Design of IRNSS Tracking System using 1.5 bit ADPLL and Correlator

Abstract

IRNSS is an indigenous satellite navigation system consisting of 7 satellites that provide accurate positioning in the Indian sub-continent region. Each IRNSS satellite transmits a signal which contains information regarding satellite orbital and clock parameters (known as navigation message). The purpose of the receiver is to demodulate the satellite signal and extract navigation message, the receiver must know certain parameters of the signal like its doppler shift and code offset. However, in real-time, due to relative velocity of the satellite and ionospheric interference, these parameters vary with time. Therefore, the receiver must continuously perform the tracking operation to update the varying parameters. Existing tracking systems are based on SDR and SoC’s, which require high-performance processors and iterative algorithms to perform both carrier and phase tracking. Though they are highly accurate, these designs are complex and expensive. In this paper, 1.5-bit ADPLL is used to track the carrier. This design does not require numerous computational loops to perform tracking of the carrier, thus reducing the complexity of the design. This work includes simulation results for 1.5-bit ADPLL. In this work, 2-bit, 1.5-bit, and modified 1.5-bit correlators are simulated and synthesized. It was found that modified 1.5-bit correlator architecture is less complex compared to 2-bit correlator and offers better SNR compared to 1.5-bit correlator. Therefore, modified 1.5-bit correlator is used for code tracking. The IRNSS signal tracking is performed in ModelSim. The system utilizes 77 standard LUTs and exhibit maximum settling time of 714μs and 31.28ms for carrier tracking and code tracking, respectively.