| Passive radar permits inexpensive and stealthy detection and tracking of aerospace and geoscience targets. Transmitters of opportunity such as commercial FM broadcast, DTV broadcast, and cell phone towers are already illuminating many populated areas with continuous power. Passive radar receivers can be located at a distance from the transmitter, and can sense this direct transmission as well as any reflections from ground clutter, aircraft, ionospheric turbulence and meteor trails. The 100% duty cycle allows for long coherent integration, increasing the sensitivity of these instruments greatly. Traditional radar receivers employ analog front end downconverters to translate the radio frequency spectrum to an intermediate frequency (IF) for sampling and signal processing. Such downconverters limit the spectrum available for study, and can introduce nonlinearities which limit the detectability of weak signals in the presence of strong signals. With suitably fast digitizers one can bypass the downconversion stage completely. Very fast digitizers may have relatively few bits, but precision is recovered in subsequent signal processing. We present a new passive radar receiver designed to utilize a broad spectrum of commercial transmitters without the use of a front end analog downconverter. The receiver centers around a Reconfigurable Open Architecture Computing Hardware (ROACH) board developed by the Collaboration for Astronomy Signal Processing and Electronics Research (CASPER) group. Fast sampling rates (8-bit samples as fast as 3 GSps) combined with 640 multiply/addition operations on the Virtex-5 FPGA centered on the ROACH allows for coherent processing of broad spectrum and dynamic decision-making on one device all while sharing a single front end, putting this device on the cutting edge of wideband receiver technology. The radar is also designed to support mobile operation. It fits within a 19'' rack, it is equipped with solid state hard drives, and can run off an uninterruptible power supply (UPS) for up to 1 hour of continuous operation. In this document we provide technical details of the hardware, firmware, and software of the system and design strategies and decisions. We cover the topic of coherent processing for passive radar, specifically an overview of the cross-ambiguity function as a detection mechanism. While the applications of a system like this are incredibly broad, the initial validation and performance analysis was applied specifically to detection of aircraft using Digital Television (DTV) broadcast as an illuminator. We present results of both stationary and mobile operation. In stationary operation, the same helicopter has been detected using two different DTV transmissions. Early mobile operation results show the Doppler-spread ground clutter and possible detection of aircraft. In addition to the fully-functional aircraft detection signal chain, alternative FPGA designs are presented with modes for fast sampling on two antennas or four antennas, with access to an aggregate 240 MHz of spectrum, with 8-bit samples. At these extremely high data rates, moderate data loss occurs while saving this data to disk, but as detailed within this document, it can be accounted for and the effects minimalized, still allowing for detection of aircraft. With these modes, FM transmission and DTV transmission can be captured synchronously from a single antenna and digitizer feed, an exciting result that offers promise for both aerospace and geoscience applications. |
