Global navigation satellite system (gnss) navigation and timekeeping is a technology enabler for a wide range of future space architectures, including spacecraft formation-flying, on-orbit…

This is a Cooperative Agreement awarded by the National Aeronautics and Space Administration (NASA) to The Leland Stanford Junior University (Stanford University) for the development of a reusable, miniaturized, absolute/relative navigation subsystem called DIGITAL (Distributed GNSS Timing and Localization System). The goal is to mature spaceborne carrier-phase differential GNSS (CPDGNSS) technology beyond the state-of-the-art to enable precise distributed timekeeping and real-time relative positioning at sub-centimeter-level accuracy for decentralized distributed spacecraft platforms as small as nanosatellites. The contract has a ceiling value of $199,966.84 and a period of performance ending on May 27, 2020. This award is not set-aside and represents an advancement of NASA's technology roadmaps in areas such as communications, navigation, and autonomous systems.

Global navigation satellite system (gnss) navigation and timekeeping is a technology enabler for a wide range of future space architectures, including spacecraft formation-flying, on-orbit servicing, swarming, spacecraft fractionation, and distributed spacecraft missions. Especially its potential to precisely recover the 3D-baseline between CO-orbiting satellites at a broad range of separations (zero to hundreds of kilometers), makes gnss an ideal candidate for a number of future applications such as synthetic aperture radar interferometers, differential gravimeters, starshade/telescopes, distributed space telescopes, sparse aperture telescopes, in-situ atmosphere measurements, and on-orbit assembly of large structures. In this context, carrier-phase differential gnss (cpdgnss) has been used in the past to determine the relative position between coorbiting satellites with sub-centimeter-level accuracy, see the grace (USA/germany) and tandem-x (germany) missions. However, these systems are based on the post-facto ground-processing of telemetry data collected over multiple days and make use of expensive hardware that can only fit large spacecraft (>5 KG,>10 w). To date, the only demonstration of on-board autonomous cpdgnss is represented by the prisma technology demonstration mission (sweden/germany/france) which achieved relative positioning at the decimeter-level accuracy using a navigation system CO-developed by the stanford's principal investigator. However, prisma employs a single-frequency GPS-only receiver in a centralized processing scheme and does not exploit the full accuracy potential of carrierphase measurements whose ambiguities are estimated as float numbers. In an attempt to enable new scientific and technological applications, increase autonomy and reduce the inherent time delays of a ground-based approach, we propose to mature spaceborne cpdgnss beyond state-of-the-art employing multi-frequency multiconstellation gnss for autonomous navigation of multiple formation-flying nanosatellites with a one to two order of magnitude improvement in accuracy. In particular, we propose a reusable, miniaturized, absolute/relative navigation subsystem called digital (distributed gnss timing and localization system). Digital will provide precise distributed timekeeping and real-time relative positioning at sub-centimeter-level accuracy for decentralized distributed spacecraft platforms as small as nanosatellites. Digital will be designed to address the limitations of the state-of-the-art. IT will be largely based on the prisma and gsfc's MMS flight heritage with the following key advancements: 1) integration and packaging of commercial-off-the-shelf (COTS) hardware for nanosatellite platform, 2) decentralization of algorithms for a peer-to-peer configuration while minimizing communication bandwidth and computational effort, 3) use of adaptive EKF for reduced operational effort during different formation-flying scenarios including maneuver activities, 4) real-time carrier-phase integer ambiguity resolution for improved accuracy, 5) usage of multiple gnss signals (rather than GPS only), and 6) integration with a chip scale atomic clock (csac) for faster convergence and gnss impaired scenarios (e.g., high altitude orbits). Starting from TRL-3, the project team will develop two TRL-6 prototypes of digital that are ready for integration into a target NASA nanosatellite system. Functionality and performance of the units will be validated in the high-fidelity gnss signal simulation facilities available at stanford university and NASA gsfc. Digital is a high-impact cross-cutting technology which paves the way for future NASA space missions with unrivaled science and exploration capabilities. The addressed NASA technology areas are TA5: communications, navigation, and orbital debris tracking and characterization systems (5.4.2, 5.4.4, 5.4.5, 5.4.1) and TA4: robotics and autonomous systems (4.6.1, 4.6.2).