Relative Range Estimation using SDR for Space Traffic Management

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.