Attempt to improve Mars’ time variable gravity with ESA’s Exomars2016 (TGO) tracking data

Abstract:

The ESA’s Exomars2016 (TGO) spacecraft has no radio-science PI-ship. However, tracking data are regularly performed for the purpose of orbit navigation determination. As the TGO orbit is near-circular, 400 km altitude and 74 degrees inclination, it theoretically offers the opportunity to improve the determination of the seasonal variations of the second-degree zonal harmonics of Mars gravity field. These variations are indeed poorly resolved using tracking data of near-polar orbiters. Here, we attempt to improve these second-degree harmonics variations using the tracking data of TGO collected from ESOC.

 

Theoretical basis:

The orbital perturbations of Martian spacecraft due to the seasonal variations of low degree zonal harmonics are expected to be small. An amplitude of up to 70 cm is expected on polar orbiters like Mars Odyssey for the odd zonal harmonics and only 5 cm for the even zonal harmonics. Today, the best orbit accuracy is about 1 meter, which makes possible to retrieve the odd harmonics but not the even harmonics (e.g. Marty et al., 2009; Konopliv et al., 2006; 2016; Genova et al., 2016). For a TGO like orbit, the orbit perturbations from the seasonal low degree zonal harmonics are about 25 cm for both even and odd harmonics. Therefore, even harmonics could be detected with TGO tracking data if its orbit can be determined with an accuracy of 1 meter or better.

Therefore, we performed Precise Orbit Determination from tracking data and a as precise as possible model of the orbital motion of TGO using the GINS software (Marty et al., 2009).

 

Precise Orbit Determination (POD) process

Any spacecraft is tracked from the Earth using the radio-link established between the spacecraft and tracking stations on Earth (Holmes et al., 2008). These stations record the variations of the carrier frequency of this radio-link (i.e Doppler effect). However, these Doppler tracking data are not a direct measurement of the variations of the gravity of the planet, but measurements of the spacecraft orbital velocity perturbations projected on to the Earth-spacecraft line-of-sight (LOS) direction. These perturbations are induced by the gravity field (including its time variations) of the planet as well as non-gravitational forces like the atmospheric drag, the solar radiation pressure and the albedo and Infrared radiation from the planet. In addition, Wheel-Off loading maneuvers are regularly performed and generate orbit perturbations. A model of all the forces driven the orbit is performed in order to generate Doppler data to get the residual with the observed Doppler data collected at tracking stations. A least-squares fit of the force model to these Doppler residuals is then performed in order to estimate parameters of the force model including seasonal variations of the zonal harmonics, a scale factor of the non-gravitational forces and a delta-velocity at each WoL event. As the non-gravitational forces act on the faces of the spacecraft, it is represented as flat plates (for the bus, the solar arrays and the High Gain Antenna – HGA) with known optical properties as provided by ESOC. The epoch and a delta-velocity a priori estimate at each WoL are also provided. The POD process is repeated over successive data-arcs of a few days’ duration.

 

Results of POD process on TGO tracking data

TGO is tracked from the Earth 4 hours a day on average using the ESTRACK network (Holmes et al., 2008) using a coherent two-way link in X band. The WoL events are frequent, between 4 and 6 per day. Therefore, most of them are off the tracking passes and the a priori delta-velocity could not be tuned during the least-squares process. This is a limitation on the accuracy of the orbit as shown for Mars Express which shows similar poor tracking coverage (Rosenblatt et al., 2008). The orbit accuracy is estimated using the recovery method over pairs of successive data-arcs (e.g. Konopliv et al., 2006; Rosenblatt et al., 2008; Marty et al., 2009). We obtained 4 meters in the along and cross-track direction on average and about 40 cm in the radial direction (Figure2). This accuracy is worse than the accuracy on Martian polar spacecraft which is about 1-2 meters. The main source of error on TGO orbit comes probably from the frequent WoL events that are far away tracking passes. In addition to ESTRACK passes, DSN passes are daily performed adding 2 more hours of tracking each day. However, it could not compensate the lack of tracking around WoL events so that the orbit accuracy could not be improved. The current accuracy prevented us to retrieve the 25cm signal expected from the seasonal variations of both odd and even zonal harmonics.

 

Perspectives

The seasonal gravity variations solution could not be improved using TGO tracking data in spite of the theoretically promising opportunity. However, by-products of the POD provide a scale factor of the drag force, which can be interpreted as a scale factor of the atmosphere density model used in the force model. As the orbit altitude of TGO is 400 km, it thus provides an estimation of the density in the exosphere of Mars. Therefore, it can provide a monitoring of this density to study its interaction with the space environment around Mars. The Mars Odyssey (ODY) tracking data have also been used to perform such studies since this spacecraft also orbit Mars at 400 km altitude (e.g. Bruinsma et al., 2014). We plan to compare both ODY and TGO density estimation in order to assess whether the TGO estimates can also be used for exospheric studies.

 

References

Bruinsma S. et al. (2014), J. Geophys. Res. : Planets, 119, P, 210-218; Genova A. et al. (2016), Icarus 272, 228-245; Konopliv A.S. et al. (2006), Icarus 182, 23–50; Konopliv A.S. et al. (2016), Icarus 274, 253-260; Marty J.C., et al. (2009), Planet. and Space Sci., 57(3), pp. 350-363 ; Rosenblatt P., et al. (2008), Planet. and Space Sci., 56, pp. 1043-1053.