Distributed Multi-GNSS Timing and Localization for Nanosatellites

Abstract

The way humans conduct spaceflight is being revolutionized by two key trends. The first trend is the distribution of payload tasks among multiple coordinated units, referred to as Distributed Space Systems (DSS), which allow for advances in Earth and planetary science, on-orbit servicing, and space situational awareness. To mimic a gigantic spacecraft with adjustable baselines, DSS require precise knowledge of the relative states of each orbiting satellite. Centimeter-level relative positioning precision can be obtained from Global Navigation Satellite Systems (GNSS) using differential carrier-phase techniques, where synchronous measurements are shared between spacecraft and error-cancelling combinations of various data types are formed to create precise baseline knowledge. The second trend is spacecraft miniaturization, whereby micro- and nanosatellites are transitioning from being merely educational tools to a viable scientific platform. The recent advances in distribution and miniaturization motivate the Distributed Multi-GNSS Timing and Localization (DiGiTaL) system described in this paper. DiGiTaL is intended to provide nanosatellite swarms with unprecedented centimeter-level navigation accuracy in real-time and nanosecond-level time synchronization through the integration of a multi-constellation GNSS antenna and receiver, a ChipScale Atomic Clock (CSAC), and an Inter-Satellite Link (ISL). This paper describes DiGiTaL’s hardware and software design, architecture, and algorithms in detail. Specifically, the design is documented through the major trades conducted on the potential hardware, considering its compatibility with the CubeSat size and power resources, modern GNSS signal support, and flight heritage. Next, the performance of the candidate receivers is characterized in terms of measurement noise to verify their capability to perform precise relative navigation through carrier-phase differential GNSS at the centimeter level. The navigation software and core algorithms are described with their rationale. In order to render the distributed navigation system computationally tractable, the states of all swarming spacecraft are grouped into subsets through a connected graph. Differential GNSS is only performed between spacecraft within each subset by a precise orbit determination module with integer ambiguity resolution in real-time. The resulting precise orbits are then exchanged and fused by the swarm orbit determination module removing the necessity to form single- and double-differenced carrier-phase measurements between all spacecraft of the swarm. Finally, the DiGiTaL architecture is integrated into CubeSat avionics and tested with all key hardware in the loop using the Stanford’s GNSS and Radiofrequency Autonomous Navigation Testbed for DSS (GRAND). For the first time, this paper shows the capability to perform centimeter-level precise relative navigation using commercial-off-the-shelf CubeSat hardware for a swarm of multiple spacecraft.