Under certain conditions of low signal power and/or high noise, there is insufficient signal to noise ratio (SNR) to close tracking loops with individual signals on orbiting Global Navigation Satellite System (GNSS) receivers. In addition, the processing power available from flight computers is not great enough to implement a conventional ultra-tight coupling tracking loop. This work provides a method to track GNSS signals at very low SNR without the penalty of requiring very high processor throughput to calculate the loop parameters.
The Kalman Orbit-Optimized Loop (KOOL) tracking approach constitutes a filter with a dynamic model and using the aggregate of information from all tracked GNSS signals to close the tracking loop for each signal. For applications where there is not a good dynamic model, such as very low orbits where atmospheric drag models may not be adequate to achieve the required accuracy, aiding from an IMU (inertial measurement unit) or other sensor will be added. The KOOL approach is based on research JPL has done to allow signal recovery from weak and scintillating signals observed during the use of GPS signals for limb sounding of the Earth’s atmosphere. That approach uses the onboard PVT (position, velocity, time) solution to generate predictions for the range, range rate, and acceleration of the low-SNR signal. The low-SNR signal data are captured by a directed open loop. KOOL builds on the previous open loop tracking by including feedback and observable generation from the weak-signal channels so that the MSR receiver will continue to track and provide PVT, range, and Doppler data, even when all channels have low SNR.
The KOOL algorithm will also reduce the processor throughput requirements. This is enabled because the dynamic model of the receiver motion is very smooth, so that the full physical orbit model can be run at a low rate; for example, every 10 seconds. This contrasts with the signal tracking loop requirement for a much less complex set of processor activity at 50 Hz, a 500 times higher rate. Coarse benchmarks of PVT filter requirements for processor throughput, and the benchmark tracking loop’s requirements, indicate KOOL tracking will require an order of magnitude less throughput, considering both its lower rate and greater complexity.
KOOL tracking high-rate models for phase and range at shorter times will be generated within the digital logic once they are primed with model parameters from the PVT filter. The onboard oscillator must be commensurately stable, requiring (delta F)/F of about 10–11 over times up to 10 seconds.