752 AIRBORNE NAVIGATION AND GROUND SURVEILLANCE 



from vibrating equipment, back-scattering from weather and turbulent air, 

 and transmitter signal leakage. Doppler spectrum broadening, caused by 

 power supply ripple leading to transmitter amplitude or frequency modula- 

 tion, local oscillator frequency drift, transmitter tube noise, and internal 

 vibration noise, can degrade S /N ratio in all classes of doppler systems. 

 These phenomena must receive careful attention in the design to avoid 

 potentially severe degradations, particularly since they cannot be overcome 

 by increasing transmitter power. 



In summary, then, the signal altitude capability of a doppler radar is a 

 function of available transmitter power and radar circuit design, the latter 

 factor including antenna size, type of modulation, physical rigidity, etc. 

 On this basis, most doppler radars achieve the minimum doppler signal- 

 to-noise ratio which is required by the doppler frequency tracker for signal 

 acquisition (2 to 4 db) at an altitude above 70,000 ft over Douglas sea state 

 1 and at their top operational ground speed, with an average transmitter 

 power between 2 and 20 watts. 



It is worthwhile noting at this point that many doppler navigation 

 systems are equipped with an automatic wind memory circuit, which 

 becomes operative as soon as the received doppler signal-to-noise ratio goes 

 below a predetermined threshold which is selected near the minimum 

 signal-to-noise ratio required for tracking by the frequency tracker (norm- 

 ally between and 2 db). It is immaterial whether this signal drop occurs 

 when flying over extremely calm water, when making a steep bank, when 

 climbing rapidly or because of loss of transmitter power. As soon as the 

 signal drops below this threshold level, the system goes "on memory," 

 i.e. the ground speed data fed to the computer are obtained by vectorially 

 adding a true air speed signal from a true air speed meter and a "last 

 remembered wind vector" signal which has been continuously computed 

 from the difi^erence between doppler ground speed and true air speed and 

 which is instantly available as soon as the system goes "on memory." It 

 is therefore possible to continue to obtain rather accurate navigational 

 information from such a system even if the doppler signal goes below the 

 required level for a limited time. Generally speaking, this accuracy is 

 maintained as long as the wind structure does not change. When it does 

 change the remembered wind does of course become erroneous. 



14-7 LOW-ALTITUDE PERFORMANCE AND THE 

 "ALTITUDE HOLE" 



A discussion of the performance capability of a doppler radar would not 

 be complete without reference to the problems of low-altitude performance^ 

 inasmuch as certain systems are limited in this respect. This limitation can 

 Occur in certain pulse and FM-CW systems and takes two forms: 



