Pre-correlation Filter for GPS Quadrature Code Timing
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Solution Overview
Problem
GPS receivers face challenges in accurately synchronizing with satellite PRN codes due to multipath signal distortion, which is exacerbated by thermal noise, leading to inaccurate pseudorange measurements and increased complexity and cost.
Innovation Solution
A pre-correlation filter and receiver system that uses complex accumulation registers to form an average chip shape image of the in-phase signal, allowing direct detection of quadrature PRN code transitions and reducing noise levels, thereby eliminating the need for local PRN code generators and associated hardware.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If local PRN code generators and associated tracking hardware are used to achieve full P-code chip tracking, then measurement precision is improved, but device complexity and cost increase
Solution Approach 1:
The patent creates a simplified copy of the P-code tracking function by detecting quadrature code transitions through in-phase code correlations. Instead of implementing a full local P-code generator and tracking hardware, the system uses the in-phase C/A code correlator to indirectly detect quadrature P-code chip transitions, achieving equivalent measurement precision with reduced hardware complexity
Solution Approach 2:
The in-phase code correlator is made multi-functional by enabling it to serve both C/A code tracking and P-code transition detection purposes. The same correlation measurements and processing hardware are used to extract information about both code channels, eliminating the need for separate dedicated P-code tracking hardware
2Power
If correlation measurements are made on the entire received signal including multipath components, then signal strength is improved, but measurement precision deteriorates due to signal distortion
Solution Approach 1:
The patent extracts only the relevant information needed for P-code transition detection from the correlation measurements, rather than using the entire distorted received signal for synchronization. By focusing on specific features (quadrature code transitions embedded in the in-phase signal) and using selective correlation processing, the system isolates the useful signal components from multipath distortion
Solution Approach 2:
The in-phase C/A code correlation measurements serve as an intermediary to indirectly detect quadrature P-code transitions. This intermediary approach allows the system to obtain P-code timing information without directly processing the distorted quadrature signal, using the cleaner in-phase channel as a mediator to extract the needed timing information
Data Source
AI summary
A pre-correlation filter determines the timing of a second pseudorandom number (“PRN”) code, using an image of the average chip shape formed for a first PRN code. The filter collects measurements corresponding to samples of a received signal over multiple code chips of the first PRN code at sample times that are asynchronous to code rate. A code phase decoder directs the measurements to accumulation registers that are associated with code chip ranges that are fractions of a code chip of the first PRN code based on the code phase angles of the samples in the first PRN code, to accumulate measurements relating to chip transitions in the first PRN code and measurements relating to chip transitions in the second PRN code. The chip edges of the second PRN code are detected from the image of average chip shape for the first PRN code formed from the accumulated measurements.