Low probability of intercept radar using atmospheric loss

Abstract

A low probability of intercept radar apparatus and technique involving operation at the lower water vapor absorption resonance frequency of 22.2 GHz wherein the radar beam is shaped such that it is restricted to low altitude and short range coverage. The system is implemented by conventional radar techniques with the exception of the technique disclosed herein of operating at the lower water vapor absorption frequency in combination with utilizing a rotating shaped beam antenna system that restricts the radar beam to low altitude and short range coverage.

What is claimed is: 
     
       1. A method of radar surveillance with low-probability of intercept comprising the steps of: generating a modulated waveform having a midband operating frequency of approximately the lower water vapor absorption resonance frequency of 22.2 GHz;   transmitting a beam derived from said waveform out a shaped beam antenna such that the transmitted radar beam is shaped to keep the energy uniformly distributed over low altitudes where the water vapor absorption is relatively high; and   receiving, when present, and processing radar return echo signals of said transmitted waveforms.   
     
     
       2. The method of claim 1 wherein the step of transmitting a beam includes the step of limiting the range of said radar beam to a distance of 15 nm and an altitude of 3000 feet above sea level. 
     
     
       3. The method of claim 2 wherein said step of transmitting a beam includes the step of insuring that less than a -70 dbm signal will be present at a distance of 50 nm from the radar transmitting site. 
     
     
       4. A low probability of intercept radar system using atmospheric loss to attenuate the radar signals as seen by long range interceptors comprising: first means for generating a modulated radar waveform having a midband frequency approximately in the region of the lower water vapor absorption frequency of 22.2 GHz;   a shaped beam radar antenna operably coupled to said first means for transmitting a beam derived from said modulated waveform over a relatively short range low altitude area; and   radar receiver means for receiving and processing radar return signals from said transmitted modulated waveform.   
     
     
       5. The radar system of claim 4 wherein said first means include: a frequency programmed computer;   a frequency synthesizer operably coupled to the output of said frequency programmed computer for generating a CW signal;   a waveform generator means operably coupled to said frequency synthesizer for multiplying said CW signal up to said approximate 22.2 GHz midband frequency region and for modulating said CW signal; and   amplifier means operably coupled to the output of said waveform generator means for amplifying the modulated CW signal.   
     
     
       6. The radar system of claim 5 further including a duplexer connected between said amplifier means and said shaped beam radar antenna. 
     
     
       7. In a radar system including a waveform generating means for producing a modulated CW radar signal, means for transmitting a radar beam derived from said modulated CW signal and receiver means for receiving and processing radar return echo signals from said transmitted radar beam, the improvement comprising: said waveform generating means producing said modulated CW signal such that the midband frequency of said signal is approximately in the lower water vapor absorption resonance frequency of 22.2 GHz, whereby said radar system operates as a low probability of intercept radar.   
     
     
       8. The system of claim 7 wherein said improvement further comprises: said means for transmitting said radar beam is a shaped beam antenna constructed such that said transmitted radar beam is shaped to keep the energy distributed over low altitudes where the water vapor absorption is relatively high.   
     
     
       9. The system of claim 8 wherein said radar beam is limited to a distance of 15 nm beyond which, less than -70 dbm radar signal will be present and said radar beam is further limited to an altitude of 3000 vertical feet above sea level.