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A conventional radar transmits a radio signal and receives back the signal reflected from the target. The distance is estimated by measuring the propagation delay when the propagation velocity of the electromagnetic wave in the medium is known. In conventional radars transmitter and receiver are co-located and therefore use the same local oscillator to measure the propagation delay and time tag the measurement. This mode of operation is called "monostatic".In "bistatic" radars transmitter and receivers are in different locations, remote from each other. In this case a single transmitter can illuminate the target and the reflected signals are received by one or several receivers, which are passive, and therefore undetected by the opponent that can "see" only the transmitter. In this case, transmitter and/or receivers must be mutually synchronized in order for the propagation path to be measured.An extension of the method exploits signals of opportunity which are not specifically intended for detection and ranging 1. These can be signals broadcast for a variety of services (TV, mobile telephone base stations, radio-beacons, etc.).Among the signals of opportunity, an interesting possibility is being offered by the radio navigation signals broadcast by a variety of satellites composing different GNSS (Global Navigation Satellite Systems) constellations, notably GPS (the U.S. Global Positioning System), Galileo, Glonass, … The relatively low signal levels as received on the ground from these satellites make this possibility certainly less appealing that using other signals of opportunity of greater strength. However, even if somewhat limited by the low signal levels which requires relatively large antennas for detection in the back-scattering mode, GNSS signals offer various advantages:–different from other signals of opportunity, the signal broadcast by navigation satellite systems has been specifically designed for high resolution ranging;–they provide a global coverage, including the polar caps or regions lacking traditional radio services;–the illumination is from above, from different transmitters and at different angles, increasing the probability of detection, effectively countering common stealth schemes, generally aiming to minimize the forward cross section;–a large number of transmitters operating at the same frequency is available at any time, moreover their geometry is continuously changing with time proving a continuously varying geometry of observation.–the transmitters position is always known with high accuracy, and the signals transmitted are all mutually synchronized; after the target is tracked, this allows to use positioning algorithms to compute the target position and the relevant motion elements.Preliminary analyses have shown the possibility to detect backscattering from targets of the size of a medium ship at ranges in excess of 10 nautical miles with medium size antennas. Forward scattering can be detected using carrier phase measurements limited by the thermal noise of the signal (a few mm).This paper addresses the potential of using GNSS signals as illuminators of opportunity in PCL (Passive Coherent Locator) systems, exploiting both back-scattering and forward-scattering modes of operation, the latter offering interesting possibilities in detection and tracking of low-flying small drones. |
