Defence test equipment is the set of receivers and simulators that defence laboratories use to test missiles and radars. It has two common kinds. Telemetry receivers capture the test data that a missile sends down by radio during a test flight. Radar simulators feed false targets to a radar that is being tested.
Telemetry is a stream of readings that a missile sends while it flies, such as speed, height and temperature. The receiver on the ground records this stream, so engineers can study the flight afterwards. Say a defence laboratory fires a test missile over the sea from a range on India's east coast. The missile may be destroyed at the end of the test, and it is never recovered if it falls into the sea. The record on the ground is then all that is left, so the receiver must capture every reading without a gap.
A radar simulator solves a different problem. Testing a radar with real aircraft is costly. So the simulator makes the radar believe that a target exists. A radar finds the distance of a target from the time that its pulse takes to go out and come back. Radio travels about 300 metres in one microsecond. So the simulator receives the radar's pulse, waits a calculated time, and sends it back as if it were the echo of a target. The wait sets the distance. A small change in the frequency of the pulse sets the speed. The box shows one false target in numbers.
The buyers are mainly India's defence laboratories, such as those of the Defence Research and Development Organisation (DRDO). The makers are the same small companies that build counter-drone systems, because both need radar and radio skills. The measuring instruments inside belong to the same family as network test equipment. The radars that it tests are of the kind built under defence electronics. Radars with other jobs, such as a synthetic aperture radar that makes pictures of the ground, are tested in a similar way.