GALILEO - Understanding GPS: Principles and Applications - page 584

Furthermore, accurate range-rate (Doppler) measurements are possible when
the S-band transponder is operated in coherent mode. S-band TT&C operations will
be provided via two separate hemispherical-coverage helix antennas situated on
opposite sides of the satellite. Designed for orthogonal circular polarization,
together they will provide omnidirectional coverage for reception and transmission.
The S-band antennas are provided with RHCP and LHCP, respectively. Ranging
operation will be performed simultaneously with telemetry transmission.

Satellite Payload Description
The GALILEO satellite carries two payloads: a navigation payload and a SAR
payload.
The navigation payload will broadcast navigation data on four carrier frequen-
cies in the L-band, with each carrier modulated with the navigation data for a num-
ber of services. Refer to the navigation signal description in Section 10.3 for more
detailed information.
The navigation data are uploaded to the GALILEO satellite by means of a dedi-
cated CDMA C-band uplink. This enables uplinking several simultaneous signals.
The navigation payload features two different types of on-board clock technolo-
gies, a RAFS and a Passive Hydrogen Maser (PHM). Each type of technology has a
redundant unit. These are shown in Figures 10.12 and 10.13, respectively, along
with their measured stability in laboratory tests [12]. These tests indicated that both
clocks largely exceed their respective design specifications.
The SAR transponder on GALILEO satellites will detect the distress alert from
any COSPAS-SARSAT beacon emitting an alert in the 406-406.1-MHz band. The
SAR transponder will rebroadcast this information to dedicated ground stations in
the L-band (1,544 MHz). Particular emphasis is given to the SAR payload design to
avoid negative mutual effects with respect to the navigation payload.
Once the beacon information has been received by the dedicated ground seg-
ment, COSPAS-SARSAT mission control centers (MCCs) carry out the position
determination of the distress alert-emitting beacons. This is further discussed later in
this chapter.

10.5.1.2 Launch Vehicle Description
In order to enable a cost-optimized placing of the spacecraft into orbit, a launcher
family has been selected. This family is comprised of large launchers having the

Frequency stability RAFS1-R2 EQM5
December 2002

Averaging Time (Tau), seconds

100 10,000 100,000

1,000

1.0E-12

RAFS GALILEO
specification

1.0E-13

1.0E-14

Figure 10.12 GALILEO RAFS and stability results.