11:00 〜 11:15
[PPS07-07] Estimated Callisto’s gravity field using the inter-satellite observation mode for "Tianwen-IV" mission
キーワード：Callisto, precise orbit determination, simulation analysis, gravity field estimate, Love number k2, inter-satellite range-rate observation mode
China will launch the "Tianwen-IV" probe around 2030, focusing on the orbiting exploration of Callisto, a moon of Jupiter. To support this exploration mission, we proposed an inter-satellite observation mode that differs from the conventional observation mode and designed a series of simulation experiments to evaluate the role of this mode in estimating the Callisto gravity field model and the Love number k2.
The results showed that when both satellites were in polar orbits with orbital altitudes of 200 km, the gravity field coefficients of degree 80 or more than can be estimated using the two types of observation data. The accuracy of the estimate was three orders of magnitude higher than those using only the 2W observation mode. The inter-satellite observation noise level determined the resolution and accuracy of the estimate. When the noise level value was reduced from 1 mm/s to 0.01 mm/s, the resolution of the estimated gravity field model was increased from 45 to more than 80 degree and order, and the accuracy was improved by two orders of magnitude. The selection of the initial inter-satellite distance also influenced the accuracy of the gravity field model. Resonance occurred when the inter-satellite angular in-plane separation coincides with several wavelengths of a particular harmonic degree. When the inter-satellite distance was 300 km, the separation angle was about 6.6 degree, the resonant harmonic degree should be around 54. In addition, the inter-satellite distance should not be too large because of the reduced sensitivity of the inter-satellite observation data to the shallow mass distribution. There was a significant improvement in the accuracy and degree of the gravity field model as the orbital altitude of the two satellites decreased. When the two satellites were in lower orbit, gravity information in the medium-short-wavelength band was obtained, and the observation data were more sensitive to the gravity field coefficients. However, for sake of the compromise between the recovery accuracy of the gravity field and the satellite system life, the choice of orbit for "Tianwen-IV" mission will require a trade-off analysis. The accuracy of the estimate was slightly higher when the two satellites were in polar orbit. However, the low-degree coefficients of the gravity field obtained from the different orbital inclinations were significantly more accurate than the polar orbit solution. The gravity field coefficients obtained by solving the tracking data using the polar orbits showed large off-diagonal cross-correlations. Reducing the orbital inclination of the slave satellite from 90 degrees to 80 degrees or 70 degrees greatly reduced the correlation between the gravity field coefficients. When the master satellite was in polar orbit and only the orbital inclination of the slave satellite was decreased, the sensitivity of the inter-satellite observation data to the Love number k2 was increased. After 10 days of orbiting, the accuracy of the k2 estimate was improved by one order of magnitude with respect to when both satellites were in the same polar orbit. The improvement of k2 precision will provide an important constraint for investigating the presence of a subsurface ocean inside Callisto.
Nevertheless, there are still some challenges to be solved in this simulation experiment. Hubble Telescope observations confirmed the presence of an oxygen-dominated collisional atmosphere on Callisto, which affects probes. Moreover, solar radiation pressure also affects the satellites and may also influence the final covariance estimate if the uncertainty of the thermos-optical properties of the surface is not properly considered. Accurate modeling of non-conservative forces is a challenge. In addition, the high degree gravity field model of Callisto is based on the lunar gravity field model to scale. This method is representative of the gravity field of rocky bodies however, and its application to icy moons has not been demonstrated. Perhaps it is appropriate for rocky interiors of the moons, but an outer ice layer may produce a completely different gravity signal.
This work will provide a reference for the "Tianwen-IV" mission; and hopefully, the satellite-to-satellite observation mode will be implemented to improve the accuracy of the gravity field model and the Love number k2, which can enhance the understanding of the internal structure and evolution of Callisto.
The results showed that when both satellites were in polar orbits with orbital altitudes of 200 km, the gravity field coefficients of degree 80 or more than can be estimated using the two types of observation data. The accuracy of the estimate was three orders of magnitude higher than those using only the 2W observation mode. The inter-satellite observation noise level determined the resolution and accuracy of the estimate. When the noise level value was reduced from 1 mm/s to 0.01 mm/s, the resolution of the estimated gravity field model was increased from 45 to more than 80 degree and order, and the accuracy was improved by two orders of magnitude. The selection of the initial inter-satellite distance also influenced the accuracy of the gravity field model. Resonance occurred when the inter-satellite angular in-plane separation coincides with several wavelengths of a particular harmonic degree. When the inter-satellite distance was 300 km, the separation angle was about 6.6 degree, the resonant harmonic degree should be around 54. In addition, the inter-satellite distance should not be too large because of the reduced sensitivity of the inter-satellite observation data to the shallow mass distribution. There was a significant improvement in the accuracy and degree of the gravity field model as the orbital altitude of the two satellites decreased. When the two satellites were in lower orbit, gravity information in the medium-short-wavelength band was obtained, and the observation data were more sensitive to the gravity field coefficients. However, for sake of the compromise between the recovery accuracy of the gravity field and the satellite system life, the choice of orbit for "Tianwen-IV" mission will require a trade-off analysis. The accuracy of the estimate was slightly higher when the two satellites were in polar orbit. However, the low-degree coefficients of the gravity field obtained from the different orbital inclinations were significantly more accurate than the polar orbit solution. The gravity field coefficients obtained by solving the tracking data using the polar orbits showed large off-diagonal cross-correlations. Reducing the orbital inclination of the slave satellite from 90 degrees to 80 degrees or 70 degrees greatly reduced the correlation between the gravity field coefficients. When the master satellite was in polar orbit and only the orbital inclination of the slave satellite was decreased, the sensitivity of the inter-satellite observation data to the Love number k2 was increased. After 10 days of orbiting, the accuracy of the k2 estimate was improved by one order of magnitude with respect to when both satellites were in the same polar orbit. The improvement of k2 precision will provide an important constraint for investigating the presence of a subsurface ocean inside Callisto.
Nevertheless, there are still some challenges to be solved in this simulation experiment. Hubble Telescope observations confirmed the presence of an oxygen-dominated collisional atmosphere on Callisto, which affects probes. Moreover, solar radiation pressure also affects the satellites and may also influence the final covariance estimate if the uncertainty of the thermos-optical properties of the surface is not properly considered. Accurate modeling of non-conservative forces is a challenge. In addition, the high degree gravity field model of Callisto is based on the lunar gravity field model to scale. This method is representative of the gravity field of rocky bodies however, and its application to icy moons has not been demonstrated. Perhaps it is appropriate for rocky interiors of the moons, but an outer ice layer may produce a completely different gravity signal.
This work will provide a reference for the "Tianwen-IV" mission; and hopefully, the satellite-to-satellite observation mode will be implemented to improve the accuracy of the gravity field model and the Love number k2, which can enhance the understanding of the internal structure and evolution of Callisto.