Frequency Discriminator for Unstable Oscillators in GNSS Receivers
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Solution Overview
Problem
Modern positioning and communications devices face challenges with unstable frequency sources, particularly in GNSS applications, where a stable frequency reference is costly and difficult to maintain due to factors like temperature and vibrations, affecting the accuracy of signal reception and processing.
Innovation Solution
A method and apparatus for determining frequency-related parameters, such as frequency error or offset, in a receiver using motion-compensated correlation techniques, allowing for phase compensation and joint analysis of signals from multiple sources to correct for frequency instability, enabling accurate signal processing even with unstable local frequency sources.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a low-cost quartz oscillator is used as the frequency source, then the device cost is reduced, but the frequency reference stability deteriorates over long time periods and under changing operating conditions
Solution Approach 1:
The patent introduces a frequency discriminator as an intermediary component that measures the frequency error of the low-cost oscillator. This discriminator acts as a mediator between the unstable local oscillator and the signal processing system, providing error information that can be used to compensate for frequency instability without requiring a high-precision oscillator throughout the entire system.
Solution Approach 2:
The patent replaces the mechanical/physical requirement for a high-stability oscillator with a signal processing approach. Instead of relying on the physical stability of a expensive temperature-compensated or atomic oscillator, the system uses digital signal processing techniques including phase compensation and coherent integration to achieve the same stability effect, thereby substituting hardware quality with algorithmic correction.
2Reliability
If a high-stability frequency source is used for GNSS positioning, then the frequency reference stability is improved, but the device cost increases significantly
Solution Approach 1:
The patent employs a low-cost quartz oscillator that does not need to maintain stability over long periods or under all conditions. The system accepts that this cheap oscillator will drift and requires frequent recalibration using the frequency discriminator and signal processing techniques, effectively using a disposable/low-stability component that is periodically corrected rather than relying on an expensive long-stability component.
Solution Approach 2:
The patent changes the operating parameters of the frequency measurement system by using coherent integration over multiple signal periods and applying phase compensation based on frequency error measurements. This allows the system to achieve high effective stability not through the oscillator's inherent stability parameters, but through signal processing parameters such as integration time and phase correction algorithms.
3Device complexity
If the frequency source stability is insufficient, then the device complexity is reduced, but the positioning accuracy in poor environments deteriorates
Solution Approach 1:
The patent implements continuous frequency error measurement and phase compensation during the signal integration process. The frequency discriminator continuously monitors the oscillator drift, and the system continuously applies phase compensation to the correlation process, ensuring that useful signal integration continues uninterrupted despite frequency instability, thereby maintaining positioning accuracy in challenging environments.
Solution Approach 2:
The patent performs preliminary frequency error measurement and phase compensation before the final signal correlation and positioning calculation. By pre-compensating for frequency errors using the discriminator feedback and applying phase corrections to the correlation process, the system prepares the signal in advance to counteract the effects of oscillator instability, ensuring accurate positioning results.
Data Source
AI summary
A method and apparatus for determining a frequency related parameter of a frequency source. In some embodiments, the method includes receiving a plurality of signals from a plurality of remote sources; generating motion compensated correlation results using a determined receiver motion, the received signals, and a local signal derived from the local frequency source; phase compensating the motion compensated correlation results to produce phase compensated correlation results using a plurality of phasor sequences that represent frequency error of the local frequency source; and jointly analysing the phase compensated correlation results associated with the plurality of remote sources to determine a frequency related parameter of the local frequency source.