Abstract
Space traffic is considered a complex contemporary issue originated from the difficulty inherited in the nature of space missions. Satellites and space objects are actively tracked using Earth-based surveillance networks or insitu sensors like the GPS. The data collected from these Earth-based surveillance networks albeit their public availability, they remain only valuable for trajectory and state verification due to their limited positional accuracy. This is exhibited in the poor accuracy (within 2-5 km) of public two-line elements (TLEs), which worsen when propagated in the future to more than tens of kilometers. Consequently, precision orbit data obtained from onboard GPS systems, are commonly used to alleviate, to a degree, the uncertainty regarding space conjunctions. However, the lack of GPS data standardization and sharing regarding critical satellite conjunctions together with the limited deployment of the GPS system onboard nano-satellites and its susceptibility to Doppler shifts in low Earth orbits, inhibits the full exploitation of the GPS system in space traffic.
Targeting instant inter-satellite ranging for improving space situational awareness (SSA), this research explores the benefits of the proposed custom signature sequence to optimize a signal acquisition and detection for coherent and asynchronous synchronisation. This comes with the advantage of accurate propagation delay estimation and bias errors correction. Data sharing also becomes possible using the proposed custom direct sequence code division multiple access (DS-CDMA) and Reed-Solomon forward error correction for a futureproof inter-satellite ranging and data sharing solution. Coherent detection, acquisition and synchronisation are proposed based on data-assisted correlation which reduces the false- and miss-alarms respectively by 27% and 97%. This CDMA frame is based on orthogonal dual-channel separation which offers considerable noise and interference resilience for data recovery, in addition to precise frequency and timing errors correction during synchronisation. Results show that the ranging accuracy approaches meter-level at no expense of increased bandwidth beyond 2 MHz. Occupying the same bandwidth, the ranging resolution scales linearly with the spreading factor contributed by the Kronecker filter length. Additionally, the delay measurement solution is proven unsusceptible to the Doppler frequency errors caused by the high relative velocity between the satellites and Doppler errors within 16 kHz were corrected. Further, the coding gain increases by 16 dB compared to narrow-band communications, using CCSDS Reed-Solomon encoder coupled with data sequence spreading, offering considerable data transfer resilience in low signal-to-noise conditions.