TABLE OF CONTENTS In a way, threshold detection only determines the presence of a signal. To provide useful intelligence, signal parameters must be measured. Often, just as in radar, threshold detection provides more information than mere existence of a signal. With varying degrees of precision, the threshold crossing gives some information about the following:
When the signal begins and ends;
The minimum signal amplitude;
The bearing angle, if a directional antenna is used;
The carrier frequency at least that it is in the band covered by the receiver at the time the signal occurred.
As Woodward [14] points out, radar systems cannot divorce simple target existence information from target location information. The radar receiver works best when separate range and velocity resolution cells are provided and when a directional antenna is used. The existence threshold is then the same as the resolution threshold.
An ELINT receiver has somewhat similar properties. A receiver that sought to intercept only one signal in an otherwise empty environment might find excess resolution of time, frequency, angle, and amplitude to be a burden. In practice, in an environment containing many signals, the interceptor needs parameter resolution to make sense of the energy being received.
In signal reception, the word information has been given a precise meaning by information theory. In particular, if the initial uncertainty about some signal is U 1, and after some energy has been received the uncertainty is U 2
