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Aviation History
1977
1977 - 0075.PDF
FLIGHT International, 8 January 1977 69 inmagfflHB •HHJi^HHHffi > -r^- Lli !^j SMHfflBg DTA, @©©©0® MSB Area navigation In an area navigation (RNav) system, bearing and distance, position or velocity information from such aids as VOR is processed in a computer to produce steering information between pre-set waypoints. A large range of equipment is available. Simple units—usually produced for general-aviation applica tions—use two or more receiver frequencies simul taneously (any combination of VORs or DMEs) and a computer. The system can provide a simple cross-track error and position readout, or a flight director can be connected to the computer to produce guidance equiva lent to that of point-source aid lying directly beneath the chosen route. The most sophisticated RNavs embody larger computers—usually with a large back-up memory capable of storing details of the operator's route struc ture—and are mostly used by overland civil operators. Autopilot coupling and 'flight-director outputs are com mon in such systems, and there may be electronic data displays and a cathode-ray tube topographical presenta tion. to him. But as the computer's abilities have expanded, so it has usurped the navigator's authority. Essentially, it is not the absolute accuracy or novelty of the automatic system which has displaced the navigator. The computer-based navigation system also usually wins hands-down because it is cheaper. The human navigator was expensive, representing a heavy expenditure on wages, training and overheads, and duty-time limits meant that several navigators were needed for each aircraft. Although his equipment costs were not high, the human navigator was expensive to run. Automatic systems, on the other hand, tend, over a typical service life, to cost less than the navigators which they replace. Although a triple-INS set can cost around £150,000, it can be highly cost-effective when compared with, say, three navigators over 10 years. The comparison is even more flattering if one takes into account the extra revenue which results from being able to carry an extra passenger. INS cost of ownership does however include very high maintenance costs and high depreciation, so running costs are far from being negligible. Although many navigation systems could have replaced the human operator several years ago, the airlines were not prepared to commit themselves before acceptable reliability had been demonstrated. Triple-INS sets are now common on airliners—even though one set could do the job adequately, the possibility of a failure rules out complete reliance on it. When three INS are fitted, a majority vote singles out the defective set in the event of a failure. An Omega or VLF navigation system costs less than one-third as much as an INS, an apparently fatal imbalance for inertial. But though Omega and VLF win on costs, have comparable reliabilities, and appear to be as accurate, it is possible for a radio-based system such as Omega to "slip" a lane. Even if three systems are installed, it remains possible for all three to register the same slip. Triple Omega or VLF are unlikely ever to replace triple INS as the standard long-range system, but mixes such as INS/Omega/Omega or INS/INS/Omega offer many advantages. An event significant for the future of navigation systems takes place on December 29 of this year. International standards which have stood the test of time have governed the lateral separations between aircraft tracks across the North Atlantic ever since oceanic operations became commonplace. The distance between adjacent tracks should be as little as possible, but the track-keeping accuracy of aircraft using old-style navigation techniques did not permit separations of less than 120 n.m. Since INS was introduced as standard on wide-body airliners, track-keeping accuracy has improved so much that a cross- track error of just 30 n.m. after a six-hour flight on INS only is now seen as worthy of investigation. The terminal error on a North Atlantic crossing now averages 5 n.m. When Loran A is withdrawn at the end of the year, new minimum navigational performance specifications (MNPS) will be introduced. The new standards call for aircraft to be equipped with INS or Omega/VLF, but as 70 per cent of North Atlantic aircraft are already so fitted the effect will not be dramatic. Many narrow-body opera tors feel that they cannot afford to retrofit INS and Omega and VLF are now being extensively evaluated. Inertial navigation A typical inertial navigation system (INS) measures inertial forces, or the effects of accelerations, with accelerometers mounted on a stabilised platform. Basic stabilisation is achieved gyroscopically, but corrections have also to be made for the effects of the Earth's rotation, the system's position on the Earth, and coriolis and centrifugal forces. Accelerations are resolved into north-south and east-west components (and sometimes also vertically) and then integrated successively to provide, at any time during the flight, instantaneous speed and accumulated distance. Given the co-ordinates of its starting point, an INS can provide a position readout at any time during flight. Accuracy deteriorates with time, but in a good system error accumulates at less than 1 n.m./hr. Speed and distance data can be processed to provide true airspeed, wind velocity, track, heading, cross-track error, and distance to go. INS can be coupled to the autopilot to provide automatic track-keeping. For fail-safe operation up to three independent INS sets may be fitted, with the crew comparing the output of individual sets to obtain an accurate position.
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