In environments without access to Global Positioning System (GPS) signals, the capability to accurately detect the direction of arrival (DOA) and track the location of a source is useful for a wide variety of applications. In GPS-denied environments, such as indoor and urban scenarios, tracking an RF source is even more challenging due to multipath conditions. A team of researchers from the University of Michigan at Ann Arbor has proposed a source-tracking technique based on a combination of radio triangulation and a direction retrieval approach utilizing a compact receive (RX) antenna array in the high-frequency (HF) range.

In the proposed technique, signals are received by an array of antennas closely positioned on a rotating platform. The direction of the source is provided by the magnitude and phase differences between the received fields on the array elements. A system prototype was implemented and tested. The prototype’s receiver end consists of two antennas operating in the HF range that are very close to each other in terms of wavelength. A problem arises because the phase difference between the received signals becomes too small to be accurately measured at such low frequencies.A phase difference amplification circuit was designed to amplify small phase differences. The system prototype was tested for various scenarios with different levels of multipath. See “A Sub-Wavelength RF Source Tracking System for GPS-Denied Environments,” IEEE Transactions On Antennas And Propagation, April 2013, p. 2252.