SBAS Signal in Space Performance Improvement through Code Phase Resolution Error Correction

S.D. Ericson

Abstract:	During the course of ongoing Wide-Area Augmentation System (WAAS) performance evaluation, it was noted that oscillations affect pseudorange measurements from geostationary earth orbit (GEO) satellites. Investigation by Zeta and NovAtel determined that oscillations attributable to systematic receiver errors occur with amplitudes of ± 0.1 to 0.3 meter and frequency dependent on Doppler, with each PRN code having a unique signature. It was also found that different oscillations exist on L1 and L5, with L5 oscillations generally being smaller in magnitude. Later, during analysis of open-loop data, it was observed that the oscillation frequency is related specifically to pseudorange rate of change, not carrier Doppler. Using a prototype signal generator (SIGGEN) and a NovAtel WAAS GUS receiver for further characterization, it was determined that the phase of the oscillation is dependent on the pseudorange measurement, i.e., the oscillation phase is determined using a modulo operation on the pseudorange (PR) measurement with the oscillation wavelength (?osc, equivalent to the numerically controlled oscillator (NCO) wavelength). This behavior was tested on multiple GUS receivers, receiver sections and channels to confirm that the same oscillation phase pattern exists independent of the receiver or channel under test. However, because of carrier aiding in the code tracking loop, the shape of the oscillation varies with pseudorange rate of change (P•R) and DLL bandwidth. Still, tables of corrections based on P•R and mod(PR, ?osc) can be generated using data from a GUS receiver set to the WAAS L1 and L5 DLL bandwidth settings. Such tables were developed using the prototype SIGGEN, which progressively covered ranges of P•R typically observed from the WAAS GEOs CRE (PRN 138), CRW (PRN 135) and AMR (PRN 133). In order to demonstrate the ability of the correction tables to consistently mitigate pseudorange oscillations, archived open-loop data files from GUS receivers at various sites were retrieved and evaluated. In all cases the oscillations were mitigated to the point where residual error was less than the measurement noise, providing evidence that the appropriate correction table is applicable to any GUS receiver at any site provided the DLL bandwidth setting is consistent. Further investigation to identify the root cause of the oscillations revealed that they are based on code phase resolution error related to differences in digital sampling of incoming and replica codes. That is, the incoming signal and the replica match each other only when the incoming samples taken at the NCO frequency align with those of the replica. Most of the time the samples are misaligned, which generally does not cause a problem with GPS tracking because of randomness induced by an asynchronous NCO relative to the code chipping rate along with relatively fast pseudorange rates of change and carrier smoothing [1]. However, with the slow relative motion of GEO satellites, code phase resolution error becomes significant as pseudorange rates of change approach zero, causing slowly changing biases and oscillations on the measurements. So although different types of SBAS receivers may be affected by this phenomenon, the nominal effects can be modeled by PRN code and NCO frequency, as demonstrated in this paper. To assess the potential value of mitigating pseudorange oscillations during GUS signal in space (SIS) generation control (also known as the “control loop”), L1 and L5 oscillation correction tables are embedded in a Raytheon MATLAB Simulink control loop simulation to induce realistic oscillations on the pseudoranges output by the GUS receiver. After completing simulation runs with and without the oscillations in the control loop, it was observed that the runs without oscillations exhibited faster time to convergence and better code-carrier coherence when compared to the runs with oscillations. Results of these simulated tests using different SBAS PRN codes and GEO satellite motion profiles are presented in this paper. Collectively, the results shown in this paper demonstrate that pseudorange oscillation correction tables can be used to reliably mitigate SBAS pseudorange measurement error. Continued use of the corrections in WAAS SIS evaluation has shown consistent applicability as well. Study of the simulated interaction between the oscillations and the WAAS signal generation control loop indicates an SBAS signal in space improves when control loop pseudoranges are corrected in real time. Future study and tests during initial on-orbit testing of upcoming SBAS GEO satellites will be useful to confirm the potential benefit.
Published in:	Proceedings of the ION 2013 Pacific PNT Meeting April 23 - 25, 2013 Marriott Waikiki Beach Resort & Spa Honolulu, Hawaii
Pages:	995 - 1002
Cite this article:	Ericson, S.D., "SBAS Signal in Space Performance Improvement through Code Phase Resolution Error Correction," Proceedings of the ION 2013 Pacific PNT Meeting, Honolulu, Hawaii, April 2013, pp. 995-1002.
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