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Project: Distributed Multi-GNSS Timing and Localization System (DiGiTaL)

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The Distributed Timing and Localization (DiGiTaL) system provides nanosatellite formations with unprecedented centimeter-level navigation accuracy in real-time and nanosecond-level time synchronization through the integration of a multi-constellation, multi-frequency Global Navigation Satellite System (GNSS) receiver, a Chip-Scale Atomic Clock (CSAC), and a dedicated Inter-Satellite Link (ISL). In comparison, typical single nanosatellite GNSS navigation solutions are accurate to the meter-level due to the sole usage of single-frequency pseudorange measurements. To meet the strict requirements of future miniaturized distributed space systems, DiGiTaL exploits powerful error-cancelling combinations of synchronous carrier-phase measurements which are exchanged between the swarming nanosatellites through a peer-to-peer decentralized network. A reduced-dynamics estimation architecture on-board each individual nanosatellite processes the resulting millimeter-level noise measurements to reconstruct the full formation state with high accuracy.

Although carrier-phase observables offer millimeter-level noise, they are subject to an offset corresponding to an unknown integer number of cycles, the so-called integer ambiguity. These ambiguities must be resolved in real-time on-board to meet the accurate relative positioning goals of this project. This is a very computationally intensive task, often beyond the capability of spaceborne microprocessors. In contrast to standard offline approaches, DiGiTaL leverages diverse combinations of measurements from new GPS frequencies (L2 and L5), as well as from Galileo and BeiDou, to efficiently resolve integer ambiguities in real time using the Modified Least-Squares Ambiguity Decorrelation Adjustment (mLAMBDA). The estimation architecture is embedded in a distributed network of nanosatellites that is intended to support all operational scenarios, while coping with data handling and communication constraints. To this end, each DiGiTaL instance processes carrier-phase measurements from only a limited number of satellites simultaneously. The resulting state-covariance estimates produced by each nanosatellite are then fused through a dedicated swarm relative orbit determination algorithm to provide full formation orbit knowledge. Contingency scenarios are aided by a nearly omni-directional antenna system and a CSAC which allows accurate orbit propagation and faster convergence times in GNSS-impaired scenarios.

DiGiTaL is motivated by two key technologies which are revolutionizing the way humans conduct spaceflight: the miniaturization of satellites and the distribution of payload tasks among multiple coordinated units. The combination of these techniques is leading to a new generation of space architectures, so-called distributed space systems, which promise breakthroughs in space, planetary, and earth science, as well as on-orbit servicing and space situational awareness. However, current technologies for satellite navigation and timekeeping are insufficient to support NASA’s future mission concepts. DiGiTaL responds to this need by providing the navigation and timing accuracy required for multiple nanosatellites to act in unison as a large aperture spacecraft. Some specific mission applications include but are not limited to synthetic aperture radar interferometers, differential gravimeters, starshade-telescope systems for the direct imaging of the star vicinity, and autonomous assembly of larger structures in space.

DiGiTaL is currently being developed by the Space Rendezvous Laboratory (SLAB) at Stanford University’s Department of Aeronautics and Astronautics. It plans to undergo flight demonstrations for the VISORS mission (on-board) and SWARM-EX mission (on-ground). The project leverages algorithms, software, and hardware developed by the proposing team and demonstrated on formation-flying missions such as PRISMA (SSC, DLR, CNES), MMS (NASA), and CPOD (NASA).

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Related Publications

Giralo V., D'Amico S.;
Precise Real-Time Relative Orbit Determination for Large Baseline Formations Using GNSS;
Institute of Navigation, International Technical Meeting, Virtual Event, January 25-28 (2021).

Giralo V.;
Precision Navigation of Miniaturized Distributed Space Systems using GNSS;
Stanford University, PhD Thesis (2021).

Giralo V., Chernick M., D'Amico S.;
Guidance, Navigation, and Control for the DWARF Formation-Flying Mission;
2020 AAS/AIAA Astrodynamics Specialist Conference, South Lake Tahoe, California, August 9 - 13 (2020).

Giralo V., D'Amico S.;
Distributed Multi-GNSS Timing and Localization for Nanosatellites;
Navigation: Journal of The Institute of Navigation, Vol. 66, No. 4, pp. 729-746 (2019). DOI: 10.1002/navi.337

D'Amico S., Giralo, V.;
Evaluation of GPS Altitude Accuracy of the LX9000 High Altitude Flight Recorder;
Technical Note, Stanford Space Rendezvous Lab (SLAB), September 17 (2019).

Giralo V., D'Amico S.;
Distributed Multi-GNSS Timing and Localization for Nanosatellites;
ION GNSS+ 2018, Miami, FL, September 24-28 (2018).

Giralo V., D’Amico S.;
Development of the Stanford GNSS Navigation Testbed for Distributed Space Systems;
Institute of Navigation, International Technical Meeting, Reston, Virginia, January 29-February 1 (2018).

Giralo V., Eddy D., D'Amico S.;
Development and Verication of the Stanford GNSS Navigation Testbed for Spacecraft Formation Flying;
Technical Note, Stanford Space Rendezvous Lab (SLAB), July 18, 2017.

Ardaens J.-S., D’Amico S., and Sommer J.;
GPS Navigation System for Challenging Close-Proximity Formation-Flight;
24th International Symposium on Spaceflight Dynamics, 5-9 May 2014, Laurel, USA (2014).