GALILEO - Understanding GPS: Principles and Applications - page 581

10.5.1 Space Segment
The GALILEO space segment will comprise 27 operational satellites in a Walker
constellation (see Section 2.3.2.3) with three orbital planes, equally spaced and with
56º nominal inclination. Each plane will contain nine satellites, nominally 40º
apart. It is planned to keep a spare, nonoperational satellite in each orbit plane, so
that a failure in the constellation may be repaired quickly by moving the spare to
replace the failed satellite, which can be done in a matter of days, rather than wait-
ing for a new launch to be arranged, which could take many months.
The orbit altitude above Earth of 23,222 km has been chosen so that the constel-
lation has a repeat cycle of 10 orbits in 17 days. This is short enough to allow repeat-
ability of measured characteristics while being long enough to avoid gravitational
resonances; thus, after initial orbit optimization, station-keeping maneuvers will not
be needed during the lifetime of a satellite. The position constraints for individual sat-
ellites are set by the need to maintain a uniform constellation, for which it is specified
that each satellite should be within

2º of its nominal position relative to the adja-
cent satellites in the same orbit plane and should be within 2º of the orbit plane. The
in-plane accuracy is equivalent to a relative tolerance of over 1,000 km but requires
very careful adjustment of the satellite velocity to ensure that the orbit period of all of
the satellites is kept precisely the same. The across-track tolerance allows the inclina-
tion 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 [11]. Table 10.9 lists the primary constellation parameters.

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10.5.1.1 Satellite Description
The GALILEO satellite is a 680-kg/1,500-W class satellite. The satellite dimensions
in launch configuration are shown in Figure 10.10.
The main drivers for the satellite design have been:

• Direct injection in MEO orbit;
• Optimization of the satellite size and mass for multiple launch;
• Modularity for assembly, integration, and verification (AIV) and series pro-
duction;

Table 10.9 Primary Constellation Parameters
Constellation Parameters*
Constellation

Walker 27/3/1 + 3 spare satellites

Orbit 10 day/17 orbits repeat cycle

Orbit parameter Semimajor axis a 29,600.318 km

Eccentricity e 0.002

Inclination i 56º

*For altitudes above 23,000 km, the minimum number of satellites to
achieve the required vertical accuracy is 24, with no major benefit
beyond 30 satellites. For this range of altitude and number of satellites,
three-plane constellations show the best performance. Furthermore, lim-
iting the number of planes allows lower deployment costs and easier
maintenance and replenishment strategies. The chosen inclination of 56º
ensures a good coverage at Northern Europe latitudes.