GPS System Segments - Understanding GPS: Principles and Applications - page 90

Fundamentally, GPS navigation accuracy is derived from a coherent time scale,
known as GPS system time, with one of the critical components being the satellite's
AFS, which provides the stable reference for the satellite clock. As discussed earlier,
each satellite carries multiple AFSs. The MCS commands the satellite AFSs, moni-
tors their performance, and maintains estimates of satellite clock bias, drift, and
drift rate (for rubidium only) to support the generation of clock corrections for the
NAV Data message. As stated in Section 2.6, GPS system time is defined relative to
an ensemble of all active SV and MS AFSs. The ensemble or composite AFS
improves GPS time stability and minimizes its dependency on any single AFS failure
in defining such a coherent time scale.
Another important task of the MCS is to monitor the integrity of the navigation
service. Throughout the entire data flow from MCS to satellite and back, the MCS
ensures that all NAV Data message parameters are uploaded and transmitted cor-
rectly. The MCS maintains a complete memory image of the NAV Data message and
compares each downlink message (received from its monitor stations) against the
expected message. Significant differences between the downlink versus expected
navigation message result in an alert and corrective action by 2SOPS. Along with
navigation bit errors, the MCS monitors the L-band ranging data for consistency
across satellites and across monitor stations. When an inconsistency is observed
across satellites or monitor stations, the MCS generates an L-band alert within 60
seconds of detection [12].
The CS depends on several external data sources for coordination with the UTC
(USNO) absolute time scale, precise monitor station coordinates, and Earth-
orientation parameters. NGA and USNO provide the CS with such external data.
3.3.1.2 Monitor Station Description
To perform the navigation tracking function, the CS has a dedicated, globally dis-
tributed, L-band monitor station network. At the time of this writing, the CS net-
work consisted of six Air Force monitor stations: Ascension Island, Diego Garcia,
Kwajalein, Hawaii, Colorado Springs, and Cape Canaveral. These stations are
located near the equator to maximize L-band coverage and are shown in Figure
3.16.
Each monitor station operates under the control of the MCS and consists of the
equipment and computer programs necessary to collect satellite-ranging data, satel-
lite status data, and local meteorological data. This data is forwarded to the MCS
for processing. Specifically, a monitor station consists of a single dual-frequency
receiver, dual cesium AFSs, meteorological sensors, and local workstations and
communication equipment. Each receiver's antenna element consists of a conical
ground plane with annular chokes at the base to produce a 14-dB multipath-
to-direct signal rejection ratio for signal paths above 15° elevation. (An in-depth dis-
cussion on multipath is contained in Section 6.2.) The HP5071 cesium AFSs provide
a 5-MHz reference to the receiver. Continuous-phase measurements between the
AFSs are provided to the MCS for independent monitoring of the active atomic
clock and for support of AFS switchovers. The MCS maintains a coherent monitor
station time scale. At AFS switchovers, the MCS provides the phase and frequency
difference estimates (between AFSs) to the CS Kalman filter to minimize any time