Satellite conjunctions in space are highly challenging because of the lack of space situational awareness solutions and orbit data sharing schemes. Two Line Elements set (TLEs) are commonly used to define satellite state on orbit but are highly inaccurate. Similarly, Global Positioning System (GPS) used for positioning and tracking purposes is not, as a standalone solution, optimised for satellite traffic management. Therefore, an autonomous system specifically designed for space traffic management is needed. A new approach has been adapted for different satellite conjunction scenarios and investigated in a way that each satellite is equipped with a radio measurement instrument operating in multiple low-noise bands taking advantage of Software Defined Radio (SDR) concepts. Relative range between satellites has been obtained from the Received Signal Strength (RSS) by implementing adaptive changes in operating frequencies. Doppler frequency shifts have also been obtained which also have a significant importance on tracking satellites. Our results show that for a two-satellite scenario, it is possible to receive a signal with 20 dB signal to noise ratio from approximately 1800 km when operating at High Frequency (HF) and 600 km at Very High Frequency (VHF). Consequently, in a 600-satellite scenario, more than 79 satellites were detected by a main satellite observer when operating at 30 MHz whereas only 10 and 5 satellites were detected when operating at 140.8 MHz and 440.01 MHz respectively. Operating at the higher frequencies (2499 and 5088 MHz) yielded two dangerous close approaches with a maximum relative range of 2 km using both RSS and Doppler. Further, a second method of estimating range based on time of flight (ToF) has been implemented showing directly dependant ranging errors from signal processing and propagation time delays. Combining different ranging methods and altering between transmitting frequencies by using enabling SDR technologies helps to develop a new highly accurate collision detection system which can complement the existing systems and define the nature of satellite conjunctions in space.
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.