Sub-microsecond Time Transfer via Weak GPS Signal Cross-Correlation
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional GPS/GNSS receivers struggle to achieve sub-microsecond time transfer using weak GPS/GNSS signals, especially in indoor or urban environments where signal attenuation is severe, as existing methods require strong signal strengths to maintain low bit error rates.
Innovation Solution
A method and apparatus for sub-microsecond time transfer using a GPS/GNSS receiver that involves receiving a digitized complex baseband signal, generating a PN code, using a delay-locked loop to align the PN code, and cross-correlating it with the received signal to generate a navigation data stream, locating data bit boundaries, and detecting target segments within the navigation message to determine transmission time, even with signal levels as low as -160 to -170 dBm.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional GPS/GNSS receiver methods are used to demodulate timing information, then time transfer can be achieved, but the signal strength must be sufficiently strong to maintain an acceptably small bit error rate
Solution Approach 1:
The patent applies preliminary action by performing code alignment using a delay-locked loop before attempting navigation message demodulation. This preliminary code synchronization enables the receiver to process weak signals more effectively, allowing time transfer to be achieved from signals as weak as -160 to -170 dBm that would be insufficient for conventional demodulation methods
2Reliability
If signal strength is increased to maintain low bit error rates, then demodulation accuracy improves, but the system cannot operate in indoor or urban environments where signal attenuation is severe
Solution Approach 1:
The patent changes the operational parameters by using a delay-locked loop for code alignment and processing only the timing information from the PN code correlation peak, rather than attempting to demodulate the full navigation message. This parameter change allows the system to achieve sub-microsecond time transfer from weak signals in challenging environments like indoors and urban areas where signal attenuation is severe
3Loss of information
If conventional demodulation methods are used, then navigation message can be decoded, but the process cannot maintain time accuracy when signals drop below tracking thresholds
Solution Approach 1:
The patent extracts only the essential timing information from the GPS/GNSS signal by using a delay-locked loop to align the local PN code with the received signal and measuring the time of arrival at the correlation peak. This extraction approach separates the time transfer function from the full navigation message decoding process, enabling continuous time transfer even when signals are too weak for conventional tracking and demodulation
Data Source
Figure 1
Figure 2A~2B
Figure 2C~2E
AI summary
Sub-microsecond time transfer in a GPS/GNSS receiver using a weak GPS/GNSS signal is provided. The digitized complex baseband signal and the generated PN code are cross-correlated for each code period so as to output a complex correlation value at each code epoch of the generated PN code, where a sequence of the output correlation values form a data stream representing the navigation message. Bit synchronization generates bit sync pulses at bit boundaries. The location of a target segment having a known sequence at a known bit location in the navigation message is detected by searching through a plurality of subframes and accumulating search results for the plurality of subframes. Transmission time of the target segment is determined from the detected location of the target segment, with a certain time ambiguity. Accurate local time is determined by solving the time ambiguity using approximate time obtained from an external source.