Communications: directive radio wave systems and devices (e.g. – Radar ew
Reexamination Certificate
1999-06-11
2001-03-20
Pihulic, Daniel T. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Radar ew
Reexamination Certificate
active
06204797
ABSTRACT:
STATEMENT OF GOVERNMENT INTEREST
The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon.
BACKGROUND OF THE INVENTION
The present invention relates to radar applications, and, in particular, to waveform diversity methods that enhance radar performance and/or prevent unauthorized utilization of emissions that result from the application of bistatic technology to radar and communication systems.
Applying bistatic technology to radar can lend itself to impermissible use of sidelobe emissions through unfriendly or unauthorized detection and tracking of targets. Prior efforts at controlling radar signatures have focused on developing ultra-low sidelobe antennas. Prior-art methods attempt to defeat signal acquisition by affecting the temporal aspect of sidelobe energy. Although sidelobe masking has shown some success in defeating non-cooperative bistatic operation, sophisticated systems can still use sidelobe energy to decode and eventually detect radar transmission signals.
What is needed is to increase traffic in multiple access schemes (e.g., a greater opportunity to reuse frequencies in space-division multiple access) while maintaining the security aspects of a transmitted radar waveform. In communications systems the benefits of waveform diversity are similar to those in simple radar systems. Preventing the acquisition and manipulation of signals is key to maintaining communication security.
SUMMARY OF THE INVENTION
Spatial waveform manipulation is the alteration of a signal's sidelobe waveform so that it differs from that of the mainlobe signal. Unlike temporal manipulation, where a sidelobe's signature is merely obscured, spatial manipulation can change the information contained within a signal with respect to the main beam. Although spatial manipulation alone of a waveform would make it more difficult to decipher than temporal manipulation, a prominent radar sidelobe is still problematic, as its signature can still be acquired. The present invention eliminates this possibility by joint spatial-temporal modulation.
The present invention diversifies radar and/or communication signal signatures by modulating the spatial and temporal attributes of a transmitted waveform in the main beam differently from its sidelobes. This diversification accomplishes three objectives. First, a non-cooperative receiver intercepting sidelobe energy observes a signal that is devoid of information contained in the main beam response. Second, since multipath signals no longer cohere to the main beam signal, channel fading and scintillation are mitigated (this is also true for the radar application). Third, sidelobe suppression is enhanced, since sidelobe signals can be more readily recognized and separated from the response from the main beam. Further, angle of arrival estimates are no longer subject to extreme error from multiple paths. This result also makes the present invention useful for precision navigation (i.e., an improved global positioning system (“GPS”)).
The present invention allows monostatic radars and communications systems to operate while it eliminates the interception of sidelobe energy and its exploitation for non-cooperative applications, including bistatic radar and wireless communications transmission. Modem radar systems typically require 40-60 dB of sub-clutter visibility (“SCV”) for operation (Skolnik). The present invention limits the adversary's SCV on conventional targets to less than 10 dB and on weak targets to well under 10 dB. Specific calculations show that SCV can be limited, in fact, to 1.58 dB, where SCV is defined as the ratio of the signal plus interference before filtering to the signal plus interference after filtering.
Additionally, embedded communications (a new type of multiple access) and precision navigation signals may now be impressed upon the radiated waveform of an otherwise classically designed radar or communications system.
In communications systems the present invention can mitigate multi-path transmission problems, suppress sidelobes, and provide for highly accurate estimates of angle of arrival. The present invention thus enhances existing multiple access systems (e.g., Space-Division Multiple Access) by allowing greater reuse of frequencies, thereby increasing capacity for traffic.
Briefly stated, the present invention uses spatial-temporal waveform diversity methods to vary or modulate the far-field radiated waveform of a radar and/or communication antenna as a function of look direction (sidelobe structure). Radar detection performance and angle of arrival estimation are enhanced to deny a coherent reference to non-cooperative bistatic radars and coherent repeater jammer systems. In the most general case, the spatial-temporal modulated waveform varies as a function of angle, based upon the principles of multi-dimensional Fourier synthesis. Spatial-temporal denial is achieved with as few as two auxiliary antennas bracketing a main antenna. The same methods for spatial-temporal waveform diversity can also embed communications signals into the transmitted radar waveform for one-way simulcast of both waveform types. These same directionally dependent simulcast waveforms can incorporate navigation signals for enhanced precision engagement.
According to an embodiment of the invention, a method of substantially preventing the interception of information associated with radar or communication waveforms that comprise a main signal and a plurality of sidelobe signals comprises: (a) manipulating a spatial waveform to cause sidelobe signals to differ from a mainlobe signal; and (b) manipulating a temporal waveform through sidelobe signal time-domain encoding to reduce sidelobe signal signature.
According to a feature of the invention, a method of substantially preventing the acquisition and use of radar or communication waveforms that comprise a main signal and a plurality of sidelobe signals comprises: (a) manipulating signatures of the plurality of sidelobe signals through temporal manipulation of waveforms of the plurality of sidelobe signals, thereby causing substantial difficulty in signal acquisition; and (b) manipulating information contained within the plurality of sidelobe signals with respect to the main signal through spatial waveform manipulation, thereby causing substantial difficulty in signal deciphering.
According to another feature of the invention, apparatus for denying the acquisition and use of radar or communication waveforms that comprise a main signal and a plurality of sidelobe signals comprises a multichannel transmit antenna system that further comprises at least two sidelobe antenna elements carrying spatial-temporal sidelobe information to accompany the transmission of a main antenna carrying said main signal.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the pertinent art from the following detailed description of a preferred embodiment of the invention and the related drawings, in which like reference numerals designate the same elements.
REFERENCES:
patent: 4580139 (1986-04-01), Weathers et al.
Bolen Scott M.
Brown Russell D.
Wicks Michael C.
Burstyn Harold L.
Pihulic Daniel T.
The United States of America as represented by the Secretary of
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