WAAS GUS Clock Steering for L1/L5 Signal Coherence
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Solution Overview
Problem
Conventional Wide Area Augmentation System (WAAS) GEO Uplink Subsystem (GUS) clocks face challenges in accurately tracking WAAS Network Time (WNT) and maintaining signal-in-space coherence, leading to potential deviations that affect navigation accuracy and integrity.
Innovation Solution
A clock steering mechanism is implemented using WAAS Type 9 messages and a PID controller to synchronize the GUS clock with GPS time, decoupling it from orbit errors and enhancing observability, while maintaining L1 and L5 signal coherence through advanced frequency control and error correction techniques.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional WAAS GUS clocks are used without advanced steering mechanisms, then the system structure remains simple, but clock synchronization accuracy with GPS time deteriorates
Solution Approach 1:
The patent implements a feedback mechanism where the GUS clock is continuously monitored against GPS time references, and steering corrections are applied based on measured deviations. The system uses GPS time stamps and WAAS message time tags to detect clock errors and applies corrective steering signals to maintain synchronization, resolving the contradiction between maintaining simple hardware and achieving high synchronization accuracy.
Solution Approach 2:
The patent replaces mechanical clock synchronization methods with software-based processing of GPS time signals and WAAS messages. Instead of using complex mechanical gear systems or physical time transfer mechanisms, the system uses digital signal processing of electromagnetic signals (GPS L1/L5 carriers and WAAS data messages) to achieve precise clock steering with minimal hardware complexity.
2Reliability
If the GUS clock is coupled with orbit error corrections, then comprehensive error compensation is achieved, but observability of clock errors deteriorates
Solution Approach 1:
The patent segments the error correction process by separating clock error estimation from orbit error correction. The system independently processes clock synchronization using GPS time stamps and WAAS message time tags, while orbit errors are corrected separately through differential GPS calculations. This segmentation allows clear observability of clock errors without the confounding effects of coupled orbit corrections.
Solution Approach 2:
The patent introduces an intermediary time reference system using GPS time stamps embedded in WAAS messages as a mediator between the GUS clock and GPS satellite orbits. This intermediary allows the system to measure clock errors relative to a stable external time reference independent of orbital variations, maintaining both comprehensive error compensation and clear clock error observability.
3Stability of the object's composition
If L1 and L5 signals are transmitted without advanced frequency control, then the signal generation process is simpler, but signal-in-space coherence deteriorates
Solution Approach 1:
The patent merges the frequency control mechanisms for L1 and L5 signals into a unified system driven by the GPS-synchronized GUS clock. Both signals share the same time reference and frequency steering mechanism, ensuring their frequencies remain coherent with each other and with GPS time. This unified approach maintains signal coherence while avoiding the need for separate complex frequency control systems for each signal.
Solution Approach 2:
The patent dynamically adjusts the frequencies of L1 and L5 signals based on real-time clock error measurements from the GPS synchronization system. The frequency parameters are continuously modified through steering corrections to compensate for drift and maintain coherence, resolving the contradiction between simple signal generation and high signal stability.
Data Source
AI summary
Methods and apparatus for receiving an estimate of L1 iono delay and L1 iono delay rate, receiving GEO slant ionospheric delay, receiving user iono vertical error information for a GEO satellite, and computing a L1/L5 bias estimate from the estimate of L1 iono delay and L1 iono delay rate and GEO slant ionosperic delay. A wide area augmentation system (WAAS) includes L1/L5 bias estimation. The L1/L5 bias is the differential bias between the WAAS GEO satellite L1 and L5 signals. The data used in the L1/L5 bias estimation include the GEO L1 ionospheric delay estimated from a Kalman filter, and the GEO slant ionospheric delay calculated from the Ionospheric Grid Point (IGP) information in a WAAS message. The Sigma User Ionospheric Vertical Error (UIVE) is used as one of the conditions for performing the L1/L5 bias estimation.