Galileo Space Segment

GALILEO
Title	Galileo Space Segment
Author(s)	GMV
Level	Basic
Year of Publication	2011

The Galileo global component will provide the GALILEO Space segment: a constellation of Galileo satellites, each of which will broadcast navigation timing signals together with navigation data signals which will contain not only the clock and ephemeris correction data essential for navigation but also integrity signals which provide a global space-based augmentation service.

The GALILEO space segment will comprise 30 satellites in a Walker constellation with three orbital planes at 56° nominal inclination. Each plane will contain nine operational satellites, equally spaced, 40° apart, plus one spare satellite to replace any of the operational satellites in case of failures. The orbit altitude of 23 222 km results in a repeat a constellation repeat cycle of ten days during which each satellite has completed seventeen revolutions.

Constellation features

Galileo Space Segment

The altitude of the satellites has been chosen to avoid gravitational resonances so that, after initial orbit optimisation, station-keeping manoeuvres will not be needed during the lifetime of a satellite. The altitude chosen also ensures a high visibility of the satellites.

The position constraints for individual satellites are set by the need to maintain a uniform constellation, for which it is specified that each satellite should be within +/- 2° of its nominal position relative to the adjacent satellites in the same orbit plane and should be within 2° of the orbit plane.

The in-plane accuracy is equivalent to a relative tolerance of over 1000 km but requires very careful adjustment of the satellite velocity to ensure that the orbit period of all the satellites is kept precisely the same. The across-track tolerance allows the inclination and RAAN of each satellite to be biased at launch so that natural drifts remain within the tolerance without the need for orbit plane changes requiring major expense of fuel.

The spare satellite in each orbit plane ensures that in case of failure the constellation can be repaired quickly by moving the spare to replace the failed satellite. This could be done in a matter of days, rather than waiting for a new launch to be arranged which could take many months. The satellites are designed to be compatible with a range of launchers providing multiple and dual launch capabilities.

There are good reasons for choosing such a structure for the Galileo constellation. With 30 satellites at such an altitude, there is a very high probability (more than 90%) that anyone anywhere in the world will always be in sight of at least four satellites and hence will be able to determine their position from the ranging signals broadcast by the satellites. The inclination of the orbits was chosen to ensure good coverage of polar latitudes, which are poorly served by the US GPS system.

From most locations, six to eight satellites will always be visible, allowing positions to be determined very accurately – to within a few centimeters. Even in high rise cities, there will be a good chance that a road user will have sufficient satellites overhead for taking a position, especially as the Galileo system will be interoperable with the US system of 24 GPS satellites.

Galileo satellites

Galileo satellite

The Galileo constellation is composed of a total of 30 Medium Earth Orbit (MEO) satellites, of which 3 are spares, in a so-called Walker 27/3/1 constellation. Each satellite will broadcast precise time signals, ephemeris and other data. The Galileo satellite constellation has been optimised to the following nominal constellation specifications:

• circular orbits (satellite altitude of 23 222 km)
• orbital inclination of 56°
• three equally spaced orbital planes
• nine operational satellites, equally spaced in each plane
• one spare satellite (also transmitting) in each plane

The Galileo satellite is a 700 kg/1600 W class satellite.

The image shows an artist's impression of a Galileo spacecraft in orbit with solar arrays deployed. The spacecraft rotates about its Earth-pointing axis so that the flat surface of the solar arrays always faces the Sun to collect maximum solar energy. The antennas, shown on the underside of the body in the picture, always point towards the Earth. The spacecraft body will measure 2.7 m x 1.1 m x 1.2 m and the deployed solar arrays span 13 m.

Galileo Satellite components

The L-band antenna transmits the navigation signals in the 1200-1600 MHz frequency range.

The SAR (Search and Rescue) antenna picks up distress signals from beacons on Earth and transmits them to a ground station for forwarding to local rescue services.

The C-band antenna receives signals containing mission data from Galileo Uplink Stations. This includes data to synchronise the on-board clocks with a ground-based reference clock and integrity data which contains information about how well each satellite is functioning. The integrity information is incorporated into the navigation signal for transmission to users.

Two S-band antennas are part of the telemetry, tracking and command subsystem. They transmit housekeeping data about the payload and spacecraft to ground control and, in turn, receive commands to control the spacecraft and operate the payload. The S-band antennas also receive, process and transmit ranging signals that measure the satellite's altitude to within a few metres.

The infrared Earth sensors and the Sun sensors both help to keep the spacecraft pointing at the Earth. The infrared Earth sensors do this by detecting the contrast between the cold of deep space and the heat of the Earth's atmosphere. The Sun sensors are visible light detectors which measure angles between their mounting base and incident sunlight.

The laser retro-reflector allows the measurement of the satellite's altitude to within a few centimetres by reflecting a laser beam transmitted by a ground station. The laser retro-reflector is used only about once a year, as altitude measurements via S-band antenna ranging signals are otherwise accurate enough.

The space radiators are heat exchangers that radiate waste heat, produced by the units inside the spacecraft, to deep space and thus help to keep the units within their operational temperature range.