GALILEO - Understanding GPS: Principles and Applications - page 589

to uplink the OCCD. That is, RAFS corrections are valid for 100 minutes and,
therefore, need to be uploaded to the satellites for dissemination to the users in
100-minute intervals.

Table 10.11 summarizes the navigation control drivers.
The GCC also hosts the precision timing facility (PTF), which represents an
ensemble of high-performance atomic clocks of different types (currently two active
hydrogen maser and four cesium) operated in a carefully controlled environment to
ensure maximum clock stability. GST is computed by the OSPF, based on averaging
of the different clock contributions (onboard and on the ground) with different
weights. The clocks of the PTF participate in the establishment of the GST with the
highest weight due to their high stability.
GALILEO time receivers are also installed with UTC(k) laboratories participat-
ing with the time service provider to determine the offset of GST with regard to their
representation of UTC for steering of GST to TAI.

GALILEO Integrity Determination
In addition to SIS authentication, integrity determination for GALILEO will be
achieved through the GSS network. The GSS network will monitor the navigation
signal transmissions from all GALILEO satellites. With the known positions of the
GSSs, the actual position of the SV can be determined. This SV position will enable
the maximum error on the range measurement—the signal-in-space error

(SISE)—to be estimated.
It is assumed that the prediction of the SISE distribution (which may not neces-
sarily be Gaussian) can be overbounded by a nonbiased Gaussian distribution with
the minimum standard deviation, called signal-in-space accuracy (SISA). With this
distribution, the difference between the actual four-dimensional position (orbit and
clock) of the SV and the predicted four-dimensional position on the navigation mes-
sage is described in Figure 10.15.
The estimation of the SISE is also a statistical process. The assumption made in
this case is that the distribution of the actual SISE around the value of the estimated
SISE can be described with a Gaussian distribution with the standard deviation
called signal-in-space monitoring accuracy (SISMA). The determination of the

Table 10.11 Design Drivers for Navigation Control

Design Driver Value

Measurement data acquisition
• Sampling frequency

30 seconds

• Minimum elevation angle

12.5°

• Number of receivers

1 nominal and 1 backup

• Number of carriers acquired

All (E5a, E5b, E6, L1)

• Maximum number of receiver channels

15

Navigation message generation
• Clock and ephemeris contribution to UERE

65 cm

• Ephemeris prediction validity time

12 hours

• Clock prediction validity time

100 minutes (RAFS); 8.3 hours (PHM)