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Aviation History
1992
1992 - 0035.PDF
accuracy at a rate of up to 2nm (3.5km) in an hour. The low-frequency Omega and Loran C ground- based systems are availa ble for some non-preci sion approaches in the USA. Neither is available consistently worldwide because of propagation anomalies. "The advantage of GPS is that it is entirely space- based; it will be available practically anywhere, al most 24h a day; and it is very accurate most of the time," says Capt Fintan Ryan, group leader for applications development in the aeronautical services division of Inmarsat. "It provides three-dimensional position more accurately than any other current navigation system aid, except per haps the microwave landing system." Both US and Soviet systems are advertised as "under development", with full constella tions not yet complete. Ten of 24 planned Navstar Block 2 satellites are replacing a Block 1 fleet which now numbers an operational five, so that there will be 21 satellites in position eventually, with three back-ups. With 24 satellites in orbit, six to ten are visible at any time anywhere, giving the system massive redundancy because only four are required for three-dimensional position determination. NETWORK LAUNCH Satellites are launched one at a time every two months or so by Delta 2s from Cape Canaveral into 20,200km circular, 55°- inclination orbits. Full operational capabil ity is planned for 1993. A similar constellation is planned for GLONASS. The design life for GPS is seven years and GLONASS five years. Three Glonass satel lites at a time are launched by Proton boosters from Baikonur into 19,000km circular, 64°-inclination orbits. A total of 12 are now availa ble. Full deployment is due in 1995. "GLONASS is in an earlier stage of development and is funded to be operational in 1995," says Jim Nagle, navigation service group leader at Inmarsat. Aircraft are already using GPS and airlines are evaluating it non-opera- tionally. Stringent safety requirements required by air-transport authorities will delay its operational use for at least two years. Airlines may have to use United Airlines Boeing 777s, with service entry in 1995, will have GPS recei GPS and GLONASS to achieve the reliability levels and coverage until full operational coverage of both systems is achieved. For a start, GPS and GLONASS satellites can provide only a minimum of 98% availability. There are still several hurdles to overcome. Receiver specifications have yet to be approved and have not received airframe certificates. Procedures are still being adopted to allow aircraft to take GPS. Initially, GPS will be most useful for long-haul flights, working alongside INS, but it could lead eventually to the with drawal of terrestrial systems such as VOR, DME and NDB. GPS is capable of support ing non-precision approaches and, with differential corrections becoming available locally and through communications satel lites, it may be able eventually to support precision approaches and landings, already demonstrated under carefully controlled conditions by the Swedish and the US aviation authorities. GPS and GLONASS do have disadvan tages. Although the US GPS offers two accuracies of service, only the lesser accu racy is available to civilians and, despite the US commitment to ten years' use, there is Northwest has tested the automatic dependent surveillance system operationally no absolute guarantee that the system will not be degraded unexpectedly. Even so, an airline-stan dard GPS receiver will still be able to give a position report accurate to about 100m. Perhaps the biggest problem is that, as GPS use increases, it will re place some terrestrial sys tems in countries which effectively will be surren dering control to a foreign country. This is a real un certainty. Continued reli ance on GPS and GLO NASS and a withdrawal of terrestrial systems would lead to nowhere, particularly if either sys tem were to be unavailable for political or military reasons. NOT SOLE SENSOR GPS/GLONASS, however, is unlikely to be approved as a sole sensor in any airspace, as it will be complemented by other sys tems, primarily INS, with INS updated continually by GPS. No airline is going to put all its eggs in the GPS basket, probably for a long time. Neither could there be an entirely space-based system, without several "health-monitoring schemes". Airlines have to know that GPS is accu rate at times when it is used within a sole-means system for navigation. The present GPS system is not yet availa ble at all times since it is still being built up and there are some points of the globe where its satellites cannot be seen. Also, there has to be a system for knowing if a satellite malfunctions. GPS and Glonass cannot guarantee the user warning of sys tem degradation until about 30min after failure. ICAO requires a warning within 10s for non-precision approaches and 2s for precision approaches. Received autonomous integrity monitoring could isolate data from a misbe having satellite. Wide band integrity monitoring uses ground stations mon itoring information trans mitted to GEO satellites for relay to GPS users at the same frequency. In marsat 3s will be equipped with an integrity-moni tored navigation package, which could bring forward the day when GPS/GLO NASS is regarded as the major navigation system for airlines, providing the basis for a FANS 2 concept CNS system in the next century. fj 40 FLIGHT INTERNATIONAL 8 - 14 January, 1992
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