Hardware-in-the-loop verification of GNSS-based orbit determination, time synchronization, and oscillator disciplining for LEO-PNT

Positioning, navigation and timing (PNT) services from low Earth orbit (LEO) present an important alternative or complement to established global satellite navigation systems (GNSSs). Within a LEO-PNT system, proper knowledge of the instantaneous LEO satellite position and onboard time is essential for the generation of navigation signals. Among the currently proposed LEO-PNT systems, GNSS-based onboard orbit determination and time synchronization (ODTS) is a widely considered concept for the autonomous generation of navigation signals and satellite ephemerides. To demonstrate the feasibility of autonomous real-time ODTS and assess the achievable performance, a realistic hardware-in-the-loop simulation for a LEO environment is employed. As a device under test, a miniature satellite payload developed for the ATOMIC (Autonomous Time and Orbit Determination for Micro-satellite Constellations) experiment is utilized, which involves a multi-constellation GNSS receiver, a chip-scale atomic clock (CSAC) and a micro-processor executing the GNSS-based real-time orbit determination and clock disciplining process. For maximum realism, the simulation replicates the real-world orbital dynamics of a representative LEO satellite at 1336 km altitude along with the true GPS and Galileo orbits and clocks as well as the actual broadcast navigation messages transmitted by those constellations on the day of interest. The results for a 24 h data arc show that a 3D root mean square (RMS) position error of 9.5 cm can be achieved, while the clock phase of the onboard oscillator can be synchronized to the GNSS system time scale with a precision of 1.0 ns. While the carrier-phase-based real-time ODTS enables a significantly better clock synchronization than traditional GNSS-disciplined oscillators, the achieved performance is ultimately limited by the broadcast ephemeris accuracy and the correlation between the receiver clock offset and the float-valued carrier phase ambiguities in the real-time ODTS process.