CONNECTI V IT Y
Satellites live in different orbits around the Earth,
at different heights and running along different
paths. Each factor has an impact on the connectivity
possible between aircraft and internet.
Low Earth Orbit (LEO) satellites operate at
1,200 miles (1,931km) or less above sea level –
approximately a third of the Earth’s diameter – or
have an orbital period of 128 minutes or less. LEOs
provide the lowest communications latency, as they
are closest to Earth, but are subject to the drag of
the very outer reaches of the atmosphere – so they
are inevitably going to fall to Earth at some point.
They do however require the least amount of energy
to launch, so are cheaper.
Medium Earth Orbit (MEO) – sometimes called
intermediate circular orbit (ICO) – is used by a
number of artificial satellites, most commonly for
navigation, communication and geodetic/space
environment science. The most common altitude
is approximately 12,552 miles (20,200km), yielding
an orbital period of 12 hours, as used by the Global
Positioning System for example. Other MEO
satellites include Glonass, with an altitude of 11,900
miles (19,100km) and Galileo, with an altitude of
14,429 miles (23,222km).
Communications satellites that need to cover the
North and South Poles also use MEO space. The
orbital periods of MEO satellites range from about
two to nearly 24 hours. Telstar 1, an experimental
satellite launched in 1962, orbited in MEO space.
Geosynchronous Earth Orbit (GEO) matches the
period of the Earth, so its satellites rotate once
every 23 hours, 56 minutes and four seconds. This
effectively means that GEO satellites appear to
hover over a particular spot on the Earth’s surface.
A circular GEO has a constant altitude of 22,236
miles (35,786km), and all GEO satellites share
that semi-major axis. A special case of GEO is the
geostationary orbit, which is a circular GEO in Earth’s
equatorial plane. A satellite in GEO remains in the
same position in the sky to observers on the surface.
Loosely, the term geosynchronous may be used
interchangeably with geostationary.
Communications satellites are often given
geostationary or close to geostationary orbits so that
the antennas that communicate with them do not
have to move, but can be pointed permanently at the
fixed location in the sky where the satellite appears.
024 APRIL 2020
“ Consistency is important
to our customers, and that’s
why they tend to choose
to use GEO satellites”
orbit far enough out and fast enough to appear stationary
above a point on Earth. While their distance from Earth
may add a slight latency, overall Inmarsat sees ‘stationary’
satellites as the perfect solution for aircraft connectivity.
“The aircraft is moving pretty fast, and LEO and
MEO satellites would also be moving, even faster,” says
Kai Tang, senior vice president of business and general
aviation at Inmarsat. “The aircraft’s antenna has to
follow the satellites, constantly switching back and
forth. Consistency is important to our customers, and
that’s why they tend to choose to use GEO satellites.”
SPOILT FOR CHOICE
There are no simple answers as to which way of doing
things is best. Craig Foster of Valour Consultancy warns
all suppliers that to ensure the success of their business,
quality is vital: “Those with ambitions to stay relevant
in the long term need to ensure that they are best-in-class
and not pursue an un-winnable race to the bottom on
price, especially if it comes at the expense of a goodquality
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