An analytical Mars entry guidance method for higher deployment altitude considering uncertainties

The atmospheric entry phase is challenging and vital for the whole Mars entry, descent and landing (EDL) process. The Entry Terminal Point Controller (ETPC) adapted from the Apollo guidance law has helped the Mars Science Laboratory mission to land the Curiosity rover on Mars surface successfully with greatly improved landing accuracy and altitude compared with the former landing missions. In this research, the authors develop an improved analytical entry guidance method based on ETPC, by considering improving the deployment altitude and eliminating the tracking error caused by the parameter uncertainties. According to the controllability of the entry vehicle, the original range control phase is further divided into two sub-phases: range control phase and altitude/range control (ARC) phase. The trigger for the ARC phase is carefully designed to make a compromise between the final range performance and the altitude performance. The ETPC is improved in terms of two aspects. Firstly, the state is extended to consider the parameter uncertainties in the ballistic coefficient and reference atmospheric density. By taking uncertain parameters as augmented state variables, the final range or altitude errors caused by the parameter deviation can be calculated analytically. Secondly, the influence coefficients in the ARC phase are redesigned according to the new guidance objective. To evaluate the performance of the proposed guidance method, Monte Carlo tests are designed and carried out in MATLAB environment. The results indicate that when the altitude control is involved in the improved ETPC, the terminal 3σ altitude distribution will decrease and the average deployment altitude increases. However, as the entry vehicle has to use its limited control ability to eliminate the altitude and range tracking errors simultaneously, the 3σ range distribution increases compared with the original ETPC. By modulating the trigger for the ARC phase, as well as the weights of the final range and altitude in the performance index, mission designers may get compromised altitude and range performance, which might increase the possibility for the final successful landing.