A "classic" ranging system uses radar to time how long it takes for a radio signal to travel to a target, in this case a rocket, and back. However there are a number of problems with this method that make it accurate to only perhaps 1%. Primary among these problems is creating a sharp "pulse" of radio so that the start of the signal can be accurately defined, there are both practical and theoretical limits to the sharpness of the pulse. In addition the timing often introduced inaccuracies of its own, building super-accurate clocks in the 1960s was not the science it is today.
In MISTRAM this was avoided by broadcasting a continuous signal. The basic system used a ground station located downrange from the launch site (at Valkaria, Florida and Eleuthera Island, Bahamas), and a transponder on the vehicle. The tracking station transmits a carrier signal which the transponder responds to by re-broadcasting it on another (shifted) frequency. By slowly changing the frequency of the carrier broadcast from the station, and comparing this with the phase of the signal being returned, you can measure the distance to the vehicle very accurately. Even with the analog circuitry used, MISTRAM is accurate to less than 1km at the distance of the moon).