Mars orientation parameters determination based on direct-to-Earth measurement.

The Mars orientation parameters (MOPs) are important information for constraining Martian interior properties, ice caps sublimation, and condensation process. Considering the future Mars landing mission, we performed numerical simulations of MOP determination using direct-to-Earth radio signals. The results suggest that after 200 d of tracking using high-quality levels of both two-way Doppler and two-way range measurements, the MOP uncertainties will be significantly reduced. Then, after about 800 mission days the accuracy of precession parameters will be improved by 5–10 times compared with the present. Also, the accuracy of nutation parameters will reach 10–30 mas, and the accuracy of length-of-day and Chandler wobble parameters will reach 5–10 mas, which meets the need to investigate the internal structure of Mars and the dynamics of the atmosphere. By analyzing the solution results of different latitude landers, two-way range data can provide a good complement to two-way Doppler data. Therefore, it is necessary to perform a two-way range measurement for a high-latitude lander. In addition, the study has also found the limit of the ephemeris error of Mars for MOP determination. When using the two-way Doppler data, the ephemeris error should not be more than 75 m; and for the two-way range measurement, the error should be less than 0.15 m.