By 2020, four separate GPS-like constellations of navigation satellites forming the Global Navigation Satellite System (GNSS) will be circling the earth, operated separately by the U.S. (GPS), Europe (Galileo), Russia (Glonass) and China (BeiDou/Compass). Each constellation will be made up of between 24 and 36 individual satellites, with the four constellations well separated in altitude between 10,000 nm and 12,500 nm above the earth’s surface. Each constellation’s satellites will transmit their navigation and other data on its own dedicated, and occasionally encrypted, UHF frequencies, although all four constellations and their satellites have a common “open” frequency for future inter-constellation applications.
At present, only GPS and Glonass constellations are complete and operationally available, while Galileo and Bei Dou are progressively building their constellations. Of the four, only Galileo is privately owned and operated, the other three being owned and operated by their respective military authorities.
Augmentations
All four constellations provide–or will provide when fully operational–worldwide basic position accuracies on the order of plus or minus 10 meters. While that level of accuracy is adequate for en route operations, augmentation techniques are required to refine those errors for landing approaches. For GPS, these come with its wide area augmentation system (Waas), which measures GPS errors at the earth’s surface and then uplinks that data to geostationary satellites (geos) orbiting at 24,000 nm. In turn, the geo retransmits those errors back down to Earth–but now as position corrections–where they are automatically and differentially applied to all GPS receivers over a wide area below (typically from around 70 North to 70 South latitudes and out to proportional east/west distances) to bring position accuracies down to plus or minus one to two meters. Europe, Russia and China have Waas-like augmentation systems, as does India for the non-global regional satnav constellation it is currently building.
Two further augmentations, also developed by U.S. researchers, can enhance Waas performance to provide, via supplementary avionics, precision approach guidance comparable to ILS. These are the space-based and ground-based augmentation systems (Sbas and Gbas). As of January the FAA had published 3,522 Sbas localizer precision with vertical guidance (LPV) procedures, of which 2,385 were to non-ILS runways, and 1,137 were to ILS runways. Of those approaches, 1,840 were to Category 1 limits, with decision heights of either 200 or 250 feet. There are also several hundred Sbas procedures operational or in development in Western Europe, and smaller numbers in Canada and Mexico.
However, while Sbas can cope with Category 1’s maximum time-to-alert warning of six seconds (the data’s roundtrip delay via the geo,) that’s too slow for safe Category 2 or 3 operations, which normally require a failure warning in one second, but never greater than 2.5 seconds. Gbas handles these shorter warning times by dispensing with the roundtrip to the geo, relying instead on a dedicated GPS antenna field at each airport offering Category 2 or 3 approach service. Those local GPS corrections are then data uplinked to the approaching aircraft’s onboard GPS receiver. United, Lufthansa, Emirates, British Airways and Cathay have been flying Category 1 familiarity approaches at Houston and Newark, both of which have interim Gbas installations.
Farewell ILS?
The end of ILS is still years away, since Sbas, Gbas and ADS-B are vulnerable to GPS jamming and spoofing. We will probably slowly lose less-used ILS installations as time goes by, but the FAA has suggested that ILS will remain as backup to all Category 2 and 3 approach systems in the NAS, and possibly for Cat 1 systems at busier locations. Unfortunately, no jamming- or spoofing-resistant ILS replacements have been developed yet.