CNS/ATM 2- How satellite navigation works


In Australia today, global
navigation satellite systems, or GNSS, make up a major
part of the navigation component of CNS/ATM and
this will only increase as more ground-based
nav-aids are decommissioned. Satellite navigation systems
are used in all forms of aviation, from highly
integrated systems in airliners, to handheld mobile devices. Most people associate GNSS with GPS, the US government’s
satellite constellation. But over the years,
more have come on line, including the Russian
GLONASS, European Galileo, and China’s Bei-Dou. And
more and more constellations will come online in the future. But at this stage,
Australian TSO aviation receivers are single frequency, so
can only use the US GPS system. There’s a big difference between
aviation and non-aviation receivers. Aviation receivers are
certified according to a technical standard order, or TSO, and fitted according to
a CASA advisory circular. Uncertified receivers,
including mobile devices, have no way of detecting errors. Unbelievably, they can be out
by more than 500 nautical miles and they do not meet any of the
requirements for IFR navigation. So how does it all work? Well, there are
three elements to GNSS. Firstly, there’s the
satellites. And with the US’s GPS there’s 30 and four spares. And they’re traveling
in six orbital planes about 18,000 km above the earth. And each one of
them has an atomic clock and is transmitting GPS signals. Secondly, there’s the GPS
receiver, giving the pilot positioning, velocity, and
precise timing information. This means the satellites
transmit data about their current and predicted
location, timing, and health and the receivers
interpret the information to estimate their location on
earth relative to the satellites. Finally, there’s ground-based
control. A network of monitoring stations that checks the
accuracy of the satellite positions and their atomic clocks. GNSS works out a
navigation solution for an aircraft from the differences in the
time in takes for the radio waves from the various
satellites to reach the receiver. It takes four satellites
to tell you where you are. But before you go find yourself, you should be aware the
system does have errors. Solar radiation and the gravitational
pull of the earth and moon can cause wobbles in
the satellites’ orbits. The ionosphere, a
layer of charged particles about 200 km from
the earth’s surface, can slow the radio signals down, skewing the position
and time information. All of these errors tend to
amplify or cancel each other out depending on the
geometry of the satellites. But they limit GNSS
accuracy to about ten metres However, the accuracy
can be improved, by using an augmentation system. Augmentation corrects
the errors in the GPS system using a range of technologies. SBAS, or satellite-based
augmentation systems, use dedicated, high-orbit,
geo-stationary satellites to get ranging, integrity,
and correction information from a GNSS ground
monitoring network. Currently, the government
is running a test bed project through Geoscience
Australia to implement SBAS into the Asia-Pacific region. They’ll be trialling next
generation SBAS that’ll ensure accurate positioning and
vastly improve the precision of GNSS. The benefit is that
most of the currently mandated receivers in Australia are already capable of using SBAS. There’s also GBAS, or
ground-based augmentation systems. It’s where a ground
station is installed at an airport to monitor GPS satellites
and transmit corrections, integrity parameters,
and approach data through a VHF uplink
on board the aircraft. GBAS is coming on line at
major airports around the world. As the capabilities
of GNSS grow, they’ll be tremendous benefits to
civil aviation worldwide, including the opportunity
to continue decommissioning costly ground-based
nav-aids and the further enabling of performance
based navigation, PBN, to allow for more
efficient use of airspace.

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