navigated Mars-related velocity drops to

1700 m/s, the range control capability is greatly reduced,

and the commanding bank angle is easy to saturate. At

this time, the range control algorithm is ceased, and the

heading alignment algorithms begin to minimize the

cross-range error until the trim wing is deployed. Then,

the zero-bank phase begins, in which the bank command

is set to 0°

If the error between the navigated and nominal states at

entry interface (EI) was too large, it might not be possible to

track the reference trajectory, which may saturate the guidance bank command and make it impossible to meet the

parachute deployment requirements. To cope with this case,

a range compensation algorithm is designed to implement at

the beginning of the AOA-trim mode such that the parachute deployment altitude constraint is satisfied at the

expense of parachute deployment ellipse.

The flow chart of the entry guidance algorithm is summarized in Figure 5.

4.2. Powered Descent Guidance. The main purpose of powered descent guidance is to reduce the vehicle’s velocity

and execute hazard avoidance and backshell evasion. As

mentioned in Section 3.2, the GNC operates in five modes

in the powered descent process, i.e., powered deceleration,

hover and imaging, hazard avoidance, slow descent, and

no-control. There is no trajectory guidance for the nocontrol mode. The powered descent guidance is mainly

inherited from the ChangE-3 [16, 17], except that the backshell evasion should be considered for the Tianwen-1 during

the powered deceleration phase.

Here, we only introduce the powered deceleration guidance. For the guidance algorithms of other four modes,

readers may refer to Refs. [16, 17]. The main tasks of the

powered deceleration are reducing the lander’s velocity

using the MLE, executing a backshell evasion maneuver,

and performing coarse hazard avoidance through the safe

landing zone detection by the OOAS. The powered deceleration guidance consists of two segments. In the first segment, the lander’s velocity is reduced, and a backshell

evasion maneuver may be executed, depending on the magnitude of the lander’s navigated velocity at the time of backshell separation. When the lander reaches a specified altitude

with an almost vertical attitude, the OOAS begins to work,

trying to determine a safe landing zone. Once the safe landing zone is determined, the second segment begins, of which

a hazard avoidance maneuver is executed with a throttleable

thrust explicit guidance [19] until the lander is hovered at a

100 m altitude above the Mars surface slowly.

5. Navigation

The radar-updated inertial navigation strategy is used for the

Tianwen-1 EDL phase, whereas the GNC system relies on

the inertial navigation system (INS) only before the heatshield jettison and the radar-derived states are used to correct the INS-derived states once the heatshield is separated,

such that accurate altitude and velocity estimates can be provided. In the slow descent stage, in order to avoid the

adverse effects of engine plumes on the landing radars, pure

inertial navigation is restored at this stage.

The Tianwen-1 EDL navigation framework after heatshield separation is mainly inherited from the ChangE-3

lander [16, 17, 20]. Note that at most eight beams can be used

for correction. For each measurement time, at most eight

Space: Science & Technology 5

(OOAS), which consists of only a single optical imaging lens.

The other is called the multifunction obstacle avoidance sensor (MOAS), consisting of an optical imaging lens and a

laser imaging lens. During the powered deceleration mode,

2D images of the landing area are taken by the OOAS and

then a safe area for coarse hazard avoidance is selected. During the hover and imaging mode, 3D images are taken and a

safe landing site for precise hazard avoidance is selected. An

Heatshield

jettison

Backshell

separation

Optical

imaging

(Not in actual proportion)

Parachute

deployment

(1.8 mach)

Trim wing deployment

(2.8 mach)

0 km

AOA-trim

~740 km

~125 km

~60 km

~10 km

~100 m

Slow

descent

Parachute descent

~20 m

control Powered

decelearation

Hover and

imaging Hazard

avoidance

maneuver

No-control

Lift control

Mars surface

Figure 3: Schematic and GNC modes of Tianwen-1 EDL [11].

Navigation

algorithm

Estimated state

Attitude

command

Guidance

law

Attitude

control

IMU & landing

radars

Dynamic &

environment

models

Actuators

(MLE & thrusters)

Estimated attitude

GNC

computer

Landing site selection

Hazard avoidance sensors

Thruster

command

MLE command

Figure 4: The Tianwen-1 EDL GNC system scheme.

4 Space: Science & Technology

additional option for precise hazard avoidance is that the

OOAS can work in conjunction with the optical imaging

mode of the MOAS.

3.5. Actuators. 6 × 25 N thrusters and 20 × 250 N thrusters

are mounted for attitude control, of which 8 × 250 N

thrusters are additionally used for translation control in

the horizontal plane after the hover and imaging phase. In

addition, a MLE for translation control is carried out to produce either a constant thrust with 7500 N or a throttleable

thrust in the range from 1500 N to 5000 N.

3.6. GNC Computer. The GNC computer, referred to as

entry and descent control unit (EDCU), collects and processes data from the sensors, actuators, and On-Board Data

Handling (OBDH) system, performs real-time GNC calculation, and then sends control signals for orbit and attitude

control. During the powered descent phase, the EDCU is

also responsible for hazard recognition and landing site

selection by analyzing the images obtained from the hazard

avoidance sensors.

4. Guidance Algorithm

The Tianwen-1 performed a guided entry, an unguided parachute descent, and a guided powered descent in succession.

The entry guidance is based on the MSL [12, 13] and

ChangE-5 reentry probe [14, 15], and the powered descent

guidance is based on ChangE-3 [16, 17].

4.1. Entry Guidance. For the entry phase, early missions

adopted the unguided ballistic trajectory, leading to a large

landing error ellipse. The MSL was the first mission that flew

the guided lifting entry at Mars. Because of the displacement

of the center of mass of the entry with its axis of symmetry, a

nonzero trim angle-of-attack can be generated to produce a

lift force. By modulating the bank angle to change the direction of the lift vector, the entry trajectory can be controlled

such that the parachute deploy ellipse is minimized. To minimize size of the parachute deploy ellipse, the Tiawen-1 also

adopted active guidance during the entry phase.

Depending on the bank angle command, the entry guidance of the Tianwen-1 is divided into four phases: prebank,

range control, heading alignment, and zero-bank.

In the AOA-trim mode, the prebank guidance is executed by commanding a constant nominal bank angle.

When the AOA-trim mode is switched to the lift control

mode, the range control begins. In this mode, the descent

module flies with its trim angle-of-attack. Based on the estimated drag accelerations, altitude rate, and range errors with

respect to a reference trajectory [18] stored on-board, an

analytic predictor-corrector guidance algorithm calculates

the commanding bank angle and then sends it to the attitude

control system such that the range error can be minimized.

When the navigated Mars-related velocity drops to

1700 m/s, the range control capability is greatly reduced,

and the commanding bank angle is easy to saturate. At

this time, the range control algorithm is ceased, and the

heading alignment algorithms begin to minimize the

cross-range error until the trim wing is deployed. Then,

the zero-bank phase begins, in which the bank command

is set to 0°

If the error between the navigated and nominal states at

entry interface (EI) was too large, it might not be possible to

track the reference trajectory, which may saturate the guidance bank command and make it impossible to meet the

parachute deployment requirements. To cope with this case,

a range compensation algorithm is designed to implement at

the beginning of the AOA-trim mode such that the parachute deployment altitude constraint is satisfied at the

expense of parachute deployment ellipse.

The flow chart of the entry guidance algorithm is summarized in Figure 5.

4.2. Powered Descent Guidance. The main purpose of powered descent guidance is to reduce the vehicle’s velocity

and execute hazard avoidance and backshell evasion. As

mentioned in Section 3.2, the GNC operates in five modes

in the powered descent process, i.e., powered deceleration,

hover and imaging, hazard avoidance, slow descent, and

no-control. There is no trajectory guidance for the nocontrol mode. The powered descent guidance is mainly

inherited from the ChangE-3 [16, 17], except that the backshell evasion should be considered for the Tianwen-1 during

the powered deceleration phase.

Here, we only introduce the powered deceleration guidance. For the guidance algorithms of other four modes,

readers may refer to Refs. [16, 17]. The main tasks of the

powered deceleration are reducing the lander’s velocity

using the MLE, executing a backshell evasion maneuver,

and performing coarse hazard avoidance through the safe

landing zone detection by the OOAS. The powered deceleration guidance consists of two segments. In the first segment, the lander’s velocity is reduced, and a backshell

evasion maneuver may be executed, depending on the magnitude of the lander’s navigated velocity at the time of backshell separation. When the lander reaches a specified altitude

with an almost vertical attitude, the OOAS begins to work,

trying to determine a safe landing zone. Once the safe landing zone is determined, the second segment begins, of which

a hazard avoidance maneuver is executed with a throttleable

thrust explicit guidance [19] until the lander is hovered at a

100 m altitude above the Mars surface slowly.

5. Navigation

The radar-updated inertial navigation strategy is used for the

Tianwen-1 EDL phase, whereas the GNC system relies on

the inertial navigation system (INS) only before the heatshield jettison and the radar-derived states are used to correct the INS-derived states once the heatshield is separated,

such that accurate altitude and velocity estimates can be provided. In the slow descent stage, in order to avoid the

adverse effects of engine plumes on the landing radars, pure

inertial navigation is restored at this stage.

The Tianwen-1 EDL navigation framework after heatshield separation is mainly inherited from the ChangE-3

lander [16, 17, 20]. Note that at most eight beams can be used

for correction. For each measurement time, at most eight

Space: Science & Technology 5

beams can be available which can provide a much more

redundancy. Therefore, a multiple-beam fault detection, isolation, and recovery (FDIR) algorithm for the landing radars has

been designed. Noting that the working beams for the PAS are

not fixed, the velocity corrections in the presence of coplanarity have also designed for the Tianwen-1. In addition, high

dynamic oscillatory motion at the beginning of the parachute

descent phase may saturate the gyroscope and produce large

attitude estimation errors, thereby producing a high landing

risk. To address this problem, an online INS reinitialization

algorithm [21] is designed by combining the data from the

IMU and the radars. The flow chart of the EDL navigation

algorithm is summarized in Figure 6.

5.1. Inertial Navigation Algorithm. The descent module’s

navigated states are initialized a few minutes before the

orbiter/descent module separation, where position and

velocity are uploaded by ground tracking and attitude

knowledge is provided by star sensors. Since then, the

descent module’s attitude is propagated by the gyroscope

measurements with the aid of the star sensors, and its

position and velocity are propagated using attitude information and high-order orbit model, where the accelerometers are used to detect and compensate for

nongravitational acceleration. Because of the block of the

Mars, the star sensors are not available about 10 seconds

prior to the entry interface. After that, the descent module

relies on the INS to propagate the navigation state using

the IMU measurements.

To compensate for the high dynamical motion during

the parachute descent phase, based on ChangE-3’s algorithm

[20], a four-interval strapdown algorithm with recursive

form attitude propagation and lever arm compensation

based on least-square methods is designed.

Bank angle = 0°

Mach < 2.8 (deploy

trim-wing)

Heading alignment algorithms

Range compensation

algorithms

Analytic predictor-corrector guidance

algorithm

Output

Navigated parameters

Altitude < 125 km

Yes

Bank angle = 52°

Get reference values of

drag, altitude rate, downrange and

gains from reference trajectory

Get the reference vertical L/D at the

velocity

Calculate bank angle magnitude

command

Command a bank reversal

Bank angle filtering and limiting

Yes

Initial longitude and

latitude error >

threshold

Yes

V < 1700 m/s

Calculate bank angle

magnitude and sign

Bank angle filtering

and limiting

Drag

acceleration > 0.2 g

Figure 5: The flow chart of the Tianwen-1 entry guidance.

6 Space: Science & Technology

5.2. Altitude Correction. Because INS errors accumulate over

time, once the heat shield is jettisoned, the landing radars

can provide surface-relative measurements to correct the

INS-derived altitude and surface-relative velocity. Before

altitude correction, an FDIR algorithm for slant-range measurements is implemented to detect and remove multiplebeam outliers.

Given the selected beams for altitude correction, a distributed fusion architecture is used [20]. Firstly, the

second-order state equation for altitude and vertical velocity

is formulated, and local estimates of altitude corrections for

each beam are updated using the Kalman filter, the gain of

which is approximated as a function of altitude, which is calculated offline according to the predefined landing

INS

propagation

Altitude &

velocity

correction

IMU

Initialization before

entry

Multiple-beam

FDIR Radar

Guidance

and

control

INS online

re-initialization

Gyroscope saturation

Figure 6: Flow chart of the EDL navigation algorithm of Tianwen-1.

Rate limiter region

PID control region

Rate limiter region

Nominal

rate control

region

Parabolic

target rate

control

region

??d = d??r

??d = –d??r

–??mL ??mL

–??mS ??mS

Parabolic

target rate

control

region

Nominal

rate control

region

Figure 7: The attitude phase plane partition.

Space: Science & Technology 7

beams can be available which can provide a much more

redundancy. Therefore, a multiple-beam fault detection, isolation, and recovery (FDIR) algorithm for the landing radars has

been designed. Noting that the working beams for the PAS are

not fixed, the velocity corrections in the presence of coplanarity have also designed for the Tianwen-1. In addition, high

dynamic oscillatory motion at the beginning of the parachute

descent phase may saturate the gyroscope and produce large

attitude estimation errors, thereby producing a high landing

risk. To address this problem, an online INS reinitialization

algorithm [21] is designed by combining the data from the

IMU and the radars. The flow chart of the EDL navigation

algorithm is summarized in Figure 6.

5.1. Inertial Navigation Algorithm. The descent module’s

navigated states are initialized a few minutes before the

orbiter/descent module separation, where position and

velocity are uploaded by ground tracking and attitude

knowledge is provided by star sensors. Since then, the

descent module’s attitude is propagated by the gyroscope

measurements with the aid of the star sensors, and its

position and velocity are propagated using attitude information and high-order orbit model, where the accelerometers are used to detect and compensate for

nongravitational acceleration. Because of the block of the

Mars, the star sensors are not available about 10 seconds

prior to the entry interface. After that, the descent module

relies on the INS to propagate the navigation state using

the IMU measurements.

To compensate for the high dynamical motion during

the parachute descent phase, based on ChangE-3’s algorithm

[20], a four-interval strapdown algorithm with recursive

form attitude propagation and lever arm compensation

based on least-square methods is designed.

Bank angle = 0°

Mach < 2.8 (deploy

trim-wing)

Heading alignment algorithms

Range compensation

algorithms

Analytic predictor-corrector guidance

algorithm

Output

Navigated parameters

Altitude < 125 km

Yes

Bank angle = 52°

Get reference values of

drag, altitude rate, downrange and

gains from reference trajectory

Get the reference vertical L/D at the

velocity

Calculate bank angle magnitude

command

Command a bank reversal

Bank angle filtering and limiting

Yes

Initial longitude and

latitude error >

threshold

Yes

V < 1700 m/s

Calculate bank angle

magnitude and sign

Bank angle filtering

and limiting

Drag

acceleration > 0.2 g

Figure 5: The flow chart of the Tianwen-1 entry guidance.

6 Space: Science & Technology

5.2. Altitude Correction. Because INS errors accumulate over

time, once the heat shield is jettisoned, the landing radars

can provide surface-relative measurements to correct the

INS-derived altitude and surface-relative velocity. Before

altitude correction, an FDIR algorithm for slant-range measurements is implemented to detect and remove multiplebeam outliers.

Given the selected beams for altitude correction, a distributed fusion architecture is used [20]. Firstly, the

second-order state equation for altitude and vertical velocity

is formulated, and local estimates of altitude corrections for

each beam are updated using the Kalman filter, the gain of

which is approximated as a function of altitude, which is calculated offline according to the predefined landing

INS

propagation

Altitude &

velocity

correction

IMU

Initialization before

entry

Multiple-beam

FDIR Radar

Guidance

and

control

INS online

re-initialization

Gyroscope saturation

Figure 6: Flow chart of the EDL navigation algorithm of Tianwen-1.

Rate limiter region

PID control region

Rate limiter region

Nominal

rate control

region

Parabolic

target rate

control

region

??d = d??r

??d = –d??r

–??mL ??mL

–??mS ??mS

Parabolic

target rate

control

region

Nominal

rate control

region

Figure 7: The attitude phase plane partition.

Space: Science & Technology 7

trajectory, statistical characteristics of the IMUs and radars,

and the terrain characteristics of the Mars. Then, local estimates are fused to form a global estimate of the altitude correction, and the fusion coefficients are determined using the

information allocation principle.

5.3. Velocity Correction. Before implementing the velocity

correction, an FDIR algorithm is used for velocimeter measurement validation and outlier removal. As mentioned in

Section 5.2, at most eight beams can be available at each

update, which allows for multiple-beam FDIR. The velocimeter FDIR algorithm is based on parity equations of five

beams. For the case when eight beams are available, 56 (C5

) combinations should be evaluated. Because the PAS’s

working beams are not fixed, the 56 combinations should

be calculated online, exceeding the limited on-board computational capability. Therefore, an FDIR velocimeter algorithm based on hybrid fault tree and parity equations is

designed for the Tianwen-1, in which details will be presented in an additional paper.

Once the velocimeter FDIR algorithm is implemented,

the beams that used for velocity correction can be selected.

For velocity correction, the first-order equation for the

ground-relative velocity along each beam is established,

and the velocity correction for each beam is updated using

the Kalman filter, the gain of which is approximated as a

function of velocity magnitude. These corrections are then

fused to form a 3D velocity correction. Note that the number

of selected beams for correction is not fixed, and multiple

beams may be coplanar or almost coplanar. The fusion strategy depends on the number of selected beams. If only one

beam is selected, then only the velocity along the beam is

corrected. For the case when two and more beams are

selected, if these beams are coplanar or almost coplanar,

then a plane is constructed by two beams, and the velocity

along the plane is corrected using a least-square method;

otherwise, the 3D velocity correction is performed.

5.4. INS Online Reinitialization. At the beginning of the

parachute descent phase, high dynamic oscillatory

motion, such as parachute inflation and area oscillation

[22], may cause a high angular rate and saturate the

gyroscope. The saturation of the gyroscope would produce large attitude knowledge errors and thereby cause

large errors of the altimeter-derived altitude, estimated

vertical and horizontal velocities, and the landing attitude,

which may cause serious consequences such as a complete system loss. To cope with the gyroscope saturation,

an online INS reinitialization algorithm is designed. The

key for online initialization is the determination of the

nadir vector in a redefined inertial frame using the measurements from the IMU and the radars. Given the nadir

vector, the INS can be reinitialized, and the gravitational

acceleration can be modeled such that the INS navigation

equation can be propagated, and finally, crucial parameters

used for guidance and control systems can be derived. Once

the online initialization is accomplished, the radar-updated

inertial navigation algorithm presented in Sections 5.2 and

5.3 can be implemented.

Altitude and mach

100

120

140

100

Mach

Atmospheric entry

Entry interface

Switch to lift-control mode

Altitude (km)

Deploy trim-wing

Jettison heatshield

Deploy parachute

Separate backshell

Hover and imaging

Hazard avoidance

Slow descent

Parachute descent Powered descent

200 300

Time (s)

400 500 600

Figure 8: The EDL altitude, Mach number, and key events trigger time.

8 Space: Science & Technology

6. Attitude Control Algorithm

In this section, the attitude control algorithms for the

Tianwen-1 EDL phase will be summarized, details of which

will again be presented in an additional paper.

6.1. Attitude Control for Entry Phase. In the AOA-trim

mode, the controller commands the RCS thrusters on the

backshell to track the predicted bank command, the predicted trim angle-of-attack, and the zero sideslip. Here, the

3-axis attitude is decoupled into three independent channels,

and a phase plane logic controller with straight switching

lines is used for each channel.

In the lift control mode, the controller tracks the

guidance bank command using a proportional–integral–

derivative (PID) controller with a pulse-width-modulation

(PWM) technique. It is realized that the powerful capability of the 250 N thrusters may result in angular rate resonance phenomenon. To cope with this problem, an

attitude planning technique is proposed to improve the

tracking capability. For the attitude control of the angles

of attack and sideslip, rate damping controllers are

adopted to stabilize them around their trim values. It

should be noted that, for the case when the trim wing is

not deployed properly, the trim values of the angle-ofattack and sideslip angle are still not around zero, which

cannot meet the requirements of parachute deployment.

In this case, a PID controller is used for keeping the

angle-of-attack being zero. In view of its self-stabilizing

characteristics, the rate damping controller is still used

for the sideslip angle control.

6.2. Attitude Control for Parachute Descent Phase. In the

parachute descent, the attitude controller begins to work

after a few seconds of parachute deployment. The control

strategy in this phase is similar to the one in the lift control

mode. The only difference is that the commanded attitude in

the roll channel here is calculated according to the local upsouth-east frame.

0 100 200 300 400 500 600

Time (s)

Altitude (m)

×104

0 100 200 300 400 500 600

Time (s)

–0.5

0.5

–0.5

0.5

–0.4

–0.2

0 100 200 300 400 500 600

Time (s)

–100

100

x (deg)

–100

–50

y (deg)

–100

100

z (deg)

0 100 200 300 400 500 600

Time (s)

–20

x (deg/s)

–20

y (deg/s)

–20

z (deg/s)

0 100 200 300 400 500 600

Time (s)

(a) (b)

Figure 9: The EDL altitude, quaternion, attitude angles, and angular rate. (a) Altitude. (b) Vector parts of quaternion. (c) Three-axis attitude

angles with respect to the J2000 frame in a “xyz” rotation sequence. (d) Angular rate of the lander.

Space: Science & Technology 9

trajectory, statistical characteristics of the IMUs and radars,

and the terrain characteristics of the Mars. Then, local estimates are fused to form a global estimate of the altitude correction, and the fusion coefficients are determined using the

information allocation principle.

5.3. Velocity Correction. Before implementing the velocity

correction, an FDIR algorithm is used for velocimeter measurement validation and outlier removal. As mentioned in

Section 5.2, at most eight beams can be available at each

update, which allows for multiple-beam FDIR. The velocimeter FDIR algorithm is based on parity equations of five

beams. For the case when eight beams are available, 56 (C5

) combinations should be evaluated. Because the PAS’s

working beams are not fixed, the 56 combinations should

be calculated online, exceeding the limited on-board computational capability. Therefore, an FDIR velocimeter algorithm based on hybrid fault tree and parity equations is

designed for the Tianwen-1, in which details will be presented in an additional paper.

Once the velocimeter FDIR algorithm is implemented,

the beams that used for velocity correction can be selected.

For velocity correction, the first-order equation for the

ground-relative velocity along each beam is established,

and the velocity correction for each beam is updated using

the Kalman filter, the gain of which is approximated as a

function of velocity magnitude. These corrections are then

fused to form a 3D velocity correction. Note that the number

of selected beams for correction is not fixed, and multiple

beams may be coplanar or almost coplanar. The fusion strategy depends on the number of selected beams. If only one

beam is selected, then only the velocity along the beam is

corrected. For the case when two and more beams are

selected, if these beams are coplanar or almost coplanar,

then a plane is constructed by two beams, and the velocity

along the plane is corrected using a least-square method;

otherwise, the 3D velocity correction is performed.

5.4. INS Online Reinitialization. At the beginning of the

parachute descent phase, high dynamic oscillatory

motion, such as parachute inflation and area oscillation

[22], may cause a high angular rate and saturate the

gyroscope. The saturation of the gyroscope would produce large attitude knowledge errors and thereby cause

large errors of the altimeter-derived altitude, estimated

vertical and horizontal velocities, and the landing attitude,

which may cause serious consequences such as a complete system loss. To cope with the gyroscope saturation,

an online INS reinitialization algorithm is designed. The

key for online initialization is the determination of the

nadir vector in a redefined inertial frame using the measurements from the IMU and the radars. Given the nadir

vector, the INS can be reinitialized, and the gravitational

acceleration can be modeled such that the INS navigation

equation can be propagated, and finally, crucial parameters

used for guidance and control systems can be derived. Once

the online initialization is accomplished, the radar-updated

inertial navigation algorithm presented in Sections 5.2 and

5.3 can be implemented.

Altitude and mach

100

120

140

100

Mach

Atmospheric entry

Entry interface

Switch to lift-control mode

Altitude (km)

Deploy trim-wing

Jettison heatshield

Deploy parachute

Separate backshell

Hover and imaging

Hazard avoidance

Slow descent

Parachute descent Powered descent

200 300

Time (s)

400 500 600

Figure 8: The EDL altitude, Mach number, and key events trigger time.

8 Space: Science & Technology

6. Attitude Control Algorithm

In this section, the attitude control algorithms for the

Tianwen-1 EDL phase will be summarized, details of which

will again be presented in an additional paper.

6.1. Attitude Control for Entry Phase. In the AOA-trim

mode, the controller commands the RCS thrusters on the

backshell to track the predicted bank command, the predicted trim angle-of-attack, and the zero sideslip. Here, the

3-axis attitude is decoupled into three independent channels,

and a phase plane logic controller with straight switching

lines is used for each channel.

In the lift control mode, the controller tracks the

guidance bank command using a proportional–integral–

derivative (PID) controller with a pulse-width-modulation

(PWM) technique. It is realized that the powerful capability of the 250 N thrusters may result in angular rate resonance phenomenon. To cope with this problem, an

attitude planning technique is proposed to improve the

tracking capability. For the attitude control of the angles

of attack and sideslip, rate damping controllers are

adopted to stabilize them around their trim values. It

should be noted that, for the case when the trim wing is

not deployed properly, the trim values of the angle-ofattack and sideslip angle are still not around zero, which

cannot meet the requirements of parachute deployment.

In this case, a PID controller is used for keeping the

angle-of-attack being zero. In view of its self-stabilizing

characteristics, the rate damping controller is still used

for the sideslip angle control.

6.2. Attitude Control for Parachute Descent Phase. In the

parachute descent, the attitude controller begins to work

after a few seconds of parachute deployment. The control

strategy in this phase is similar to the one in the lift control

mode. The only difference is that the commanded attitude in

the roll channel here is calculated according to the local upsouth-east frame.

0 100 200 300 400 500 600

Time (s)

Altitude (m)

×104

0 100 200 300 400 500 600

Time (s)

–0.5

0.5

–0.5

0.5

–0.4

–0.2

0 100 200 300 400 500 600

Time (s)

–100

100

x (deg)

–100

–50

y (deg)

–100

100

z (deg)

0 100 200 300 400 500 600

Time (s)

–20

x (deg/s)

–20

y (deg/s)

–20

z (deg/s)

0 100 200 300 400 500 600

Time (s)

(a) (b)

Figure 9: The EDL altitude, quaternion, attitude angles, and angular rate. (a) Altitude. (b) Vector parts of quaternion. (c) Three-axis attitude

angles with respect to the J2000 frame in a “xyz” rotation sequence. (d) Angular rate of the lander.

Space: Science & Technology 9

6.3. Attitude Control for Powered Descent Phase. Due to the

uncertainties of the parachute descent phase, the state dispersion from the nominal values may be large at the beginning of the powered descent phase. Therefore, the attitude

controller should have a strong robust capability. For example, the controller in the powered deceleration mode should

track the guidance command with a maximum angular rate

of 15 deg/s. Meanwhile, the rapid attitude tracking may suffer from large disturbance. As shown in Figure 7, the attitude

phase plane is partitioned into four regions: a PID control

region, a nominal rate control region, a rate limiter region,

and a parabolic target rate control region. In the nominal

rate and parabolic target rate control regions, the proportional–integral (PI)+PWM controllers are used to track the

desired rate. In the rate limiter region, when the angular rate

exceeds the threshold, the available thrusters work fully on

control to drive the angular rate to the set range. In the

PID control region, the traditional PID+PWM controller is

used to track the desired attitude angle and angular rate.

Several additional strategies are also designed. Firstly,

observers are designed for disturbance online identification

and compensation. Secondly, the commanded thrust direction is decoupled from the attitude and is sent directly to

the pitch and yaw channels, which makes the tracking of

the commanded thrust direction as fast as possible. Thirdly,

an attitude controller based on multiple-level thruster

switching logic is designed based on the level of attitude

error. Finally, an FDIR algorithm is designed for the

Tianwen-1 to identify fault thrusts and rearrange the

remaining ones, which makes the controller more robust in

case that any thrust cannot be opened.

7. Flight Results

In this section, the actual flight results for the Tianwen-1

EDL process are presented. The descent module was separated from the orbiter at 04:18:54 a.m. BJT on May 15,

2021. About three hours later, the descent module entered

the Mars atmosphere at 07:08:54 a.m. BJT on May 15,

2021. Through 537-second EDL process, the lander landed

successfully on the surface of Mars. The longitude and latitude of the actual landing site are 109.925° E and 25.066°

N, respectively, and the downrange and cross-range errors

of which with respect to the predefined landing site are

–15

–10

–5

Angle of attack (deg)

–50

Bank angle (deg)

0 50 100 150 200 250 300

Time (s)

–4

–2

Sideslip angle (deg)

Figure 10: Angles of bank, sideslip, and attack during the atmospheric entry.

10 Space: Science & Technology

3.1 km and 0.2 km, respectively. The actual EDL trajectories

collected from telemetry are shown in Figures 8 and 9, the

angles of bank, sideslip, and attack during the atmospheric

entry are shown in Figure 10, and the telemetry Marsrelated velocity during the powered descent phase is given

in Figure 11.

It can be seen that the atmospheric entry phase lasted

279 seconds, in which the AOA-trim mode took 68 seconds.

When the descent module’s sensed acceleration magnitude

exceeded 1.96 m/s2 at a navigated altitude of about 63 km

and navigated velocity of about Mach 24, the lift control

mode began. The trim wing was deployed at a navigated

velocity of Mach 2.8, after which the trim values of the total

angle-of-attack approached to around zero.

The parachute was deployed at a navigated velocity of

Mach 1.8 when the navigated altitude is about 13 km. After

20 seconds when the descent module’s velocity reduced to

about Mach 0.5, the heatshield was jettisoned, and then the

landing legs were deployed, and the two radars began to provide Mars-related measurements to correct the INS errors.

At a navigated altitude of 1.3 km and navigated velocity

of Mach 0.25, the backshell was separated, implying the

beginning of the powered descent phase, which lasted 90

seconds. About 1 second after backshell/lander separation,

the MLE was ignited, the lander’s velocity was reduced

–60

–40

–20

Up (m/s)

–10

–5

South (m/s)

440 450 460 470 480 490 500 510 520 530 540

Time (s)

East (m/s)

Figure 11: Mars-related velocity in the up-south-east frame during the powered descent phase.

Figure 12: Image of the actual landing positions of the lander,

backshell, and heatshield taken by the Tianwen-1 orbiter.

Figure 13: Image of the actual landing site taken by the Zhurong

rover.

Space: Science & Technology 11

6.3. Attitude Control for Powered Descent Phase. Due to the

uncertainties of the parachute descent phase, the state dispersion from the nominal values may be large at the beginning of the powered descent phase. Therefore, the attitude

controller should have a strong robust capability. For example, the controller in the powered deceleration mode should

track the guidance command with a maximum angular rate

of 15 deg/s. Meanwhile, the rapid attitude tracking may suffer from large disturbance. As shown in Figure 7, the attitude

phase plane is partitioned into four regions: a PID control

region, a nominal rate control region, a rate limiter region,

and a parabolic target rate control region. In the nominal

rate and parabolic target rate control regions, the proportional–integral (PI)+PWM controllers are used to track the

desired rate. In the rate limiter region, when the angular rate

exceeds the threshold, the available thrusters work fully on

control to drive the angular rate to the set range. In the

PID control region, the traditional PID+PWM controller is

used to track the desired attitude angle and angular rate.

Several additional strategies are also designed. Firstly,

observers are designed for disturbance online identification

and compensation. Secondly, the commanded thrust direction is decoupled from the attitude and is sent directly to

the pitch and yaw channels, which makes the tracking of

the commanded thrust direction as fast as possible. Thirdly,

an attitude controller based on multiple-level thruster

switching logic is designed based on the level of attitude

error. Finally, an FDIR algorithm is designed for the

Tianwen-1 to identify fault thrusts and rearrange the

remaining ones, which makes the controller more robust in

case that any thrust cannot be opened.

7. Flight Results

In this section, the actual flight results for the Tianwen-1

EDL process are presented. The descent module was separated from the orbiter at 04:18:54 a.m. BJT on May 15,

2021. About three hours later, the descent module entered

the Mars atmosphere at 07:08:54 a.m. BJT on May 15,

2021. Through 537-second EDL process, the lander landed

successfully on the surface of Mars. The longitude and latitude of the actual landing site are 109.925° E and 25.066°

N, respectively, and the downrange and cross-range errors

of which with respect to the predefined landing site are

–15

–10

–5

Angle of attack (deg)

–50

Bank angle (deg)

0 50 100 150 200 250 300

Time (s)

–4

–2

Sideslip angle (deg)

Figure 10: Angles of bank, sideslip, and attack during the atmospheric entry.

10 Space: Science & Technology

3.1 km and 0.2 km, respectively. The actual EDL trajectories

collected from telemetry are shown in Figures 8 and 9, the

angles of bank, sideslip, and attack during the atmospheric

entry are shown in Figure 10, and the telemetry Marsrelated velocity during the powered descent phase is given

in Figure 11.

It can be seen that the atmospheric entry phase lasted

279 seconds, in which the AOA-trim mode took 68 seconds.

When the descent module’s sensed acceleration magnitude

exceeded 1.96 m/s2 at a navigated altitude of about 63 km

and navigated velocity of about Mach 24, the lift control

mode began. The trim wing was deployed at a navigated

velocity of Mach 2.8, after which the trim values of the total

angle-of-attack approached to around zero.

The parachute was deployed at a navigated velocity of

Mach 1.8 when the navigated altitude is about 13 km. After

20 seconds when the descent module’s velocity reduced to

about Mach 0.5, the heatshield was jettisoned, and then the

landing legs were deployed, and the two radars began to provide Mars-related measurements to correct the INS errors.

At a navigated altitude of 1.3 km and navigated velocity

of Mach 0.25, the backshell was separated, implying the

beginning of the powered descent phase, which lasted 90

seconds. About 1 second after backshell/lander separation,

the MLE was ignited, the lander’s velocity was reduced

–60

–40

–20

Up (m/s)

–10

–5

South (m/s)

440 450 460 470 480 490 500 510 520 530 540

Time (s)

East (m/s)

Figure 11: Mars-related velocity in the up-south-east frame during the powered descent phase.

Figure 12: Image of the actual landing positions of the lander,

backshell, and heatshield taken by the Tianwen-1 orbiter.

Figure 13: Image of the actual landing site taken by the Zhurong

rover.

Space: Science & Technology 11

further, and a backshell evasion maneuver was also performed. Then, the OOAS obtained images of the predefined

landing area used for coarse hazard avoidance. When the

lander’s altitude reduced to about 100 m, the GNC switched

to the hover and imaging mode. In this mode, the MOAS

obtained the 3D images of Mars surface and determined

the final landing site. Then the GNC switched to the hazard

avoidance mode. When the lander was at an altitude of 20 m

above the landing site with a 1.5 m/s vertical velocity and

0 m/s horizontal velocity, the GNC switched to the slow

descent mode. Finally, the lander landed on the Mars softly

with a stable vertical attitude. The touchdown horizontal

velocity is less than 0.16 m/s, and the attitude error is less

than 0.1 deg.

The image of the actual landing positions of the lander,

backshell, and heatshield is shown in Figure 12, and the

image of the lander taken by the Zhurong rover is shown

in Figure 13. Therefore, the effectiveness of the backshell

evasion and hazard avoidance was demonstrated.

8. Conclusions

According to the Tianwen-1 EDL GNC requirements, the

GNC modes, GNC architecture, and key GNC algorithms

have been described in this paper.

The effectiveness of the GNC system design was demonstrated by the successful landing of the Tianwen-1, which

landed on the Mars with a small landing ellipse, a soft touchdown velocity, and a stable vertical attitude.

It should be noted that the Tianwen-1 landed at a site

with a low MOLA elevation around a relative flat area. In

the future, China will target areas that have higher scientific

value, more rugged terrain, and higher MOLA elevation.

This puts forward new requirements for the EDL GNC technology, e.g., the GNC system must have high precision navigation capability and have stronger maneuverability or

deceleration capability.

Abbreviations

AOA: Angle-of-attack

BJT: Beijing time

EDCU: Entry and descent control unit

EDL: Entry, descent, and landing

EI: Entry interface

FDIR: Fault detection, isolation, and recovery

GNC: Guidance, navigation, and control

INS: Inertial navigation system

MLE: Main landing engine

MOAS: Multifunction obstacle avoidance sensor

MSL: Mars Science Laboratory

OBDH: On-Board Data Handling

OOAS: Optical obstacle avoidance sensor

PAS: Phased array sensor.

Data Availability

The data used to support the findings of this study are

included within the article.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This work is supported by the Chinese National Space

Administration (CNSA). Parts of the work are supported

by the National Natural Science Foundation of China (Grant

No. 61503023, No. 61673057, No. 61803028, and No.

61903032).

References

[1] R. N. Ingoldby, “Guidance and control system design of the

Viking planetary lander,” Journal of Guidance and Control,

vol. 1, no. 3, pp. 189–196, 1978.

[2] M. P. Golombek, “The mars pathfinder mission,” Journal of

Geophysical Research: Planets, vol. 102, no. E2, pp. 3953–

3965, 1997.

[3] R. Roncoli and J. Ludwinski, “Mission design overview for the

Mars exploration rover mission,”in AIAA/AAS Astrodynamics

Specialist Conference and Exhibit, Monterey, California,

August 2002.

[4] M. R. Grover, B. D. Cichy, and P. N. Desai, “Overview of the

Phoenix entry, descent, and landing system architecture,”

Journal of Spacecraft and Rockets, vol. 48, no. 5, pp. 706–712,

2011.

[5] M. S. Martin, G. F. Mendeck, P. B. Brugarolas et al., “In-flight

experience of the Mars Science Laboratory guidance, navigation, and control system for entry, descent, and landing,”

CEAS Space Journal, vol. 7, no. 2, pp. 119–142, 2015.

[6] T. Hoffman, “InSight: mission to mars,” in 2018 IEEE Aerospace Conference, pp. 1–11, Big Sky, MT, USA, March 2018.

[7] K. A. Farley, K. H. Williford, K. M. Stack et al., “Mars 2020

mission overview,” Space Science Reviews, vol. 216, no. 8,

p. 142, 2020.

[8] P. J. Ye, Z. Z. Sun, W. Rao, and L. Z. Meng, “Mission overview

and key technologies of the first Mars probe of China,” Science

China Technological Sciences, vol. 60, no. 5, pp. 649–657, 2017.

[9] Z. Yu, P. Cui, and J. L. Crassidis, “Design and optimization of

navigation and guidance techniques for Mars pinpoint landing: Review and prospect,” Progress in Aerospace Sciences,

vol. 94, pp. 82–94, 2017.

[10] J. Dong, Z. Sun, W. Rao et al.,“Mission profile and design challenges of Mars landing exploration,” Planetary Remote Sensing

and Mapping, vol. XLII-3/W1, pp. 75–87, 2018.

[11] X. Y. Huang, C. Xu, R. H. Hu, M. D. Li, M. W. Guo, and J. C.

Hu, “Research of autonomous navigation and control scheme

based on multi-information fusion for Mars pinpoint landing,” Journal of Deep Space Exploration, vol. 6, no. 4,

pp. 348–357, 2019.

[12] G. F. Mendeck and L. Craig McGrew, “Entry guidance design

and postflight performance for 2011 Mars Science Laboratory

mission,” Journal of Spacecraft and Rockets, vol. 51, no. 4,

pp. 1094–1105, 2014.

[13] A. M. S. Martin, S. W. Lee, and E. C. Wong, “The development

of the MSL guidance, navigation, and control system for entry,

descent, and landing,” in Presented at the 23rd AAS/ AIAA

space flight mechanics meeting, AAS 13-238, Kauai, Hawaii,

2013.

12 Space: Science & Technology

[14] J. Hu, “Adaptive predictive guidance: a unified guidance

method,” Aerospace Control and Application, vol. 45, no. 4,

pp. 53–63, 2019.

[15] J. Hu and Z. Zhang, “A study on the reentry guidance for a

manned lunar return vehicle,” Control Theory & Applications,

vol. 31, no. 12, pp. 1678–1685, 2014.

[16] H. H. Zhang, Y. F. Guan, X. Y. Huang et al., “Guidance navigation and control for Chang’E-3 powered descent,” Scientia

Sinica Technologica, vol. 44, no. 4, pp. 377–384, 2014.

[17] X. Y. Huang, H. H. Zhang, D. Y. Wang, J. Li, Y. F. Guan, and

P. J. Wang, “Autonomous navigation and guidance for

Chang’e-3 soft landing,” Journal of Deep Space Exploration,

vol. 1, pp. 52–59, 2014.

[18] M. W. Guo, M. D. Li, X. Y. Huang, and D. Y. Wang, “On guidance algorithm for Martian atmospheric entry in nonconforming terminal constraints,” Journal of Deep Space Exploration,

vol. 4, no. 2, pp. 184–189, 2017.

[19] D. Y. Wang, X. Huang, and Y. Guan, “GNC system scheme for

lunar soft landing spacecraft,” Advances in Space Research,

vol. 42, no. 2, pp. 379–385, 2008.

[20] H. H. Zhang, J. Li, Y. F. Guan, and X. Y. Huang, “Autonomous

navigation for powered descent phase of Chang’E–3 lunar

lander,” Control Theory & Applications, vol. 31, no. 12,

pp. 1686–1694, 2014.

[21] M. D. Li, X. Huang, D. Wang et al., “Radar-updated inertial

landing navigation with online initialization,” IEEE Transactions on Aerospace and Electronic Systems, vol. 56, no. 5,

pp. 3360–3374, 2020.

[22] J. R. Cruz, D. Way, J. Shidner et al., “Parachute models used in

the Mars Science Laboratory entry, descent, and landing simulation,” in AIAA Aerodynamic Decelerator Systems (ADS)

Conference, p. 1276, Daytona Beach, Florida, March 2013.

Space: Science & Technology 13

further, and a backshell evasion maneuver was also performed. Then, the OOAS obtained images of the predefined

landing area used for coarse hazard avoidance. When the

lander’s altitude reduced to about 100 m, the GNC switched

to the hover and imaging mode. In this mode, the MOAS

obtained the 3D images of Mars surface and determined

the final landing site. Then the GNC switched to the hazard

avoidance mode. When the lander was at an altitude of 20 m

above the landing site with a 1.5 m/s vertical velocity and

0 m/s horizontal velocity, the GNC switched to the slow

descent mode. Finally, the lander landed on the Mars softly

with a stable vertical attitude. The touchdown horizontal

velocity is less than 0.16 m/s, and the attitude error is less

than 0.1 deg.

The image of the actual landing positions of the lander,

backshell, and heatshield is shown in Figure 12, and the

image of the lander taken by the Zhurong rover is shown

in Figure 13. Therefore, the effectiveness of the backshell

evasion and hazard avoidance was demonstrated.

8. Conclusions

According to the Tianwen-1 EDL GNC requirements, the

GNC modes, GNC architecture, and key GNC algorithms

have been described in this paper.

The effectiveness of the GNC system design was demonstrated by the successful landing of the Tianwen-1, which

landed on the Mars with a small landing ellipse, a soft touchdown velocity, and a stable vertical attitude.

It should be noted that the Tianwen-1 landed at a site

with a low MOLA elevation around a relative flat area. In

the future, China will target areas that have higher scientific

value, more rugged terrain, and higher MOLA elevation.

This puts forward new requirements for the EDL GNC technology, e.g., the GNC system must have high precision navigation capability and have stronger maneuverability or

deceleration capability.

Abbreviations

AOA: Angle-of-attack

BJT: Beijing time

EDCU: Entry and descent control unit

EDL: Entry, descent, and landing

EI: Entry interface

FDIR: Fault detection, isolation, and recovery

GNC: Guidance, navigation, and control

INS: Inertial navigation system

MLE: Main landing engine

MOAS: Multifunction obstacle avoidance sensor

MSL: Mars Science Laboratory

OBDH: On-Board Data Handling

OOAS: Optical obstacle avoidance sensor

PAS: Phased array sensor.

Data Availability

The data used to support the findings of this study are

included within the article.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This work is supported by the Chinese National Space

Administration (CNSA). Parts of the work are supported

by the National Natural Science Foundation of China (Grant

No. 61503023, No. 61673057, No. 61803028, and No.

61903032).

References

[1] R. N. Ingoldby, “Guidance and control system design of the

Viking planetary lander,” Journal of Guidance and Control,

vol. 1, no. 3, pp. 189–196, 1978.

[2] M. P. Golombek, “The mars pathfinder mission,” Journal of

Geophysical Research: Planets, vol. 102, no. E2, pp. 3953–

3965, 1997.

[3] R. Roncoli and J. Ludwinski, “Mission design overview for the

Mars exploration rover mission,”in AIAA/AAS Astrodynamics

Specialist Conference and Exhibit, Monterey, California,

August 2002.

[4] M. R. Grover, B. D. Cichy, and P. N. Desai, “Overview of the

Phoenix entry, descent, and landing system architecture,”

Journal of Spacecraft and Rockets, vol. 48, no. 5, pp. 706–712,

2011.

[5] M. S. Martin, G. F. Mendeck, P. B. Brugarolas et al., “In-flight

experience of the Mars Science Laboratory guidance, navigation, and control system for entry, descent, and landing,”

CEAS Space Journal, vol. 7, no. 2, pp. 119–142, 2015.

[6] T. Hoffman, “InSight: mission to mars,” in 2018 IEEE Aerospace Conference, pp. 1–11, Big Sky, MT, USA, March 2018.

[7] K. A. Farley, K. H. Williford, K. M. Stack et al., “Mars 2020

mission overview,” Space Science Reviews, vol. 216, no. 8,

p. 142, 2020.

[8] P. J. Ye, Z. Z. Sun, W. Rao, and L. Z. Meng, “Mission overview

and key technologies of the first Mars probe of China,” Science

China Technological Sciences, vol. 60, no. 5, pp. 649–657, 2017.

[9] Z. Yu, P. Cui, and J. L. Crassidis, “Design and optimization of

navigation and guidance techniques for Mars pinpoint landing: Review and prospect,” Progress in Aerospace Sciences,

vol. 94, pp. 82–94, 2017.

[10] J. Dong, Z. Sun, W. Rao et al.,“Mission profile and design challenges of Mars landing exploration,” Planetary Remote Sensing

and Mapping, vol. XLII-3/W1, pp. 75–87, 2018.

[11] X. Y. Huang, C. Xu, R. H. Hu, M. D. Li, M. W. Guo, and J. C.

Hu, “Research of autonomous navigation and control scheme

based on multi-information fusion for Mars pinpoint landing,” Journal of Deep Space Exploration, vol. 6, no. 4,

pp. 348–357, 2019.

[12] G. F. Mendeck and L. Craig McGrew, “Entry guidance design

and postflight performance for 2011 Mars Science Laboratory

mission,” Journal of Spacecraft and Rockets, vol. 51, no. 4,

pp. 1094–1105, 2014.

[13] A. M. S. Martin, S. W. Lee, and E. C. Wong, “The development

of the MSL guidance, navigation, and control system for entry,

descent, and landing,” in Presented at the 23rd AAS/ AIAA

space flight mechanics meeting, AAS 13-238, Kauai, Hawaii,

2013.

12 Space: Science & Technology

[14] J. Hu, “Adaptive predictive guidance: a unified guidance

method,” Aerospace Control and Application, vol. 45, no. 4,

pp. 53–63, 2019.

[15] J. Hu and Z. Zhang, “A study on the reentry guidance for a

manned lunar return vehicle,” Control Theory & Applications,

vol. 31, no. 12, pp. 1678–1685, 2014.

[16] H. H. Zhang, Y. F. Guan, X. Y. Huang et al., “Guidance navigation and control for Chang’E-3 powered descent,” Scientia

Sinica Technologica, vol. 44, no. 4, pp. 377–384, 2014.

[17] X. Y. Huang, H. H. Zhang, D. Y. Wang, J. Li, Y. F. Guan, and

P. J. Wang, “Autonomous navigation and guidance for

Chang’e-3 soft landing,” Journal of Deep Space Exploration,

vol. 1, pp. 52–59, 2014.

[18] M. W. Guo, M. D. Li, X. Y. Huang, and D. Y. Wang, “On guidance algorithm for Martian atmospheric entry in nonconforming terminal constraints,” Journal of Deep Space Exploration,

vol. 4, no. 2, pp. 184–189, 2017.

[19] D. Y. Wang, X. Huang, and Y. Guan, “GNC system scheme for

lunar soft landing spacecraft,” Advances in Space Research,

vol. 42, no. 2, pp. 379–385, 2008.

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Space: Science & Technology 13

Research Article

Study on Dynamic Characteristics of Mars Entry Module in

Transonic and Supersonic Speeds

Qi Li,1 Rui Zhao,2 Sijun Zhang,3 Wei Rao,1 and Haogong Wei 1

Beijing Institute of Spacecraft System Engineering, CAST, Beijing, China 100094

School of Aeronautics and Astronautics, Beijing Institute of Technology, Beijing, China 100081

Advanced Simulation and Modeling, Inc. Madison, AL 35758, USA

Correspondence should be addressed to Qi Li; qi-ge-ge@163.com

Received 10 August 2021; Accepted 27 February 2022; Published 24 March 2022

Attribution License (CC BY 4.0).

The aerodynamic configuration of the Tianwen-1 Mars entry module that adopts a blunt-nosed and short body shape has obvious

dynamic instability from transonic to supersonic speeds, which may bring risk to parachute deployment. The unsteady detached

eddy of the entry module cannot be accurately simulated by the Reynolds-Averaged Navier-Stokes (RANS) model, while the

computational cost for direct numerical simulation (DNS) and large eddy simulation (LES) is huge. It is difficult to implement

these methods in the coupled engineering calculation of unsteady flow and motion. This paper proposes the integrated

numerical simulation method of computational fluid dynamics and rigid body dynamics (CFD/RBD) based on detached eddy

simulation (DES) and calculates and studies the dynamic characteristics of attitude oscillation of the Mars entry module in free

flight from transonic to supersonic speeds with one degree of freedom (1-DOF) at small releasing angle of attack. In addition,

the unstable range of Mach number and angle of attack are determined, and the effect of different afterbody shapes on

dynamic stability is analyzed.

1. Introduction

The Tianwen-1 probe successfully landed on the predetermined landing area of the southern Utopia Planaria on

May 15, 2021, opening a new era of China’s landing and

exploring on Mars.

Mars is the planet with the most Earth-like environment

that has ever been detected, attracting the most interest from

human race. The history of using space probes to explore

Mars almost runs through the whole aerospace history of

human beings. Since the 1960s, nearly 50 Mars exploration

missions have been launched by the Soviet Union, the

United States, Japan, Russia, India, Europe, and other countries, but more than half failed [1]. Only ten probes from the

United States and China succeeded in the exploration missions of the surface of Mars.

The atmosphere of Mars is thin and its atmospheric density on the surface is only 1%~10% of that of the Earth [2, 3].

Therefore, the process of Mars entry, descent, and landing

(EDL) requires the combined action of aerodynamic shape,

parachute, and other deceleration methods to ensure the safe

landing of a probe and carry out the next step of work.

Tianwen-1 adopts three deceleration methods, namely, the

aerodynamic shape, supersonic parachute, and thrust

reverser, which makes the probe decelerate to 0 m/s after

entering the Martian atmosphere at a speed of 4.8 km/s

and achieve a soft landing on the surface [1] of Mars. Therefore, whether the multiple deceleration methods can be

safely connected is the key to the success of the mission of

Mars EDL.

Due to the rarefied atmosphere of Mars, for the higher

efficiency of aerodynamic deceleration, the aerodynamic

configuration of a Mars entry module is generally designed

as a blunt-body with large angle for higher drag [4–7]. However, such blunt-body presents dynamic instability when it

decelerates to below Mach 3.5 and becomes more unstable

with the decrease in the Mach number. In extreme cases,

there may even be a risk that the parachute cannot be safely

opened due to the attitude oscillation caused by the dynamic

instability [8]. Therefore, accurately predicting the dynamic

characteristics of the entry module in free flight in transonic

and supersonic speeds and determining the range of Mach

AAAS

Space: Science & Technology

Volume 2022, Article ID 9753286, 15 pages

https://doi.org/10.34133/2022/9753286