Global Positioning System Reference
In-Depth Information
The vehicle control subsystems perform such functions as maintaining the satellite
pointing to Earth and the solar panels pointing to the Sun.
3.1.2 Control Segment (CS) Overview
The CS is responsible for maintaining the satellites and their proper functioning.
This includes maintaining the satellites in their proper orbital positions (called
stationkeeping) and monitoring satellite subsystem health and status. The CS also
monitors the satellite solar arrays, battery power levels, and propellant levels used
for maneuvers. Furthermore, the CS activates spare satellites (if available) to main-
tain system availability. The CS updates each satellite's clock, ephemeris, and alma-
nac and other indicators in the navigation message at least once per day. Updates are
more frequently scheduled when improved navigation accuracies are required. (Fre-
quent clock and ephemeris updates result in reducing the space and control contri-
butions to range measurement error. Further elaboration on the effects of frequent
clock and ephemeris updates is provided in Sections 3.3.1.4 and 7.2).
The ephemeris parameters are a precise fit to the GPS satellite orbits and are
valid only for a time interval of 4 hours with the once-per-day normal upload sched-
ule. Depending on the satellite block, the navigation message data can be stored for
a minimum of 14 days to a maximum of a 210-day duration in intervals of 4 hours
or 6 hours for uploads as infrequent as once per two weeks and intervals of greater
than 6 hours in the event that an upload cannot be provided for over 2 weeks. The
almanac is a reduced precision subset of the ephemeris parameters. The almanac
consists of 7 of the 15 ephemeris orbital parameters. Almanac data is used to predict
the approximate satellite position and aid in satellite signal acquisition. Further-
more, the CS resolves satellite anomalies, controls SA and AS (see Sections 1.3.1 and
7.2.1), and collects pseudorange and carrier phase measurements at the remote
monitor stations to determine satellite clock corrections, almanac, and ephemeris.
To accomplish these functions, the CS is comprised of three different physical com-
ponents: the master control station (MCS), monitor stations, and the ground
antennas, each of which is described in more detail in Section 3.3.
3.1.3 User Segment Overview
The user receiving equipment comprises the user segment. Each set of equipment is
typically referred to as a
GPS receiver
, which processes the L-band signals transmit-
ted from the satellites to determine user PVT. While PVT determination is the most
common use, receivers are designed for other applications, such as computing user
platform attitude (i.e., heading, pitch, and roll) or as a timing source. Section 3.4
provides further discussion on the user segment.
3.2
Space Segment Description
The space segment has two principal aspects: One aspect is the constellation of satel-
lites in terms of the orbits and positioning within the orbits. The other aspect is the
features of the satellites that occupy each orbital slot. Each aspect is described next.
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