Initial investigation of a novel passive HF radar technique using available DRM and data signals

Digitale Radio Mondiale (DRM) signals have properties that allow extraction of aircraft distance and velocity passively at distances of up to 1500km, or more depending on the DRM type. The DRM spectra are flat and typically 10kHz wide. Aircraft velocity and approximate distance can be found from the interference patterns created by the `direct´ and reflected waves received from a distant DRM transmitter as observed in a software radio SDR `waterfall´ spectrum/time display of the received DRM spectrum. Up to three aircraft can typically be observed visually `by eye´. Data FAX signals having a flat 2.5kHz spectrum can be also used when DRM signals are not available. More accurate and improved ranges for more aircraft should be possible with proposed second stage FFT and radar tracking DSP algorithms operating on the received waterfalls. At HF ionospheric reflections provide a long distance OTH capability. The ionospheric multipath can be removed by suitable processing techniques. Some fundamental information theoretic performance limits of this novel technique are examined for the case of a single receiver with an omnidirectional antenna. Aircraft direction and position can to some extent be inferred from flight profile constraints. Better positioning is obtained by combining the information from Data or DRM transmissions on more than one frequency and from different locations. Phased array reception gives target direction for single frequency DRM illumination, at some increase in cost and complexity. ADS-B `virtual radar´ plots provide data on local civil aircraft movements that can be compared with the DRM waterfall data for algorithm development in this novel passive radar technique.