Reference Signal Construction for Passive Synthetic Aperture Radar from Digital Video Broadcasting Signals with Samples Missing

Licentiatavhandling, 2026

In this thesis, an improved reference signal for making passive Synthetic Aperture Radar (SAR) images is constructed by demodulating a Digital Video Broadcasting-Terrestrial (DVB-T digital TV) signal received on a flying platform with a portion of the samples missing. The quality of the reference signal affects the quality of the passive SAR image, so the reference signal needs to be of good quality. Demodulating the DVB-T signal received in the flying platform for constructing the reference signal makes the passive SAR system less complex than using a receiver on an additional, stationary platform for that purpose. A novel algorithm for demodulating a DVB-T signal with samples missing is presented and is shown to work for constructing a good reference signal. Signals received by the airborne SAR system LORA, developed by the Swedish Defence Research Agency (FOI), are used. LORA cannot register received samples completely continuously but about 1 % of the samples are missing. The demodulated signal is then manipulated and modulated again to make a reference signal that is shown to suppress artefacts from the signal structure in an example SAR image.
Synthetic aperture radar is a useful technology for making radar images. Passive SAR does not transmit signals but instead use signals from other, possibly non-cooperative transmitters. Passive SAR has some desirable characteristics compared to conventional active SAR, for example, the cost and weight of the radar system may be lower as it does not have a transmitter. A passive SAR may also use frequencies that are not available for radar purposes and that may interact differently with the environment, e.g. frequencies sufficiently low to penetrate foliage. The passive SAR image is produced using measurements of received echo returns while the transmitter or the receiver is moving. In addition to the measured of echoes, the corresponding (measured) positions of the antennas are needed together with a measured reference signal from the transmitting source. The SAR image is formed by correlating the received echo returns with the reference signal which ideally should be the same as the transmitted signal (or an intentionally modified copy of it). There are several different kinds of possible transmitters and signals available for passive SAR. In this thesis, signals that comply with the digital TV standard Digital Video Broadcasting-Terrestrial (DVB-T) are used. DVB-T signals are attractive for use in passive radar since, for example, the transmitted power is high (tens of kilowatts), signals are available in wide areas, in several countries and on several continents, their bandwidth and frequencies and their use of Orthogonal Frequency Division Multiplexing (OFDM) modulation which can be demodulated, decoded and corrected.