Cryptography – Key management – Having particular key generator
Reexamination Certificate
1995-02-02
2001-06-26
Gregory, Bernarr E. (Department: 3662)
Cryptography
Key management
Having particular key generator
C380S001000, C380S001000, C380S046000, 36, C331S078000
Reexamination Certificate
active
06252962
ABSTRACT:
BACKGROUND
This invention relates to covert communication systems, i.e., a communication system in which the transmitted waveform exhibits no man-made detection features.
Many military operations involve platforms that wish to communicate without disclosing their presence. Covert communication systems, systems which deny the enemy even the fact that a communication system is in use by friendly units, have been designed using direct sequence pseudo-random noise (DSPN) modulation techniques. Systems employing DSPN modulation techniques produce signals which sound like noise in conventional narrow band receivers. However, if the intercept receiver is situated relatively close to the transmitter, DSPN modulation techniques do not prevent a clean reception of the wideband signal. In this situation an intercept receiver may employ feature detectors to extract tell-tale parameters of the signal, such as spectral shape, identifiable amplitude characteristics or suppressed carrier frequency, which identify it as a man-made signal, defeating the covert communication system.
Another covert communication technique is the use of a transmitted noise reference system. Under the transmitted noise reference system, two channels are broadcast: a reference noise channel and a data channel.
The two transmitted signals sound and look like noise and are correlated at the receiver to extract the data or message signal. In order to achieve covert communication, the reference noise channel must be operated at levels below ambient noise. Because the receiver must work with a reference signal that has been corrupted by external noise, operating at these levels results in degraded performance. Under these conditions, signal processing time increases dramatically.
It is therefore an object of the invention to provide a covert communication system which transmits data and/or voice communications where the transmitted waveform exhibits no man-made detection features.
It is another object of the present invention to provide a covert communication system where the signal may be reduced to much lower levels below the ambient noise levels than in prior systems.
It is also an object of the present invention to defeat all feature detections while minimizing the time it takes for the intended receiver to acquire the signal.
SUMMARY OF THE INVENTION
To achieve the foregoing and other objects, in the featureless covert communications system and method according to the invention, synchronization is first achieved between a covert transmitter and a covert receiver using two channels, one of which is a transmitted reference signal, then a single channel is used to transmit the bulk of the communication message. The single channel uses a digitally controlled noise source at both the transmitter and the receiver. The digitally controlled noise source produces a signal having pre-selected and reproducible amplitude, frequency and phase components, which exhibit random noise like properties at both locations. Since only one channel is used to transmit the message,
3
dB in power is saved. And, since the same reference signal is being generated locally at the receiver, the efficiency of the system is improved by at least another 10 dB, and corruption of the reference signal is eliminated. The featureless covert communication system operates with signals that fall below the ambient noise levels and thus, even if intercepted, these signals should be confused with the normal occurrences of noise bursts in environment. To further minimize interceptability of the system, transmission of the two channel reference subsystem is limited to short durations, typically less than one (1) second.
To further limit interceptability of the synchronization signal, additional features may be added. A time delay can be inserted into one channel at the transmitter and into the second channel at the receiver. Other features such as encryption devices that are synchronized from a global positioning satellite system, can also be added to further enhance the system.
REFERENCES:
patent: 5007087 (1991-04-01), Bernstein et al.
patent: 5048086 (1991-09-01), Bianco et al.
patent: 5245660 (1993-09-01), Pecora et al.
patent: 5291555 (1994-03-01), Cuomo et al.
Alkov Leonard A.
Gregory Bernarr E.
Lenzen, Jr. Glenn H.
Raytheon Company
Schubert William C.
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