GNSS-Based Spaceborne Inverse VLBI for Cislunar Autonomous Navigation

Increasing cislunar traffic is anticipated with more emerging commercial, scientific, exploration, and space-domain awareness activities. At present, the ground-based Very Long Baseline Interferometry (VLBI) employed by NASA’s Deep Space Network (DSN) represents the state-of-the-art technology in orbit determination and tracking for cislunar and interplanetary missions. To enable navigation on the far side and south pole of the Moon, a lunar navigation satellite system (LNSS), a constellation similar to the global navigation satellite system (GNSS) around the Earth, is planned as part of the joint effort of NASA/ESA/JAXA. A future cislunar user is likely to use GNSS (departure from the Earth), LNSS (arrival at the Moon), and both when cruising in between. However, there are two major difficulties associated with using GNSS for cislunar navigation, namely, the extremely weak signal strength due to long propagation distance and a poor positioning geometry with range and/or Doppler measurements. To address the first issue, a high-sensitivity GNSS receiver can be used, based on long coherent integration (LCI) in addition to a high gain antenna, cooled LNA, and an atomic clock so as to acquire and track weak GNSS signals. For the second issue, the concept of a spaceborne inverse VLBI is set forth in this paper in which the signals from synchronous transmitters (GNSS and/or LNSS satellites) are processed to determine the angles of departure (AOD) to produce a position vector estimate for the receiver. A GNSS-based spaceborne inverse VLBI has the potential to overcome most of the technical difficulties faced by a space-based VLBI. As the interferometry sources, synchronous GNSS signals are generated from onboard atomic clocks with a well-maintained GPS time scale. The GNSS orbits are also well maintained and are available in the form of ephemeris embedded on the signals. As an inverse operation, all computations are done at the receiving spacecraft so there is no need to transfer data from interferometric receivers to a central location as done in conventional VLBI. With near side GNSS signals to the Moon, the traverse of ionosphere and troposphere is avoided as compared to far side GNSS signals and ground-based operations. An analysis is presented in this paper, which determines the SNR levels required to generate carrier phase and cross-range measurements from the inverse VLBI to satisfy the navigation needs. The inverse VLBI angular measurements are evaluated via simulation. As shown by the simulation results, the use of an inverse VLBI improves the positioning accuracy and simplifies the overall system implementation.