Mohd Noor Islam, Clearbox Systems; Thomas Q. Wang, Clearbox Systems; Samuel Wade, Clearbox Systems; Travis Bessell, Clearbox Systems; Tim Spitzer, Clearbox Systems; Jeremy Hallett, Clearbox Systems
Keywords: Space Domain Awareness, Angle-of-arrival, Doppler, Passive RF, Satellite Signals
Abstract:
Space Domain Awareness (SDA) or Space Situational Awareness (SSA) is crucial for maintaining custody of satellites especially as the number of active satellites continues to increase rapidly. The traditional sensors for SDA are ground based radars and optical telescopes. These technologies struggle to provide comprehensive and persistent SDA due to their dependencies on weather and limitations in range. Passive Radio Frequency (RF) is a promising technology that can support the SDA mission, particularly due to its ability to operate 24/7 in all weather conditions. Passive RF sensors detect the signals transmitted from the satellites which prevents the need for an expensive transmitter and, assuming the satellite is communicating with ground, is not affected by range to the satellite. Passive RF sensors can observe the Doppler shift and Angle of Arrival (AoA) of the signals transmitted by satellites, which can be used to determine their orbit. The Doppler shift of a satellite is the difference between the transmitted frequency from a satellite and the observed frequency at the sensor site.
Satellite payloads and Telemetry, Tracking and Command (TT&C) signals are commonly digitally modulated. Satellites typically have multiple carriers for transmitting beacon and payload data that are modulated using different techniques. Line-coding techniques are also used to accommodate both telemetry and link establishment signals in the specified or approved frequency bandwidth and this bandwidth varies for different modulated signals. Therefore, estimating the accurate observed centre frequency becomes challenging. This paper describes a new method based on Welchs Power Spectral Density (PSD) method to detect the observed centre frequencies and therefore estimate the Doppler shift of a satellite for a variety of modulated signals. This method does not require any demodulation of the information in the signal, instead relying on the power spectral density estimate of the signal and the prior knowledge of a satellites transmitter frequency and modulation.
Passive RF data from Low Earth Orbit (LEO) satellites were captured using Clearbox Systems passive RF sensor network, in particular from two ground stations situated in Canberra, Australia, and Adelaide, Australia. Using this data, the Doppler shift was estimated for Frequency Shift Keying (FSK), Phase Shift Keying (PSK), and Amplitude Shift Keying (ASK)/Continuous Wave (CW) modulated satellite signals. The results were compared to both Two Line Element (TLE) sets published by the 18th Space Control Squadron (18SPCS) and accurate International Laser Ranging Service (ILRS) ephemeris data published by Crustal Dynamics Data Information System (CDDIS).
Using an array of antennas, it is possible to also estimate the Angle of Arrival (AoA) of a transmitted signal from a satellite. In this paper, azimuth and elevation are estimated using cross-correlation in the time domain as well as the interferometer method in the frequency domain. The well-known Multiple Signal Classification (MUSIC) algorithm was also used as a comparison for the results. The techniques were applied to real data from LEO satellites collected by Clearbox Systems passive RF VHF antenna array. The array consists of three omnidirectional antennas in an L-shape structure. The lengths of the two arms of the L-shape structure are designed for VHF signals and are less than half a wavelength. This paper highlights the AoA results obtained when applied to satellites with ASK modulated/CW signals. The estimated AoA is compared with the TLE data provided by the 18SPCS. The accuracy in azimuth estimation is found to be superior when compared to the elevation. This is due to the particular antenna array design which will be investigated in future work.
Date of Conference: September 14-17, 2021
Track: SSA/SDA