Intersatellite Ranging with CDMA for Space Traffic Management

The increasing technological and economical demands on space-based services has pushed the boundaries of space domain exploitation towards a new space era.Space traffic is considered a complex contemporary issue originated from the difficulty inherited in the nature of space missions. The access to space infrastructures is considerably more challenging compared to the air or ground domains. Experts view the current space traffic solutions as insufficient for accurate and consistent satellite conjunction assessment. For this reason, precision orbit data obtained from tracking systems onboard active satellites, are commonly used to alleviate, to a degree, the uncertainty regarding space conjunctions.In this research we identify the key factors in the SSA data for a successful space traffic management (STM) system. These factors are (i) SSA data collection, optimisation and sharing, and (ii) SSA data accuracy. In order to design the ranging system, we begin by exploring different radio frequency (RF) observables to reveal their relevance for a STM application. These observables are the received signal strength indicator (RSSI) and the propagation time delay. The estimation of the latter involves several communication techniques namely, two-way time transfer (TWTT) and chip-based timing error estimation using direct sequence spread spectrum (DSSS). Further, the propagation channels chosen for the ranging STM system, are line of sight inter-satellite links (ISLs). This allows for reduced RF latency with a planned support for autonomous, coherent and asynchronous ranging and data sharing capabilities. This prompted the design of a custom physical layer utilising direct sequence code division multiple access (DS-CDMA) technique. To this end, at the transmitter level, a custom CDMA sequence optimised for accurate inter-satellite ranging and reliable data sharing, is proposed. Further, at the receiver level, optimum sequence detection, acquisition and synchronisation are proposed based on data-assisted correlators. This detection algorithm reduced the detection false- and miss-alarms by 27% and 97% respectively. The proposed 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. Additionally, the spreading factor allowed sub-sample accuracy, approaching metre-level at the adequate configurations. Consequently, an encoding gain of 16 dB was perceived by combining the data and signature symbols while maintaining metre-level ranging accuracies in low signal-to-noise channels. The system also showed resilience against Doppler shifts, managing precise ranging within 16 kHz frequency errors.Ultimately, the demonstrated ranging and data sharing performance of the proposed system, is targeted to complement the existing methods and disciplines towards a standardised STM solution