GALILEO - Understanding GPS: Principles and Applications - page 579

GALILEO time service provider. GST is specified to be kept to within 50 ns (95%)
of TAI over any 1-year time interval. The offset between TAI and GST will be
known with a maximum uncertainty of 28 ns (2 sigma), assuming the estimation of
TAI six weeks in advance. Users equipped with a GALILEO timing receiver will be
able to predict UTC to 30 ns for 95% of any 24 hours of operation.
GPS System Time is the internal GPS navigation time scale, which is not
adjusted for leap seconds and which is steered to UTC (USNO) modulo 1 second.
GPS System Time is specified to be maintained to within one microsecond modulo
integral seconds, and for the past eight years it has been maintained to within
( 25 ns) of this goal. Once GALILEO is operational, it is anticipated by many

that most users will use a combined GPS and GALILEO positioning, navigation,
and timing service. There are two options for obtaining the GALILEO/GPS time
offset:

+ / −

• The user is able to determine the GALILEO/GPS time offset in the position
and navigation processing at the cost of one additional satellite tracked (fifth
satellite when determining a three-dimensional position).
• The offset could be measured by traditional time transfer techniques (e.g., two
way, common view) or precisely estimated in near real time at the monitor
station of both systems using a combined GPS/GALILEO receiver.

The latter option was adopted as part of the EU-U.S. agreement on the
GALILEO/GPS interoperability. An interface control document (ICD) was jointly
developed that will allow precise estimation of the GALILEO/GPS time offset and
inclusion of it in each system's navigation message. The accuracy of this time offset
modulo 1 second is specified to be less than 5 ns with 2-sigma confidence interval
over any 24-hour period [10].

10.5 System Architecture

The GALILEO system will comprise a number of segments as illustrated in Figure
10.9.

• The space segment will consist of 30 satellites distributed over three planes.
The design calls for nine satellites plus one (nonactive) spare satellite per
plane.
• The ground segment will comprise a network of sensor stations, the control
centers, and uplink stations. A global communication network will intercon-
nect the centers and the stations.
• The user segment will consist of the satellite navigation receivers, a variety
of which can be envisaged for exploiting the different types of GALILEO
services.

Each segment is described in detail in the following sections.