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Aviation History
1977
1977 - 0076.PDF
This VLF navigation installation in a Piper Navajo, above, illustrates the simplicity of the instruments required. The keyboard lies beneath the weather radar and radio set, and a small heading director is situated beneath the pilot's compass display. The Litton INS platforrns, below, show how far inertial navigation has progressed within a few years. Behind the latest P-4 platform (weighing 51b) can be seen, from left to right, the original P-200 (861b), a modified P-200 and a P-IOOO. The latter type is fitted to several commercial airliners VLF navigation Very-low-frequency (VLF) navigation has been developed since the introduction of VLF communica tions. VLF communications signals at frequencies of 15-25kHz are continuous transmissions carrying digital data. Because they proved very stable in frequency, the signals were investigated as a possible navigation data source, and the US Navy, which operates VLF transmitters, has recently reached agreements on signal availability. The receiver only detects changes of position from alterations in the phase of two or more VLF trans missions, and therefore lacks the corrective ability of Omega. Hybrid VLF/Omega systems are being advanced, with their proponents claiming that each will be able to fill the gaps in the other's coverage. Omega The Omega global navigation system, still under development, offers the possibility of relatively cheap, highly accurate position-fixing. Eight ground stations, located to provide near-worldwide coverage, are synchronised to generate three different pulses in a set order (pulse frequencies are 10-2kHz, 1133kHz and 13-6kHz). A complete trans mission cycle, comprising three pulses from each of the eight stations, is completed once every lOsec. An air borne or surface receiver can, depending on the pro cessing technique used, determine position to within 72 n.m. or 144 n.m., these being the maximum ambiguity distances or "lane widths." Omega is started by entering date and time (time-dependent variables are taken into account in the computer), and an approximate position to give the computer an indication of which lane it is in. An Omega-derived position will then appear in the display windows, and accuracy should thereafter be within 1-2 n.m., irrespective of time or position. As with INS, Omega offers other information, differentiating position information to obtain velocities. Omega uses very-low-frequency (VLF) signals but is not a so-called VLF navigation system. 70 FLIGHT International, 8 January 1977 THE AUTOMATIC NAVIGATOR The new MNPS should reduce cross-track errors. If the specifications succeed in this over a trial period, lateral separations over the North Atlantic will be halved. Opera tors will also be able to route their aircraft closer to the minimum-time track, so achieving not insignificant savings on fuel. General aviation has also felt the effects of the develop ment of automatic navigation. Navigators have rarely ever been evident on GA types because the emergence of radio aids happened more or less simultaneously with the post war resurgence of general aviation. Long-range types in this category are still relatively rare, and they tend to be fitted with INS and other advanced systems from the air liner range. The expression "area navigation" originally described systems such as Loran, indicating that they could provide guidance over an area instead of along a line or towards a point. But the term has taken on a new meaning in recent years. The heart of a modern area-navigation (RNav) system is a digital computer. An RNav system has any number of navigational sensors (VOR, DME, INS, Omega, Doppler), the data from which are processed in the computer and presented either as flight-director demands or on a cathode-ray tube display showing the required and achieved tracks. RNav systems are already used by a wide variety of types. Airliner operators can store full navigational data for their route structures, selected portions of which are fed into the computer on a command from the pilot. Radio frequencies will then be selected automatically as he flies the route, which need not be within range of radio aids. Corner-cutting RNav routes are most common in America (as European controlled airspace is too densely packed to accommodate new routes), and some airlines are achieving significant cost and time savings. The potential value of RNav to general-aviation opera tors is enormous. Those who operate outside controlled airspace, often between destinations lacking radio aids, should be able to use adjacent beacons to fly accurate courses between any departure point and destination. RNav is however proving a slow starter in this area, largely because the equipment is relatively expensive and most potential users are used to flying prescribed routes in controlled airspace. The navigation systems of the next few decades are unlikely to differ radically from the present generation. The ways in which they obtain their data may change, but the digital computer is almost sure to remain in control. General-purpose computers may be introduced to tackle all the decision-making tasks in an aircraft, but the com puter industry's latest little masterpiece—the cheap, highly reliable microprocessor—could win the day for distributed processing. Radio transmissions from ground or space stations, and INS, are likely to soldier on as data sources. Although radio-navigation frequencies have gone down and down, the enormous, expensive ground installations required by Omega/VLF could represent a limit. Accelerometer and gyro technology is advancing so rapidly that the INS platform could change beyond recognition, undergoing a metamorphosis into the strap-down platform and laser- gyro. The most intriguing possibility—and the one that has raised hopes so high for so many years that the pundits are becoming rather scornful—is the advent of satellite navigation. The advantages of satellites are still enormous —they can provide highly accurate surveillance over oceans—but they have begun to look less attractive as competitive systems have evolved. The cost of the long- awaited Aerosat may not be recovered commercially, with airline interest currently proving inversely proportional to the proposed user costs. Navsat, Aerosat's military equiva lent, may be put to civil use in the same way as the USN's VLF network, but at the moment this is by no means certain.
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