GPS-based Spacecraft Formation Flying Simulation and Applications to Ionospheric Remote Sensing

Abstract

GNSS-based spacecraft formation flying in LEO presents new opportunities for ionospheric remote sensing. This work demonstrates the successful application of ionospheric tomography using the newly developed Virginia Tech Formation Flying Testbed (VTFFTB), a GNSS-based HIL simulation testbed for spacecraft formation flight. Scenarios of GNSS-based satellite formation flying missions for ionospheric remote sensing have been proposed, demonstrated, and validated on the VTFFTB. The current configuration of VTFFTB consists of GNSS RF hardware signal simulators, multi-constellation multi-frequency GNSS receivers, a navigation & control system, an STK visualization system, and an ionospheric remote sensing system. GPS signals were emulated from the GNSS simulator under the scenarios that include customized ionospheric phenomena. A receiver was utilized to track a pair of spacecraft (“chief” and “deputy”) orbiting in formation. Using both differential GPS measurements and a relative motion dynamics model, an EKF was implemented in the navigation & control system to estimate the relative state between two spacecraft in real time. A flight controller uses the estimated relative state to maneuver the deputy spacecraft instantaneously, which completes the closed-loop simulation. A reference scenario of two LEO spacecraft was simulated with the initial in-track separation of 1000 m and targeted leader-follower configuration of 100 m along-track offset, which demonstrated the testbed feasibility and performance by benchmarking with past reference simulation results. Multi-constellation multi-frequency GNSS receivers were used to develop the ionospheric remote sensing system with TEC and scintillation measurement capabilities. A special scenario of ionospheric irregularities that include Equatorial Spread F with a modelled plasma bubble region was simulated, and a LEO formation flying mission with two satellites were designed to probe the irregularity region. HIL simulations using the VTFFTB were run to validate the mission concept and evaluate the measurement results. This work demonstrates that the GPS-based satellite formation is able to probe the space-based TEC, vertical electron density, S4, and identify the structure of irregularities. The entire infrastructure of GNSS-based spacecraft formation flying simulation and ionospheric remote sensing is capable of supporting more novel upper atmospheric mission designs and validation.