Current radio direction finding techniques are limited in flexibility and focus on specific applications. Commercial off the shelf systems exist for a wide range of applications from navigation to search and rescue and wildlife tracking. However these systems rely on commercially available VHF receivers and are limited in transmission modulation techniques and frequency ranges. The majority of these systems are expensive which places them outside the reach of most individuals while the current open source designs require specialized skills and knowledge to build. The goal of this work was to design a low cost system capable of determining the approximate location of a non-cooperative VHF transmitter that could easily be implemented on a variety of unmanned systems. One unmanned aerial system was designed, built, and evaluated. Existing open source hardware and software systems were utilized for the development of the pseudo Doppler direction finding system, and work was conducted utilizing recursive Bayesian techniques to estimate the VHF transmitter's location. Results and explanations of system behaviors are presented along with limitations and possible modifications to improve performance and reliability. / Master of Science / Radio direction finding uses specialized radio equipment to determine the direction that a radio signal is coming from. Commercial systems are often expense, and existing hobbyist designs require specialized skills, and both are not flexible in application or frequency. The same is true for commercially available drones, which tend to be expensive or face other limitations. In this work a low cost radio direction finding system that uses easily found open source hardware and software was built and evaluated, along with a low cost unmanned aerial system. Then using the data collected, a computer algorithm was tested that could estimate the transmitting radio’s location. After testing it was determined that all systems did work, but still had room for improvement. Future steps and system modifications are presented that could improve the system’s performance.

A low-cost radio direction finding (RDF) VHF receiver has been investigated for development into a radio direction finding network (RDFN) with a particular focus towards public service and commercial asset tracking applications. The primary design criteria were reproducibility, low-cost, and simplicity such that public service and volunteer organizations can benefit from the technology. Two receiver designs were built and tested to allow for comparison of practicality, cost, and accuracy. A pseudo-Doppler RDF and a time difference of arrival (TDOA) receiver were built as proof-of-concept for a system design based on commercial off-the-shelf (COTS) components. The pseudo-Doppler system is a less practical implementation due to the necessity for custom hardware, a large antenna system, and an increased directional error due to multipath and weak signals. The TDOA system has potential as a very simple and low-cost RDFN implementation, but requires extremely accurate time synchronization that is difficult to achieve using COTS GPS receiver modules. The final proposed solution takes advantage of the simple TDOA hardware and multiple detection techniques (including signal strength) to produce improved locational data and ultimately provide a more accurate estimate of position. Further development and improvements to this receiver design have the potential for implementation as a low-cost radio direction finding network.

A radio direction finding (DF) system is an antenna array and a receiver arranged in a combination to determine the azimuth angle of a distant emitter. Basically, all DF systems derive the emitter location from an initial determination of the angle-of-arrival (AOA). Radio direction finding techniques have classically been based on multiple-antenna systems employing multiple receivers. Classic techniques such as MUSIC [1][2] and ESPRIT use simultaneous phase information from each antenna to estimate the angle-of-arrival of the signal of interest. In many scenarios (e.g., hand-held systems), however, multiple receivers are impractical. Thus, single channel techniques are of interest, particularly in mobile scenarios. Although the amount of existing research for single channel DF is considerably less than for multi-channel direction finding, single channel direction finding techniques have been previously investigated. Since many of the single channel direction finding techniques are older analog techniques and have been analyzed in previous work, we will investigate a new single channel direction finding technique that takes specific advantage of digital capabilities. Specifically, we propose a phase-based method that uses a bank of Phase-Locked Loops (PLLs) in combination with an eight-element circular array. Our method is similar to the Pseudo-Doppler method in that it samples antennas in a circular array using a commutative switch. In the proposed approach the sampled data is fed to a bank of PLLs which track the phase on each element. The parallel PLLs are implemented in software and their outputs are fed to a signal processing block that estimates the AOA. This thesis presents the details of the new Phase-Locked Loop (PLL) algorithm and compares its performance to existing single channel DF techniques such as the Watson-Watt and the Pseudo-Doppler techniques. We also describe the implementation of the PLL algorithm on a DRS Signal Solutions, Incorporated (DRS-SS) WJ-8629A Software Definable Receiver with Sunrise™ Technology and present measured performance results. / Master of Science