A passive radio frequency (RF) geolocation solution is provided that uses a single low earth orbit (LEO) satellite to find an uncooperative earth-bound emitter. For the first time, an unambiguous solution is available for real-time, single-pass, and time-constrained acquisition scenarios where single transmissions are expected. The geolocation algorithm rapidly maps Doppler and Doppler rate measurements to an RF emitter location. Corresponding theoretic performance bounds are provided for mission analysis and optimality comparison. The proposed algorithm is a variant of the constrained Unscented Kalman Filter (cUKF), chosen for computational efficiency, modeling abilities, and ease of constraint incorporation. A novel, low-computation method of sigma point projection upon the surface of the earth is derived that drastically improves search area capabilities and convergence rates. The theoretical performance bound is in the form of the recursive constrained Posterior Cramér-Rao Bound (rcPCRB), which is uniquely suited to gauge the mean squared error optimality of iterative nonlinear estimation algorithms. Numerical analysis over measurement noise, center frequency, slant angle, and initialization error exhibit the algorithm's robustness over various mission types. The performance of the cUKF is demonstrated on raw IQ data acquired from the TDS-1 satellite operated by Surrey Satellites.
Southwest Research Institute ® (SwRI ® ) has developed a radio frequency (RF) receiver that is agile in frequency and waveform over the ultra high frequency (UHF) band. The software defined receiver is CubeSat-scale in size and power consumption, and it provides a flexible, adaptable solution for a number of RF application areas including communications, tracking and locating, and signals intelligence. Based on SwRI internal funding and previous developments for the U.S. Government, the receiver, which is currently at a technology readiness level of five, enables premission and on-orbit reconfiguration plus RF agility from 300 to 3,000 MHz with a bandwidth of up to 30 MHz. This paper describes the hardware architecture of the agile receiver and provides an application example of the geographical location of a terrestrial RF transmitter. In this example, geolocation performance is simulated using performance parameters of the receiver modeled on two or three low Earth orbit spacecraft to estimate the location of the transmitter. A number of spatial separations of the three spacecraft configuration are considered. Further, two RF frequencies are simulated, one for an RF transmitter in the lower portion of the UHF band and another for an RF transmitter in a higher portion of the UHF band. Results show that this small, low power technology, which is suitable for a CubeSat payload, provides a capability for geolocating terrestrial transmitters.
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