If we take the AN/APR-24 for an example we can break it down and see what it all means. The prefix "AN" simply lets us know we are using the 1957 JETDS (AN=Army & Navy) rather than some older designation system. - In following three letter set the first letter tells us the location of the system - here the "A" tells us we are dealing with a system that is used on a piloted aircraft.- The second letter tells us the type of equipment - so "P" tells us that we are dealing with some type of radar. - The third letter tells us the purpose of the equipment - so "R" tells us that the equipment is receiving or passive detecting. So APR stands for equipment located on a piloted aircraft, involves radar, and its purpose is receiving or passive detecting. The numbers are assigned as systems are developed, so it is possible to see a lower number supersede a higher number if the development of a product is protracted. A good example from the F-4 is when the AN/APR-25 supersedes the AN/APR-30. So don't always assume that a higher number is better (or newer) than a lower one. (For a full list of the letters used in the code see Andreas Parsch's great website - http://www.designation-systems.net/usmilav/electronics.html#_Introduction )17 MAR 2014 - Original Post