Three-Frequency GPS Carrier-Phase Ambiguity Resolution
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Solution Overview
Problem
Current GPS systems face limitations in resolving whole-cycle ambiguities in carrier-phase measurements, especially at large baseline separations, due to ionospheric refraction effects and multipath noise, which restrict the accuracy and range of differential GPS applications.
Innovation Solution
A method is proposed that uses carrier-phase measurements on three frequencies (L1, L2, and L5) to form ambiguity-resolved, refraction-corrected, and minimum-noise composite measurements by estimating refraction-corrected wavelengths, smoothing offset values, and resolving whole-cycle ambiguities, thereby overcoming the limitations of two-frequency systems.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If two carrier frequencies (L1 and L2) are used in GPS measurements, then the basic positioning function is provided, but the ability to resolve whole-cycle ambiguities is lost when the distance between reference and user receivers exceeds 10-20 kilometers
Solution Approach 1:
The patent transitions from a two-frequency system to a three-frequency system by adding the L5 signal. This dimensional expansion in frequency space enables the formation of multiple independent measurement combinations (wide-lane, narrow-lane, and ionosphere-free), which provides the additional degrees of freedom needed to resolve ambiguities over long baselines while maintaining correction of ionospheric effects.
Solution Approach 2:
The patent creates composite carrier-phase measurements by combining observations from three different frequencies (L1, L2, L5) with different weighting coefficients. These composite measurements combine the advantages of each frequency to achieve both long baseline ambiguity resolution and ionospheric correction, analogous to creating composite materials with superior properties.
2Measurement precision
If differential operations are used to eliminate error factors, then positioning accuracy is improved, but the method becomes inadequate for wide-area applications where spatial and temporal correlations of errors diminish
Solution Approach 1:
The patent changes the fundamental parameters of the measurement system by introducing a third frequency and forming multiple composite measurement types with different wavelengths and ionospheric sensitivities. This parameter expansion allows the system to maintain differential correction effectiveness over wider areas by selecting appropriate measurement combinations that are less sensitive to spatial decorrelation.
3Measurement precision
If ionospheric refraction effects are present in carrier-phase measurements, then measurement accuracy deteriorates, but adding more frequencies to correct these effects increases system complexity
Solution Approach 1:
The patent extracts and isolates the ionospheric refraction effect from the total carrier-phase measurement by forming specific linear combinations of three-frequency observations. The wide-lane combination is designed to be insensitive to ionospheric effects, allowing the ionospheric component to be separately estimated and removed from the narrow-lane and ionosphere-free combinations, thereby correcting the measurement without requiring complex real-time ionospheric modeling.
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AI summary
A new three-frequency technique for obtaining geometry free, refraction-corrected, ambiguity-resolved, carrier-phase measurements has been described. First, the ambiguities on at least two wide-lane carrier-phase measurement differences are obtained 210 by averaging the corresponding frequency weighted code measurements. These two ambiguity- resolved measurements are then combined into a composite refraction-corrected measurement 220. The resulting composite measurement is quite noisy due to the amplification of the multipath noise in the original carrier-phase measurements. But this noisy refraction-corrected carrier-phase measurement can be smoothed with another minimum-noise, refraction-corrected carrier-phase composite measurement. The minimum- noise, refraction-corrected composite measurement 230 is constructed from the primary carrier-phase measurements prior to resolving their whole-cycle ambiguities. By smoothing 240 the difference in the two refraction-corrected measurements, the noise can be reduced and the bias in the low-noise measurement (due to incorrect ambiguities) can be estimated and subsequently corrected.