Comparative Study of Interpolation Techniques for Ultra-Tight Integration of GPS/INS/PL Sensors

Abstract

Ultra-tight architecture plays a key role in improving the robustness of the integrated GPS/INS/PL (Pseudolite) system by aiding GPS receiver's carrier tracking loops with the Doppler information derived from INS (Inertial Navigation System) velocity measurements. This results in a lower carrier tracking loop bandwidth and subsequent improvement in measurement accuracy. Some other benefits using this architecture include: robust cycle-slip detection and correction, improved anti- jam performance, and weak signal detection. Typically the integration/navigation filter run at a rate of 1 to 100 Hz, which is insufficient to aid the carrier tracking loop as such loops normally run at about 1000 Hz. Two approaches were envisioned to solve this problem. One approach is to run the navigation Kalman filter at a higher rate, and the other is to run the filter at a lower rate and interpolate the measurements to the required rate. Although the first approach seems to be straightforward, it is computationally very intensive and requires a huge amount of processing power, adding to the cost and complexity of the system. The second method interpolates the low rate Doppler measurements from the navigation filter using multirate signal processing algorithms. Due to its efficiency and simpler architectures the interpolation method is adopted here. Filtering is the key issue when designing interpolators as they remove the images caused in the upsampling process. Although direct form of filtering can be adopted, they increase the computations. To reduce the computational burden, two efficient ways of implementing the interpolators are proposed in this paper: Polyphase and CIC (Cascaded Integrator Comb). The paper summarizes the design and analysis of these two techniques, and our initial results suggest that CIC is relatively better in terms of performance and computational requirements.