746 AIRBORNE NAVIGATION AND GROUND SURVEILLANCE 



The Heading Reference. The heading reference is an extremely 

 important component of a doppler navigation system, not only because it is 

 the device which transforms the navigational information into directionally 

 oriented earth coordinates but also because the heading reference error has 

 a major effect on the total system accuracy of a navigation system. In 

 terms of numbers it turns out that a 1° error in heading represents approxi- 

 mately a 1.75 per cent error in present position. It is not surprising that 

 the heading reference error tends to swamp the other errors in most modern 

 doppler navigation systems and therefore assumes extreme importance In 

 the system analysis and synthesis of such systems. 



The three most important types of heading reference are: (1) the 

 gyromagnetic compass, (2) the earth-rate directional reference or north- 

 seeking gyrocompass, and (3) the astrocompass. 



The best versions of the first two devices can achieve probable heading 

 accuracies of the order of 0.25 — 0.35° at latitudes up to 70°. In the polar 

 regions, both of these systems have to be operated in a free-gyro mode and 

 the errors are somewhat larger, increasing with the flight time. Because it 

 does not depend upon earth's magnetic field information, further improve- 

 ments in the north-seeking gyrocompass system accuracy may be antici- 

 pated as the state of the art in gyros and stabilization techniques improves. 

 The astrocompass is capable of great accuracy. However, it requires a clear 

 sky and accurate vertical reference, time and position information, and a 

 gyro for memory. 



14-5 DOPPLER NAVIGATION SYSTEM ERRORS CAUSED 



BY INTERACTIONS WITH THE GROUND AND WATER 



The foregoing pages have dealt with system errors originating within the 

 navigation system. Four major sources of random error were indicated: 

 (1) frequency measurement, (2) data conversion, (3) computation, and 

 (4) heading reference. For Janus systems in particular, the other possible 

 major sources of random error (stabilization and fluctuation) were shown 

 to be negligible if sufficient care is taken in the design. Also, it was assumed 

 that systematic errors originating in the radar (antenna misalignment, 

 boresighting, and average transmission frequency) could be "biased out" 

 in a calibration procedure. 



There is still another error which arises from interactions between the 

 radar system and the terrain or water illuminated by the radar. This error 

 — commonly called the terrain bias error — is both small and systematic for 

 overland operation. It is caused by a combination of factors: (1) a small 

 change in scattering coefficient with looking angle over the finite radiated 

 beamwidth, (2) range difference effects over the beamwidth, and (3) non- 

 linearities in converting ray angles to doppler frequencies, resulting in a 



