GALILEO - Understanding GPS: Principles and Applications - page 588

Table 10.10 Design Drivers for the GCS

Design Driver Value

Routine operations
• Telemetry data volume

1.5 MB/SV/orbit

• Telemetry downlink data rate

20 Kbps (information rate)

• Ground contact required

20-45 minutes/orbit (nominal);
60 minutes/orbit (worst case)

• Telecommand uplink data rate

1 Kbps (information rate)

Special Operations
• Number of simultaneous contact

4

• Maximum contact duration

4 hours/orbit

• Maximum visibility gap duration

2 hours

TTC masking angle 5º

10.5.2.2 GALILEO GMS
The GMS implements the functions for providing the main services of GALILEO.
A real-time worldwide-distributed network of sensors (GSSs) will permanently
monitor the GALILEO signal in space. The monitoring data obtained with these
sensor stations will be used for orbit determination, time synchronization, and
integrity monitoring.
The performance and sizing of such a network is driven by the SOL service
integrity requirements. At present, the proposed GALILEO sensor network will be
composed of approximately 40 worldwide-distributed stations.
The GSS network will collect one-way pseudorange raw measurements refer-
enced to a local atomic reference clock with navigation messages received from the
satellites. The GSS network provides this data with local meteorological and other
data (such as monitor and control information and navigation data message) to the
GCC.

Clock and Ephemeris Prediction
The GCC will host a centralized orbitography and synchronization processing facil-
ity (OSPF) that periodically computes the ephemeris data for each satellite and the
clock data for each onboard clock.
The allocated contribution of ephemeris and clock determination and models to
the UERE is predicted to be less than 65 cm (1

σ ). This results in the following main

drivers:

The prediction of orbits resulting in ephemeris data for broadcast to users will
be valid for 12 hours and are, therefore, required to be uplinked to the satel-
lites once per orbit (approximately 12 hours).

•

The prediction of the onboard clock behavior resulting in the onboard clock
correction data (OCCD) for broadcast to users depends on the stability of the
frequency standard used onboard. Both PHMs and RAFS will be installed
onboard the satellites. The use of the PHM is the current baseline. However,
RAFS are short-term stable and therefore drive the Ground Segment capability

•