Dead reckoning by computer

In modern craft, computers have proved well suited to processing the streams of data--directions, speeds, and times--involved in keeping track of position. In military land vehicles, computers are fed by compasses and signals taken from the wheels. Navigators aboard ships depend on the gyrocompass and the log; those in aircraft rely on the gyromagnetic compass and Doppler-effect speed measurements. The computers can be programmed to print periodically updated positional information. Generally, inertial guidance systems provide dead-reckoning information only, though compass and Doppler data can be included with inertial outputs. Information from radio fixing systems can be added to the dead reckoning; each item of information can be automatically compared with the others.

Radio fixing systems that can provide continuous indication of position are eliminating the distinction between position fixing and dead reckoning. Navigation is effected by supplying both the classical dead-reckoning data (speed and direction) and the continuously updated position to a computer, which determines the speed, heading, and rate of climb or descent that must be maintained to execute the flight plan. Many computers apply the technique called Kalman filtering, which weights each of the several supplied data according to its expected quality.

Instead of being mapped on a chart, position may be continuously displayed on a moving map. If the route is known beforehand, the map may be prepared in the form of a strip driven by rollers with a pointer moving to and fro to indicate exact position. When greater flexibility of route is essential, a series of maps covering a very wide area may be printed on film and projected on a ground glass screen on the bridge or in the cockpit.

Dead reckoning plays an important role in air traffic control. Not only does every air navigator have to file a complete flight plan showing courses and timings, but, during the flight, air traffic controllers keep what is virtually a dead-reckoning check on each aircraft. The flight plans are split into sections, and flight strips are prepared showing height, speed, and timings of each aircraft in a section. The air traffic controller responsible for that section can thus see which aircraft will be arriving and in what sequence. These flight strips are continually updated by reports sent by radio from the aircraft and also by changes in height and course ordered by the air traffic controller to maintain adequate separation. In modern systems, automatic flight strips are displayed on cathode-ray tubes.