| A digital estimation of Global Positioning System (GPS) Clear Access (C/A) code synchronization in real time was developed and simulated on the digital computer. The Pseudo-Noise (PN) ranging by correlation technique is used to measure the distances from the user's equipment to GPS satellites. The data from the Inphase-Quadrature (I-Q) detector is used to calculate the early-late likelihood ratios. The differences between the early-late likelihood ratios yield the tracking error signal which was used to calculate the code synchronizations.; A digital processor was used to read the data from the I-Q Squared detector using an 8-bit successive approximation analog-to-digital converter. The dwell time between data sample read was one millisecond. The early-late likelihood ratios computed by the digital processor which also controlled the PN code sweeping across the uncertainty range yielded the tracking error signal. The code synchronization was estimated from the tracking error signal by computing the centers of gravity of the positive and the negative lobes, and the average of these two centers of gravity was the estimated code synchronization.; The results of this digital estimator simulation showed that the estimator biased toward the center of the uncertainty range. At a signal-to-noise ratio of 25, the estimated code errors were -0.41, -0.15, -0.22, -0.12, 0.02, 0.09, 0.20, 0.27 and 0.39 chips for the received code delay of 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5 and 5.0 chips respectively. The 3 chip delay is the center of the 6 chip wide uncertainty range. The simulation of the other signal-to-noise ratios were also performed and their results were tabulated.; This phase-incoherent technique for estimating PN code synchronization appears very promising. Further research for refinement of the estimator design is indicated. |
