Abstract
Satellite conjunctions in space are a major problem for operators and governments due to the lack of coherent space situational awareness solutions. The tracking accuracy for two-line elements (TLEs) averages in kilometres with similar error boundaries making it limited for critical satellite collision prediction. The common practice using GPS provides high accuracy from centimetres to metres. However, satellite state data (position and velocity) are often never shared and orbit determination methods provide limited solutions at quantifying near-miss events. In the advent of mega-constellations, there is an urgent need for in-situ measurements to develop real satellite traffic management solutions and associated satellite traffic data standardisation to complement and refine the existing techniques. This research presents ToF range estimation techniques adapted for the increasing low Earth orbit satellite traffic that requires co-operative monitoring. Two techniques are investigated namely, two-way time transfer (TWTT) and two-way ranging using direct sequence spread spectrum (TWR-DSSS). Although both techniques reached centimetre-level accuracies (7 to 15 cm) in perfect communications conditions, this accuracy drops quickly when considering the real-world limitations. TWTT technique is affected by processing delay and relative clock drifts. Consequently, the ranging errors standard deviation for TWTT is 210 and 2075 m respectively for the delays 1 and 10 μs. It is also found that the relative clock drifts used for both satellites cause bias ranging errors as the best achieved accuracy is 170 m even when the delays are nullified. On the other hand, TWR-DSSS shows a robust performance against low signal-to-noise (SNR) levels. For instance, relative range is resolved with sub-kilometre accuracy for -20 dB SNR. Ultimately, inter-satellite cooperative RF ranging based on time of flight can offer real opportunities of a new measurement instrument complementing the existing satellite conjunction assessment tools.