Two-way ranging and Doppler for multiple orbiting spacecraft at Mars

Mars exploration requires multiple orbiters and surface infrastructures on Mars. Location determination is essential to support various human and robotic activities on the Mars surface and in orbits. A robust communication coverage is required for data transmission between assets on Mars surface and orbiters in addition to the communication between orbiters and Earth. In this paper a notional Mars Regional Navigation Satellite System is proposed that provides continuous or near-continuous in-situ navigation and timing services for the robotic and human missions in the vicinity of a Mars landing site. For such a system the Earth's deep space stations have to perform orbit determination (OD) for the Mars orbiters so that their locations are known to a reasonable degree of accuracy. This provides the user vehicles the ability to estimate their locations using the orbiter-vehicle range measurements. In this paper we introduce a new flight-ground collaborative architecture that provides simultaneous Doppler and ranging for multiple spacecraft in the Mars vicinity. We illustrate the simultaneous ranging and Doppler approach using an example of four Mars orbiters. We assume the two-way ranging and Doppler operate in the X-band frequency range. The operational Deep Space Network (DSN) Multiple-Spacecraft-Per-Antenna (MSPA) method is basically a static Frequency Division Multiple Access (FDMA) scheme for spacecraft in the same ground antenna beam. For the four Mars orbiters, the downlinks operate in four allocated frequency bands separated by three guard bands to prevent interferences. The guard bands have to be wide enough to accommodate the Doppler shifts between the frequency-adjacent orbiters. For the uplink, we assume the orbiters share a single X-band uplink, and each orbiter identifies its own commands by its unique spacecraft ID. The uplinks received by the four orbiters exhibit different Doppler and Doppler rate effects due to their different dynamics with respect to Earth. To perform two-way ranging and Doppler measurements a pseudo noise (PN) sequence is transmitted. A method of partial Doppler compensation for each orbiter is employed. Some preliminary results show that 45 KHz and 2.6 Hz/s upper bound the residual Doppler and residual Doppler rate respectively. In this paper we propose a new spacecraft receiver 2-way tracking architecture. A Phase Locked Loop (PLL) with sweeping frequency will be used to coherently acquire and estimate the uplink Doppler frequency on the fly. After Doppler frequency estimation, the PN ranging signal will be acquired and then passively generated to be phase modulated on the downlink X-band carrier frequency. The downlink carrier is generated coherently by PLL from the uplink carrier after applying the turn around ratio and is transmitted at downlink X-Band to the DSN station. Four Receiver Ranging Processors (RRP's) at the ground station again uses PLL with sweep frequency to acquire and to measure the Doppler frequency with some error. After acquiring the PN sequence the ground station then tracks the PN sequence for ranging measurements. This paper sets up a two-way ranging and Doppler system for analysis and simulations. The results will show the accuracy of Doppler measurements to be used for orbit determination of four orbiters at Mars. The use of DSN's Doppler-Rate-Aided Ranging Processing and how it affects the ranging accuracy is discussed. Simulation of ranging is subject of our future paper.