Visual navigation is imperative for successful asteroid exploration missions. In this study, an integrated visual navigation system was proposed based on angles-only measurements to robustly and accurately determine the pose of the lander during the final landing phase. The system used the lander's global pose information provided by an orbiter, which was deployed in space in advance, and its relative motion information in adjacent images to jointly estimate its optimal state. First, the landmarks on the asteroid surface and markers on the lander were identified from the images acquired by the orbiter. Subsequently, an angles-only measurement model concerning the landmarks and markers was constructed to estimate the orbiter's position and lander's pose. Subsequently, a method based on the epipolar constraint was proposed to estimate the lander's inter-frame motion. Then, the absolute pose and relative motion of the lander were fused using an extended Kalman filter. Additionally, the observability criterion and covariance of the state error were provided. Finally, synthetic image sequences were generated to validate the proposed navigation system, and numerical results demonstrated its advance in terms of robustness and accuracy.

To meet the requirements of the Tianwen-1 mission, adaptive entry guidance for entry vehicles, with low lift-to-drag ratios, limited control authority, and large initial state bias, was presented. Typically, the entry guidance law is divided into four distinct phases: trim angle-of-attack phase, range control phase, heading alignment phase, and trim-wing deployment phase. In the range control phase, the predictor—corrector guidance algorithm is improved by planning an on-board trajectory based on the Mars Science Laboratory (MSL) entry guidance algorithm. The nominal trajectory was designed and described using a combination of the downrange value and other states, such as drag acceleration and altitude rate. For a large initial state bias, the nominal downrange value was modified onboard by weighing the landing accuracy, control authority, and parachute deployment altitude. The biggest advantage of this approach is that it allows the successful correction of altitude errors and the avoidance of control saturation. An overview of the optimal trajectory design process, including a discussion of the design of the initial flight path angle, relevant event trigger, and transition conditions between the four phases, was also presented. Finally, telemetry data analysis and post-flight assessment results were used to illustrate the adaptive guidance law, create good conditions for subsequent parachute reduction and power reduction processes, and gauge the success of the mission.

The powered-descent landing (PDL) phase of the Tianwen-1 mission began with composite backshell—parachute (CBP) separation and ended with landing-rover touchdown. The main tasks of this phase were to reduce the velocity of the lander, perform the avoidance maneuver, and guarantee a soft touchdown. The PDL phase overcame many challenges: performing the divert maneuver to avoid collision with the CBP while simultaneously avoiding large-scale hazards; slowing the descent from approximately 95 to 0 m/s; performing the precise hazard-avoidance maneuver; and placing the lander gently and safely on the surface of Mars. The architecture and algorithms of the guidance, navigation, and control system for the PDL phase were designed; its execution resulted in Tianwen-1's successful touchdown in the morning of 15 May 2021. Consequently, the Tianwen-1 mission achieved a historic autonomous landing with simultaneous hazard and CBP avoidance.

The entry vehicle for the Tianwen-1 mission successfully landed on the surface of Mars at 7:18 AM BJT on May 15, 2021. This successful landing made China the first country to orbit, land, and release a rover in their first attempt at the Mars exploration. The guidance, navigation, and control (GNC) system plays a crucial role in the entry, descent, and landing (EDL) phases. This study focused on the attitude control component of the GNC system design. The EDL phase can be divided into several sub-phases, namely the angle of attack control phase, lift control phase, parachute descent phase, and powered descent phase. Each sub-phase has unique attitude control requirements and challenges. This paper introduces the key aspects of designing attitude controllers for each phase. Furthermore, flight results are presented and analyzed.

On May 15, 2021, the Tianwen-1 lander successfully touched down on the surface of Mars. To ensure the success of the landing mission, an end-to-end Mars entry, descent, and landing (EDL) simulator is developed to assess the guidance, navigation, and control (GNC) system performance, and determine the critical operation and lander parameters. The high-fidelity models of the Mars atmosphere, parachute, and lander system that are incorporated into the simulator are described. Using the developed simulator, simulations of the Tianwen-1 lander EDL are performed. The results indicate that the simulator is valid, and the GNC system of the Tianwen-1 lander exhibits excellent performance.