GPS Signal Acquisition via Iterative Decoding
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Solution Overview
Problem
Existing GPS signal acquisition methods face challenges in achieving low missed detection and false alarm probabilities, especially at low signal-to-noise ratios, and require long acquisition times and high complexity.
Innovation Solution
A method using iterative message passing decoding with Gold sequences generated from M-sequences, where the receiver samples the signal at the chip frequency, multiplies by the M-sequence, decodes to estimate the shift register content, and synchronizes with the satellite by comparing the estimated content to possible contents, with optional verification phases to reduce false alarms.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If correlation-based acquisition methods are used, then the receiver can detect satellite signals, but the missed detection probability and false alarm probability remain high at low signal-to-noise ratios
Solution Approach 1:
The patent segments the acquisition process into two distinct phases: a first acquisition phase using correlation methods to identify candidate satellites, and a second verification phase using iterative message passing decoding to confirm detection. This segmentation allows each phase to use optimized methods for its specific purpose, improving overall reliability at low SNR while maintaining detection capability.
Solution Approach 2:
The patent introduces an intermediary verification step between initial detection and final confirmation. The iterative message passing decoder acts as an intermediary that processes the correlation results and provides a more reliable verification, effectively mediating between the noisy initial detection and the final detection decision, thereby reducing both missed detection and false alarm probabilities.
2Reliability
If all possible spreading codes are tested one by one, then complete satellite detection is achieved, but the acquisition time becomes very long
Solution Approach 1:
The patent performs preliminary action by using correlation-based detection first to identify candidate satellites before applying the more computationally intensive iterative message passing decoding. This preliminary filtering step reduces the number of codes that need full verification, significantly reducing acquisition time while maintaining complete detection capability.
Solution Approach 2:
The patent applies partial action by using the full iterative decoding process only for candidate satellites identified in the first phase, rather than applying it to all possible spreading codes. This selective application of the more rigorous verification method maintains detection completeness while dramatically reducing the computational burden and acquisition time.
3Productivity
If parallel configuration is used to test multiple spreading codes simultaneously, then acquisition time is reduced, but the complexity increases significantly
Solution Approach 1:
The patent segments the complexity by dividing the acquisition process into two phases with different complexity levels. The first phase uses simpler correlation methods that can be implemented in parallel with lower complexity, while the second phase uses more complex iterative decoding only on a reduced set of candidates. This segmentation achieves high productivity without requiring the full system complexity to be parallelized.
Solution Approach 2:
The patent applies the complex iterative decoding process partially, only to candidate satellites identified in the first phase, rather than to all possible spreading codes. This partial application allows the system to achieve high acquisition speed through parallel processing of candidates while avoiding the excessive complexity that would result from parallelizing the full verification process for all codes.
Data Source
AI summary
The invention relates to a method of acquiring a GPS signal, spread by a Gold sequence obtained as the sum of a first M-sequence and a second M-sequence, this second M-sequence being characteristic of a satellite among a plurality of satellites in the GPS system. The acquisition method comprises a sampling step starting from a given time (410), a multiplication step for the sequence of samples obtained with the first M-sequence (420) and an iterative decoding step for the decoded values (430) making it possible to estimate the content of the second shift register of the second M-sequence at said given time. If the content thus estimated is identical to the initial content of the second register for an identified satellite (443), the receiver is synchronized with this satellite at said given time (490).