Radar altimeter

Abstract

A radar system operates at a carrier frequency near the oxygen absorption line. A selected range bin is monitored to measure the signal-to-noise ratio. The signal-to-noise ratio value is used to adjust the carrier frequency so as to maintain a preselected signal-to-noise ratio.

The embodiments of the invention in which an exclusive property or right is claimed are defined as follows: 
     
       1. A radar system comprising:
 oscillator means for providing a high frequency signal having a frequency near the oxygen absorption frequency, said oscillator means having means for adjusting said high frequency signal in response to a control signal; 
 means for transmitting said high frequency signal through the atmosphere; 
 means for receiving a reflected signal of said transmitted high frequency signal; 
 means for detecting the a signal-to-noise ratio of said reflected high frequency signal at a preselected time after said transmission of said high frequency signal; 
 means for comparing said signal-to-noise ratio with a preselected signal to noise ratio value and providing an output signal indicative of the difference thereof; 
 feedback control means for providing said control signal as a function of said comparator output signal. 
 
     
     
       2. The radar system of  claim 1  wherein said feedback control means includes means responsive to said comparator output signal for maintaining said signal-to-noise ratio substantially equal to said preselected signal-to-noise ratio. 
     
     
       3. The radar system of claim 1, wherein the high frequency signal is controlled using the control signal.  
     
     
       4. The radar system of claim 1, wherein a nominal transmitted frequency of the high frequency signal is from about 55 GHz to about 65 GHz.  
     
     
       5. The radar system of claim 1, wherein the radar system is a radar altimeter.  
     
     
       6. A radar system comprising:
 an oscillator operative to generate a carrier signal, the oscillator including a control input;   a transmitter in operative communication with the oscillator, the transmitter configured to transmit the carrier signal to atmosphere;   a receiver operative to receive a reflected return signal of the transmitted carrier signal;   a detector in operative communication with the receiver, the detector configured to determine a signal-to-noise ratio of the reflected return signal; and   a comparator in operative communication with the detector, the comparator configured to compare the signal-to-noise ratio with a preselected signal-to-noise ratio value, and to provide an output signal indicative of the difference thereof;   wherein a control signal, which is generated based on the output signal from the comparator, is sent to the control input of the oscillator to adjust a frequency of the carrier signal.    
     
     
       7. The radar system of claim 6, wherein the frequency of the carrier signal is near an oxygen absorption frequency.  
     
     
       8. The radar system of claim 6, further comprising:
 one or more antennas in operative communication with the transmitter and the receiver;   a bin selector in operative communication with the detector;   a peak-hold detector in operative communication with the bin selector, the peak-hold detector configured to hold a value of the signal-to-noise ratio upon a predetermined bin selection provided by the bin selector; and   an integrator operative to receive the output signal from the comparator, the integrator configured to generate the control signal.    
     
     
       9. The radar system of claim 8, wherein the one or more antennas are connected to the transmitter and the receiver through a duplexer.  
     
     
       10. The radar system of claim 8, wherein:
 the comparator is in operative communication with the peak-hold detector and a preselected reference level input; and   the reference level input comprises a signal representative of a predetermined acceptable signal-to-noise ratio which affords only limited detection by an intercept receiver at a predetermined range.    
     
     
       11. The radar system of claim 7, wherein the signal-to-noise ratio is maintained below a value which affords detection by an intercept receiver at a predetermined range.  
     
     
       12. The radar system of claim 7, wherein the radar system comprises a radar altimeter.  
     
     
       13. The radar system of claim 7, wherein the radar system comprises a frequency-modulated continuous-wave (FM/CW) radar system.  
     
     
       14. The radar system of claim 7, wherein the radar system comprises a pulsed radar system.  
     
     
       15. A method of operating a radar system, the method comprising:
 generating a carrier signal;   transmitting the carrier signal through the atmosphere;   receiving a reflected return signal of the transmitted carrier signal;   detecting a signal-to-noise ratio of the reflected return signal at a preselected time after transmitting the carrier signal;   comparing the signal-to-noise ratio of the reflected return signal with a preselected signal-to-noise ratio to generate an output signal indicative of the difference thereof;   generating a control signal as a function of the output signal; and   adjusting a frequency of the carrier signal in response to the control signal using a feedback control loop.    
     
     
       16. The method of claim 15, wherein the preselected time after transmitting the carrier signal is determined based on a round-trip time delay associated with the reflected return signal.  
     
     
       17. The method of claim 15, wherein the preselected signal-to-noise ratio is determined based on an acceptably low probability of detection of the transmitted carrier signal by an intercept receiver at a predetermined range.  
     
     
       18. The method of claim 15, further comprising:
 monitoring a selected range bin to measure the signal-to-noise ratio of the reflected signal.    
     
     
       19. The method of claim 15, further comprising
 controlling a frequency of the carrier signal such that the signal-to-noise ratio of the reflected return signal is substantially equal to the preselected signal-to-noise ratio.    
     
     
       20. The method of claim 15, wherein the frequency of the carrier signal is near an oxygen absorption frequency.