"Primary radar, which is the standard echo-based radar, is the only way of detecting a distant aircraft that has no transponder. It is what one would expect to use to detect "non-cooperative" objects which don't have transponder, such as UFOs!. A transponder is a transmitter on an airplane which sends out a coded signal in response to a radar pulse from the ground station. All commercial planes carry transponders as the main means of detection by the ground controllers. The transponder can carry more information than just the echo, such as the plane designation and its altitude. In this case the JAL1628 carried a transponder. Therefore the radar operator would expect to see two adjacent blips each time the radar beam, in its continuous rotatory sweep mode, would sweep past the airplane. Any primary return without a transponder return would be evidence of a radar reflective object without a transponder. The difference between primary radar and transponder signals played a roll in the FAA "explanation".
Edited by TheMacGuffin, 11 September 2012 - 08:49 PM.