Effect of different uplink and downlink range mod indices on PRN range accuracy

Abstract

Pseudorandom noise (PRN) sequences are used to provide a means to measure the ranging distance between a ground station and a satellite. Satellite Ground Link System (SGLS), Global Positioning System (GPS), and some radars are some of the examples that make use of PRN sequences. PRN ranging also provides a certain level of immunity to narrowband interference. The basic procedure involves transmitting a long binary PRN sequence, and measuring the time between its transmission and reception. A PRN ranging system will typically measure the propagation time by performing correlation of the received sequence with the locally generated sequence. A simple equation is used to convert that propagation time value to an accurate range. The length of the PRN sequence determines the maximum unambiguous distance that can be measured, while the code rate and code signal-to-noise ratio (SNR) or sometimes ranging code power-to-noise density ratio (C/No) determine the ranging accuracy. Typically a delay lock loop is used to measure the propagation delay. The SNR in the correlation receiver tracking loop bandwidth determines the phase error variance which is the range measurement accuracy. The SNR is a function of both the signal and noise power at the receiver, and the range signal power is a function of range modulation index. Normally the range mod indices are related through the turnaround ratio for the uplink and the downlink when the range signal is a coherent signal in SGLS. Range mod indices on the up- and downlinks can be different. This is more frequently found on new satellites with the need to have different uplink and downlink range mod indices to improve the link margin and the ranging accuracy, and also to reduce the interference such as from the command echo and intermodulation (IM) signals. In this paper the effect of different uplink and downlink range modulation indices on the achievable range accuracy is presented along with the details of the equations developed.