Global Satellite Navigation Inching Closer to Full Implementation
By: John Sheridan
Aviation International News (AIN online)
20 April 2015
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
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
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.