Uplink (1030 MHz) measurement results are presented, based on data recorded by the Airborne Measurement Facility of the M.I.T. Lincoln Laboratory during normal landings and take-offs at the LAX, Van Nuys, and San Diego airports. The data presented are relevant to current investigations of passive beacon-based collision avoidance systems and include: (1) the interrogator environment as received; (2) its division between FAA and other interrogators; (3) its dependence on aircraft height during landings and take-offs; and (4) the availability of P2 pulses of sufficient strength for PRF (pulse repetition frequency) tracking. The number of interrogators was found to increase with the aircraft height at the rate of 2.5 to 3 interrogators per 1000 ft. P2 pulse tracking appears to be feasible down to 2000 ft. at LAX, and lower at San Diego.