Sect_2.2.3

2.2.3 The GPS System

THE CONTROL SEGMENT

The Control Segment consists of facilities required for satellite health monitoring, telemetry, tracking, command and control, ephemeris computations and uplinking. There are five ground facility stations: Hawaii, Colorado Springs, Ascension Is., Diego Garcia and Kwajalein. They perform the following functions:

All five stations are monitor stations, tracking the satellites and sending the tracking data to the Master Control Station.
Falcon Air Force Base, Colorado Springs, is the site of the Master Control Station (MCS), where the tracking data is processed in order to compute the satellite ephemerides and satellite clock corrections. It is also the station that initiates all operations of the Space Segment, such as satellite manoeuvring, signal encryption, satellite clock keeping, etc. The MCS is managed by the 2nd Operations Squadron, 50th Space Wing, U.S. Air Force.
Three of the stations (Ascension Is., Diego Garcia, and Kwajalein), together with two other antennas in the continental U.S., are upload stations allowing for the uplink of data to the satellites. The data includes the ephemerides and clock correction information transmitted within the Navigation Message, as well as command telemetry from the MCS.

The GPS Control Segment.

Each of the upload stations can view all the satellites once a day. All satellites can therefore be viewed by an upload station three times a day. New Navigation Messages and command telemetry can be transmitted to GPS satellites approximately every 8 hours, if necessary. At present the upload rate is once (and sometimes twice) per day.

An important, latent function of the Control Segment is to maintain the WGS84 reference system. This reference system is accessible to the GPS user via the satellite ephemerides computed by the MCS, from data collected at the monitor stations. If any organisation were to compute its own satellite orbits (for example, from the post-processing GPS tracking data acquired by its own ground receiver network), the resulting reference system would be defined by the datum (or fixed) tracking stations in that system. This may not be the same as WGS84, though generally is very close to it. The basis of the International GPS Service (IGS) post-processed ephemerides is the International Terrestrial Reference System (ITRS), which is very close to the WGS84 system (to within one metre) (section 6.2.3).

As the GPS system matures the satellites will operate with greater independence from the ground-based Control Segment, without significant degradation in performance. The Block IIR and IIF satellites have a crosslink enabling between-satellite communication and ranging. The data would be processed to produce the ephemeris information within the Space Segment itself.

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