Composite Clock Weighting for Stable GPS Time Synchronization

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Solution Overview

Problem

The GPS system faces instability in its satellite clocks, leading to prediction errors and reduced accuracy, particularly due to the lack of a robust method to combine the outputs of multiple clocks effectively, which affects the synchronization with coordinated universal time (UTC) and introduces discontinuities in the GPS service.

Innovation Solution

A composite clock system is developed that uses a weighted linear combination of multiple clocks, with weights chosen based on their Allan variances to optimize stability, employing Kalman filtering techniques to separate phase and frequency contributions and produce a composite clock signal that minimizes total random error, and includes a process noise covariance shaping mechanism to avoid numerical instability.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Device complexity

If multiple clocks are combined using simple averaging, then the device complexity is reduced, but the stability and accuracy of the composite clock deteriorates

Engineering Contradiction:

Improvecomplexity of clock combination methodVSAvoidstability of composite clock

Core Design Contradiction:

Device complexityVSStability of the object's composition

Solution Approach 1:

The patent changes the parameters of clock combination by introducing weighted averaging where weights are determined by Allan variance measurements. Instead of equal weighting, each clock contributes proportionally to its measured stability, optimizing the composite clock performance while maintaining manageable complexity through systematic parameter selection.

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent implements dynamic weight adjustment based on measured stability characteristics. The weighting factors are not fixed but are determined through measurement and optimization processes, allowing the system to adapt to varying clock performances and environmental conditions, thereby improving stability without excessive complexity.

Inventive Principle:

Principle #15Dynamics

2Stability of the object's composition

If weighted averaging based on Allan variance is used, then the stability of the composite clock is improved, but the measurement precision requirements increase

Engineering Contradiction:

Improvestability of composite clockVSAvoidprecision of Allan variance measurement

Core Design Contradiction:

Stability of the object's compositionVSMeasurement precision

Solution Approach 1:

The patent performs preliminary measurements of Allan variance for each clock component before combining them. By characterizing the stability of individual clocks in advance, the system can assign appropriate weights without requiring continuous high-precision measurements during operation, thus reducing the ongoing measurement precision burden while maintaining composite stability.

Inventive Principle:

Principle #10Preliminary action

3Measurement precision

If Kalman filtering is applied to separate phase and frequency contributions, then the accuracy is improved, but the device complexity increases

Engineering Contradiction:

Improveaccuracy of clock state estimationVSAvoidcomplexity of filtering algorithm

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The patent segments the clock error analysis into distinct phase and frequency components using Kalman filtering. By separating these contributions, the system can independently optimize handling of each error type, improving overall accuracy while managing complexity through modular processing of different error sources.

Inventive Principle:

Principle #1Segmentation

4Adaptability or versatility

If clocks with non-proportional stability characteristics are combined, then the adaptability of the system is improved, but the difficulty of detecting and measuring stability increases

Engineering Contradiction:

Improveability to combine different clock typesVSAvoiddifficulty of measuring stability characteristics

Core Design Contradiction:

Adaptability or versatilityVSDifficulty of detecting and measuring

Solution Approach 1:

The patent uses Allan variance as an intermediary metric to characterize and compare clocks with different stability characteristics. This standardized measurement approach provides a common framework for evaluating diverse clock types, enabling their combination while simplifying the measurement and comparison process through a unified stability metric.

Inventive Principle:

Principle #24Intermediary (Mediator)

Data Source

PatentUS7411867B2Stable composite clock

Publication Date: 2008.08.12 LOCKHEED MARTIN CORP

AI summary

Apparatus and method for producing a composite clock signal with optimized stability characteristics from individual clocks which have different stabilities or variances. The composite clock signal includes weighted individual clock signals. In a first (PPN) case, an optimum composite clock is a scale-factor-weighted linear combination of clock signals. In another (non-PPN) case, the optimum stable composite clock is the output of a linear filter of the input clocks signals. Both the phases and frequencies of the individual clocks over time are estimated.