GPS Radio Occultation Signal Demodulation Using NDM and 4-Quadrant Phase Extraction
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Solution Overview
Problem
Current methods for demodulating open-loop GPS radio occultation signals fail to accurately process phase modulations greater than 90 degrees, leading to errors in determining atmospheric parameters due to half cycle slips when using 2-quadrant phase extractors.
Innovation Solution
The method involves externally recording the Navigation Data Modulation (NDM) bit sequence and applying it for demodulation of the occulted GPS radio signal, allowing the use of a 4-quadrant phase extractor to correctly process signals with phase modulations up to 180 degrees, thereby improving the accuracy of atmospheric parameter determination.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If open-loop tracking is used to record the occulted GPS radio signal, then the signal can be reliably recorded without distortion, but the signal-to-noise ratio is low due to aliasing of the noise into the sampling frequency domain
Solution Approach 1:
The patent applies preliminary action by pre-calculating the phase model before signal recording and using it to down-convert the signal in advance. The phase model is computed based on expected satellite positions and atmospheric conditions, allowing the system to prepare the appropriate reference signal before the actual occultation event occurs. This preliminary preparation enables accurate signal demodulation while maintaining the reliability benefits of open-loop tracking.
2Device complexity
If a 2-quadrant phase extractor is used during post-processing, then the processing is simpler, but errors occur when phase modulation exceeds 90 degrees due to half cycle slips
Solution Approach 1:
The patent resolves this contradiction by changing the parameter of the phase extractor from 2-quadrant to 4-quadrant configuration. This parameter change allows the system to handle phase modulations up to 180 degrees without half cycle slips, thereby maintaining measurement precision for signals with larger atmospheric-induced phase modulations while accepting increased processing complexity as a necessary trade-off.
3Measurement precision
If Navigation Data Modulation is removed in-receiver during PLL signal processing, then the effect of occultation on carrier phase can be determined accurately, but this cannot be done during open-loop tracking without feedback
Solution Approach 1:
The patent introduces an intermediary approach by using externally recorded NDM bit sequences as a mediator between the open-loop tracked signal and the phase extraction process. Instead of removing NDM in-receiver as in PLL processing, the externally recorded NDM serves as a reference that can be applied during post-processing to eliminate its effect, enabling accurate carrier phase determination while maintaining the simplicity of open-loop tracking during signal acquisition.
Data Source
AI summary
Methods and systems for demodulation of open-loop GPS radio occultation signals are provided. An occulted GPS radio signal where the atmosphere-induced modulation on the phase is up to 180 degrees may be recorded by a radio occultation receiver. The radio signal may be concurrently received by a second receiver where the atmosphere-induced phase modulation is below 90 degrees and where the Navigation Data Message (NDM) bit sequence can be readily extracted. The extracted NDM bit sequence may be used to demodulate the occulted GPS radio signal and a 4-quadrant phase extractor may then be used to determine the phase and amplitude of the radio occultation signal. The phase and amplitude modulations after removal of NDM may be used for inversion, e.g., retrieval of the atmospheric parameters such as the bending angle, refractivity and deriving of the meteorological parameters.