Introduction This Report compares the strengths and weaknesses of time-difference-of-arrival (TDOA) versus angle-of-arrival (AOA) methods of signal geolocation. While this Report focuses on TDOA@ it should be noted that other geolocation techniques exist1. The AOA method determines the angle of arrival of a wave at a measurement point. AOA methods have been commonly used in many direction-finding applications@ and have some advantages but also some disadvantages related to antenna requirements@ for example. TDOA methods@ on the other hand@ compute the time difference of arrival of a wave at multiple measurement points@ and calculate the source point based on timing and wave comparisons. TDOA methods have not been widely used in spectrum monitoring@ but have become increasingly useful due to the availability of inexpensive and compact computing power@ more advanced radio receiver technology@ ready availability of data links@ and accurate distributed timing signal availability. The paper will provide a short overview of TDOA technology and some comparison of the strengths and weaknesses of the TDOA method compared to more traditional AOA methods. 1 Received signal strength (RSS) uses the measured power ratio of a signal at multiple measurement points to compute the source point. RSS is often used for indoor geolocation. Frequency-difference-of-arrival (FDOA) uses the frequency Doppler shift of a moving source (and/or multiple receivers) to calculate the source point. FDOA is often used in conjunction with TDOA for airborne applications.