Optical Time-Transfer for Bistatic SAR Small Spacecraft

A spacecraft-to-spacecraft optical time-transfer simulation has been developed as a tool for informing NASA’s Surface Deformation and Change (SDC) mission architecture. The SDC mission will combine radar images from multiple spacecraft to improve understanding of the Earth’s sea-level and landscape changes. Spacecraft must be precisely synchronized in order to create sharp and phase accurate radar images. Simulation of multiple spacecraft time-synchronizing via laser communication can inform technology choices of a mission by providing sub-nanosecond precision estimates of clock error. This timing and ranging simulation has been combined with a radar system performance analysis pipeline. The simulated timing errors are used in a radar simulation to predict performance of bistatic SAR systems in the presence of oscillator noise and time synchronization inaccuracy. Precision time-transfer techniques facilitate the accurate synchronization of clocks between any combination of terminals. Most time-transfer technology for comparing two clocks at different terminals use radio frequencies (RF) to measure the time delay between the sending and receiving of signals. Laser technology offers the capability to transmit high data rates with systems that are of smaller size and lower power than comparable RF systems. The clocks on independent spacecraft will have some phase and frequency errors between them that result in clock drift. The two clock models that are included in this bi-directional MATLAB simulation are a Microchip Microsemi cesium-based Chip-Scale Atomic Clock (CSAC) and a Microchip Microsemi rubidium-based Miniature Atomic Clock (MAC). The CSAC has flown as hardware for small satellite missions such as the University of Florida’s CHOMPTT mission. A study of an example orbit, based on previous satellite laser ranging (SLR) missions and lasing rates demonstrate the impact of flight configuration parameters on the synchronization error between two spacecraft. The MATLAB timing simulation uses a Runge-Kutta 4th-order method to propagate spacecraft orbits and computes the light-travel time estimate between them. The simulation outputs the estimated clock error based on a user-defined spacecraft cluster configuration. The radar simulation is applied to evaluate a potential future bistatic SAR constellation architecture. In the proposed architecture, satellites follow each other in the same orbit at 500 km altitude, with a 250 km baseline direct line-of-sight between satellites. We also baseline the CSAC as a stable oscillator. We use NASA’s NISAR for baseline radar system parameters, but scale down the simulated antenna and radar power to represent a possible small-satellite platform. We compute a clock-system introduced phase error of 0.17 degrees with our simulated time-transfer architecture. This analysis technique could be extended or modified to evaluate the timing requirements of other geometries for other future multistatic SAR missions, or other interferometric satellite missions.