Analysis of Starlink Users' downlink for passive radar applications: signal characteristics and ambiguity function performance

New satellite systems such as Starlink have arisen as promising illumination sources for passive radar applications due to their high power and wide-band signal (transmission channels up to 250 MHz), and their almost ubiquitous presence. In this work, an acquisition system based on Radio Frequency System-on-Chip (RFSoC) with an instantaneous bandwidth of 256 MHz was employed to record complete Starlink signals at different carrier frequencies. The Starlink users' downlink was selected as the source of illumination due to its transmission characteristics. A measurement campaign was carried out at Warsaw University of Technology facilities to acquire transmitted signals at different transmission channels. A detailed analysis in time and frequency domains is carried out to identify Starlink's capabilities for passive radar purposes. Results show similar behavior of Starlink signals at different channels, with an under-demand pulse kind transmission for the users' data as well as continuous transmission of pilot carriers for synchronization purposes. The pulsed data signal was employed to compute the first ambiguity function of a Starlink signal to determine the performance of a future Starlink-based passive radar.