14-7] LOW-ALTITUDE PERFORMANCE AND THE "ALTITUDE HOLE" 753 



1. Insufficient or no usable signal because of certain range effects. 



2. Some deterioration in accuracy incident to weighting and skewing 

 of the spectrum because signal returns from certain ranges are 

 de-emphasized. 



Single-antenna continuous wave (CW) systems exhibit neither of the two 

 low-altitude limitations mentioned above, since transmission and reception 

 occur at all times. Dual-antenna CW systems can exhibit an effect similar 

 to that of (2) above at extremely low altitudes because of the physical 

 spacing of the antennas; however, the resultant error should normally be 

 small. 



Pulse systems, particularly those using TR-ATR (Transmit-Receive- 

 Anti-Transmit-Receive) duplexing, can experience both of the limitations 

 mentioned above. At very low altitudes the receiver gate simply does not 

 have tirhe to open before the entire signal pulse has returned, primarily 

 because of the finite TR recovery time. For this reason typical pulse 

 systems of this type are limited in low-altitude operation to near 200 ft. 

 For long distance navigation this is normally not a serious limitation. The 

 second limitation, the accuracy deterioration, also exists in these systems, 

 since spectrum weighting occurs at the lower altitudes incident to the 

 short-signal return time. The reason for this spectrum weighting is that 

 fewer returns from the close-in ranges than from the farther-out ranges 

 (of the beam ground intersection) have a chance to arrive at the receiver 

 after the receiver gate has opened. Hence the doppler spectrum is unduly 

 skewed and its center of gravity shifted somewhat, resulting in an error in 

 velocity measurement. 



The situation is somewhat different in certain coherent pulse systems and 

 FM-CW systems. Since TR-ATR duplexing is not used in typical coherent 

 pulse systems, it is possible to overlap intentionally the transmitting and 

 receiving gates slightly below a given altitude (say 1000 ft) in order to 

 assure sufficient signal return down to zero altitude. Only a small overlap 

 is required, since the necessary transmitter power at such low altitudes is 

 extremely small. Effectively, the system is a partial CW system in this 

 overlap mode of operation; however, because of the low altitudes involved, 

 transmitter-receiver leakage is small compared with the back-scattered 

 signal. This type of system requires a very wide dynamic-range receiver, 

 critical gating adjustments, and carrier-elimination filters. If these require- 

 ments are fulfilled, the partial CW (overlap) scheme can eliminate the low- 

 altitude signal problem and permit zero-altitude operation. If the overlap is 

 sufficiently long, considering the range differences of the beam extremities, 

 the low-altitude accuracy deterioration effect can also be avoided. 



In FM-CW systems, very low-altitude operation becomes difficult 

 because such systems have the property of inherently discriminating against 



