Improving the VERITAS orbit reconstruction using radar tie points

THE recently selected NASA Discovery mission, Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy (VERITAS), will orbit Earth’s neighboring planet and gather foundational datasets that will address many open questions on its dynamical history and active geophysical processes [1]. The scientific objectives of VERITAS will be enabled by a gravity investigation, relying on the Integrated Deep Space Transponder (IDST) operating in both X- and Ka-band ([2,3,4]), the X-band Venus Interferometric Synthetic Aperture Radar (VISAR, [5]), and an infrared spectrometer, the Venus Emissivity Mapper (VEM, [6]). To successfully acquire the high-accuracy scientific observations of VISAR and VEM, a precise knowledge of the probe’s trajectory is required. Both these instruments produce georeferenced datasets and thus the quality of the orbit prediction and the accuracy of the orbit reconstruction play a key role in successfully planning the observations and precisely georeferencing the collected data, respectively. The processing of radar echo signals requires precise knowledge of the spacecraft trajectory, particularly for geological mapping purposes where 30 m-resolution radar images are combined into mosaics. Moreover, to reduce downlinked data volume SAR image formation is done onboard requiring trajectory knowledge onboard as well as post data collection for additional processing on the ground. Relative errors between subsequent orbits can introduce discontinuities in the final products. In the early phases of the mission, the trajectory reconstruction operations will not benefit from the entire VERITAS dataset obtained at the end of the mission. Indeed, the day-to-day reconstructed orbits will invariably be more uncertain than the output of the final reprocessing, which will benefit from the much-improved determination of the Venus gravity field [3] and rotational state [2]. This indeed directly affects all the immediate science products and thus the observation planning operations. Specific data acquisition campaigns such as radar repeat-pass interferometry (RPI) require highly precise orbit reconstruction of the initial overflight and dedicated targeting of the “tube” entrance [7]. That reconstruction requires enhancing the standard Earth tracking-based orbit determination, which employs range-rate (Doppler) data collected in the context of the gravity field investigation and for on-board ephemeris generation, with VISAR data. We report on the improvement in the preliminary orbit determination performance attainable by VERITAS when combining Earth tracking data with repeated surface landmark observations (tie points) produced by the from the SAR data in greater detail than in previous work. In Section 2 we present the concept of tie points and discuss their use in the orbit determination process; in Section 3 we discuss the numerical simulations we performed for assessing the accuracy of VERITAS orbit reconstruction, while in Section 4 we present and discuss the results. Section 5 gives concluding remarks.