1. Field of the Invention
This invention relates to the detection of covert communication signals. More particularly, this invention relates to a novel circuit for detecting the presence of signals which typically are received at very low signal to interference and noise ratios.
2. Description of the Prior Art
The general class of signals to which the present invention is directed are commonly referred to as periodically keyed random modulated signals. For example, communications intelligence is often transmitted in coded form. One form or way of denying data access to the enemy is to transmit the data stream in direct sequence spread spectrum format. It is extremely difficult to detect data signals embedded or encoded in such spread spectrum format because the signal to noise ratio is so low as to make detection difficult.
Before it is possible to attempt to decode direct sequence spread spectrum coded data signals, it is necessary to determine that such coded signals are actually being transmitted. This invention is directed to the problem of detecting that such coded signals are being transmitted and is not directed to the problem of decoding such signals.
It has been suggested that radiometers or power signal detection devices be employed to determine if periodically keyed random modulated signals are being transmitted. When such signals are received at a receiver it is often impossible to distinguish them from the receiver noise, thermal background noise, other transmitted signals and interfering emission signals. It is possible that the power level of the signals which are to be detected do not exceed the background noise and interference signals mentioned above. Thus, it is often impossible to employ radiometers and power detection devices to detect the presence of low power periodically keyed random modulated signals.
When a radiometer is employed to detect the presence of a signal, then the threshold of the detector must be set very close to the signal levels. Changes in either the interference levels or the threshold levels will affect the sensitivity of the receiver which results in false alarms or reduced sensitivity. For example, if a threshold of a radiometer is set to detect the desired signal at a -20 db signal to noise ratio, then a one percent increase in interference level will cause a false alarm.
It has been suggested that since periodically keyed random modulated signals by definition change symbols at a fixed rate, it may be possible to detect the periodic repetition as a clock signal even though the data signal is not discernible.
One prior art attempt to recover the inherent clock in a periodically keyed modulated signal was to pass the received signal through a filter and then through a non-linear detector to provide power concentrated as a spectral line at the clock frequency. The output from the non-linear detector was then passed through a narrow bandpass filter tuned to the known clock frequency and that output was enveloped detected to provide an indication of the signal amplitude level as an output from the narrow bandpass filter. If the amplitude level output from the envelope detector exceeded the predetermined noise and interference reference threshold level then there is a high likelihood that a periodically keyed random modulated signal was being received.
These prior art devices have been found to require wide band filters to achieve desirable sensitivity. It is well-known that wide band width filters in such clock recovery circuits will also pass more high energy spectral lines, narrow band width signals and other noise which will be confused with the desired clock signal.
If narrow band width filters could be employed in clock recovery circuits, some of the aforementioned problems could be eliminated. Accordingly, it would be desirable to be able to detect the inherent keyed clock signal present in low signal to noise ratio periodically keyed random modulated signals employing narrow band width filters.