GPS Satellite Tracking Functionality - Implementation and Algorithm Analysis

The GPS satellite tracking functionality represents one of the receiver's core modules, primarily responsible for continuously locking onto satellite signals' carrier waves and pseudo-random code phases. Its implementation relies on two critical closed-loop systems: the carrier tracking loop and the code tracking loop.

The carrier tracking loop typically employs a hybrid architecture combining Frequency Lock Loop (FLL) assisted Phase Lock Loop (PLL). In implementation, the FLL first rapidly captures carrier frequency deviations to overcome loss-of-lock issues caused by dynamic stress; the PLL then performs fine tracking of carrier phase to ensure stability in demodulating data bit streams. This hierarchical strategy enhances both tracking sensitivity and robustness in high-dynamic environments. Code implementation often uses discriminators like cross-product for FLL and ATAN2 for PLL phase detection.

The code tracking loop commonly utilizes Delay Lock Loop (DLL), which dynamically adjusts local pseudocode phase by comparing outputs from three correlator channels: early, prompt, and late. Typical designs employ non-coherent phase discrimination algorithms to mitigate the impact of residual carrier errors, while narrow correlation intervals significantly improve multipath suppression capability. Implementation typically involves code phase detectors like early-minus-late power measurement with normalization.

In practical systems, carrier and code loops must operate cooperatively - Doppler estimates provided by the carrier loop assist the code loop in narrowing frequency search ranges, while the purified carrier wave stripped of pseudocode by the code loop reciprocally supports the carrier loop's phase discrimination. This coupling relationship forms a complete control system for GPS signal tracking, often implemented through feedback mechanisms in software-defined radio (SDR) architectures.