Patent Publication Number: US-RE49132-E

Title: Radar altimeter

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
     This Reissue application is a reissue of application Ser. No. 06/572,286, filed Jan. 20, 1984, which is incorporated by reference herein.  
     This invention is concerned with radar type altimeters. Specifically, the radar altimeter of the present invention is one which maintains a signal to noise level below a value which affords detection by an intercept receiver. 
     Radar altimeters are well known. In order to provide covert operation, i.e. not detectable by an intercept receiver, the radar signal should be a minimum to reduce the chances of radar detection by the intercept receiver. 
     The novel radar system of the present invention provides a radar altimeter which operates near the oxygen absorption line. The frequency of the transmitter is adjusted in accordance with a measurement of the signal to noise ratio of a preselected range bin (return signal) thereby minimizing detection by an intercept receiver. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a graphical representation of signal absorption versus frequency. 
         FIG. 2  is a diagrammatic representation of intercept receiver signal to noise ratio versus range measured from the transmitter. 
         FIG. 3  is a block diagram of one embodiment of the invention of the present application. 
     
    
    
     As shown in  FIG. 1 , atmospheric attenuation of a radar signal is near a maximum when the transmitter frequency is at 60 GHz. It should be noted that  FIG. 1  is presented as a representation and is not intended to be a detailed and exact graph of frequency versus absorption. Nevertheless, it can be seen that a change of 8 DB/KM (one way) attenuation occurs by changing the radar frequency from 53 GHz to 57 GHz. Thus, large changes in attenuation are obtainable from small variations in the transmitter frequency, i.e. 4 GHz. 
       FIG. 2  shows a diagrammatic representation of intercept receiver signal noise ratio versus range from the transmitter.  FIG. 2  illustrates the differences between signal noise ratio for microwave signals  20  and GHz signals  25 . A constant signal to noise ratio of 15 DB line is shown on the graph. A microwave transmitter can be detected at 15 dB signal to noise ratio at 800 K feet. However, at a frequency in the order of 55 GHz, this detection range is reduced to about 60 K feet (10 nautical miles). As noted in  FIG. 2 , by automatically shifting the transmitter frequency by 1 GHz, the intercept receiver range reduces to less than 20 K feet. In other words, the intercept detection range is reduced by over 40 times as compared with a microwave system. 
     Shown in  FIG. 3  is a schematic block diagram which illustrates the principles of the invention of the present application. Oscillator  300  provides a carrier signal to be transmitted by transmitter  305  for subsequent transmission through antenna  310  via duplexer  315 . Oscillator  300  includes a control input  301  to vary the carrier signal frequency to some extent. 
     By way of example, the radar system shown in  FIG. 3  is an altimeter. Altimeter return signals are received by antenna  310  and processed through duplexer  315  which feeds receiver  320 . The output from receiver  320  is fed into signal-to-noise detector  340 . 
     The output of detector  340  is presented to a hold circuit  342  for holding the value of the signal-to-noise ratio upon a predetermined bin selection provided by the output of bin selector  345 . The output of hold circuit  342  is then presented as one input to comparator  350 . Comparator  350  compares the output of hold circuit  342  to a preselected reference level input. The comparator provides an output signal to integrator  360  which integrates the output signal of comparator  350 . The output of integrator  360  is then presented to the control input  301  of oscillator  300  for adjusting the oscillation frequency, i.e. the carrier frequency in proportion to the output of integrator  360 . 
     The operation of the radar system of  FIG. 3  will now be described. The reference signal is a signal representative of a predetermined acceptable signal-to-noise ratio which affords only limited detection by an intercept receiver at a predetermined range. Oscillator  300  provides a signal in the order of, for example, 55 GHz which is subsequently transmitted over antenna  310  through transmitter  305  and duplexer  315 . Return signals, in time, are operated on by receiver  320  for usual altimeter range determination. Receiver  320  provides the usual amplification and detection circuits necessary for standard altimeters. An output of receiver  320  is provided to detector  340  which provides signal-to-noise detection in the usual manner well known in the art of radio signals. The output of detector  340 , a signal indicative of the signal-to-noise ratio, is presented to hold circuit  342  which will hold the value of the signal-to-noise detector  340  when the holding circuit  342  is gated by bin selector  345 . Gating occurs upon a predetermined range which usually would be the maximum range desired in the operating system. 
     The output of the holding circuit  342  is then presented to comparator  350  which is compared with the reference signal indicative of the preselected acceptable signal to noise ratio. The output of the comparator is then integrated via integrator  360  and presented to the adjustable oscillator  300  for adjusting the oscillation so as to increase or decrease as the case may be to maintain the preselected signal to noise ratio. The combination of comparator  350  and integrator  360  provides a negative feedback control means so as to maintain the signal-to-noise ratio as aforesaid. 
     It will be appreciated by those skilled in the art that the embodiment shown in  FIG. 2  is applicable to a wide range of radar systems including FM/CW, pulsed radar systems, and the like. Such systems are envisioned to be within the scope of the present application and are diagrammatically represented by a transmitter block  305 . Similarly, dependent upon the system employed, receiver  320  is constructed in the manner to be compatible with the method of transmission by transmitter  305 .