What is garbling in a secondary surveillance radar?

Because the length of a transponder code train is about 20 µs it is not always possible to decipher replies from aircraft closer than 2-3 miles of each other on a radial from the interrogator. The reply signals may garble and the decoder equipment can cause the generation of false targets between the aircraft or cause cancellation of all or part of either or both actual returns. False targets or cancellation may occur even though altitude separation between aircraft exists. Circuits in the decoder equipment are used to cancel garbled replies, and controllers will often ensure that only one aircraft within a formation has a transponder operations