One component of the Federal Aviation Administration approach to independent aircraft separation assurance is known as the Traffic Alert and Collision Avoidance System I (TCAS I), which employs passive or active techniques for the detection of nearby transponder-equipped aircraft. This paper gives the results of a study conducted by Lincoln Laboratory of simple techniques for the passive and active detection of transponders. Filter criteria that may be used to restrict passive detections to potentially threatening aircraft are described and evaluated. These techniques and criteria were used in a candidate passive detector whose performance was evaluated in flight against targets of opportunity. A candidate low-power active interrogator was also evaluated through link calculations and airborne measurements. The results indicate that a low-power active interrogator can provide more reliable detection of nearby aircraft and a lower false alert rate than any of the simple passive techniques considered. The active technique generates insignificant levels of interference and, unlike a passive system, also provides protection in regions where there are no ground interrogators.