758 AIRBORNE NAVIGATION AND GROUND SURVEILLANCE 



with 0.1 to 0.5 per cent accuracy are available. The heading reference 

 appears as the weakest link in the system. Effort to improve heading refer- 

 ence continues, and the potential use of earth rate directional references 

 (north-seeking gyrocompasses) for the navigation system holds great 

 promise. Heading reference equipment with a maximum long term heading 

 error of less than 0.75° should be available in the future. On this basis one 

 can conclude that doppler navigation systems are now and will be capable, 

 over reasonably long flights, of probable position errors (50 per cent 

 probability) near 0.65 per cent of distance traveled and a maximum error 

 (95 per cent probability) of approximately 1.5 per cent of distance traveled 

 over land and average sea state. Neglecting nonsystematic water motion 

 effects, which should be of importance only for low-speed aircraft, the 

 maximum position error over extreme sea states should generally be some- 

 where below 2 per cent of distance traveled. 



Altitude performance of doppler radar systems is a function of the 

 available microwave power and the type of transmitter-receiver design (e.g. 

 pulse or CW). It appears that most modern doppler radars, by a combina- 

 tion of these two factors (i.e. sufficiently high power and special design 

 technique) satisfy the maximum altitude requirements of both military 

 and civilian aircraft. 



Modern doppler navigation systems compute and display ground speed, 

 drift angle, present position, ground track being made good, course to 

 destination, and distance to destination and can also provide an autopilot 

 steering signal. Some of the systems also continuously compute and /or 

 display wind speed and wind direction. Systems that do this usually have 

 a wind memory feature which allows the system to operate from the true 

 air speed and "last remembered wind" when the doppler signal goes below 

 a given threshold for any reason (e.g. very smooth water). Many modern 

 systems employ great-circle course and distance computers and are capable 

 of operating in conjunction with either of the three types of heading 

 references discussed earlier. 



It bears re-emphasizing that the performance of a complete doppler 

 navigation system is a direct function of the performance characteristics of 

 all three of the major components of the system — the doppler radar, the 

 computer, and the heading reference — and that the overall system 

 accuracy is no better than that of the least accurate of the three compo- 

 nents. 



In some cases, the interconnection of a doppler radar and an inertial 

 platform, in a so-called doppler-inertial system, becomes advantageous from 

 the viewpoint of overall system performance. For example, for some appli- 

 cations extremely accurate short-term velocity information is required, in 

 addition to accurate vertical attitude and long-term velocity information. 

 In this case, the interconnection of a doppler radar and an inertial platform 



