Global Positioning System Reference
In-Depth Information
solar sensors mounted on the solar panels. The condition that the
z
axis continuously
points toward the center of the earth and the solar panels are continuously normal
to the satellite-sun direction, the satellite must yaw, i.e., rotate around the
z
axis, in
addition to rotating the antennas around the
y
axis. While the satellite passes through
the shadow region, the ACS solar sensors do not receive sunlight and, therefore,
cannot maintain the exact alignment of the solar panels. The satellite starts yawing in
a somewhat unpredictable way. Errors in yaw cause errors in GPS observations in two
ways. First, the range correction from the center of the satellite antenna to the satellite
center of mass becomes uncertain. Second, there is an additional but unknown windup
error. See Section 7.2.1 for more information on the windup error.
Bar-Sever (1996) has investigated the GPS yaw attitude problem and the compen-
sation method in detail. During shadow, the output of the solar sensors is essentially
zero and the ACS is driven by the noise of the system. Even a small amount of noise
can trigger a significant yaw change. As a corrective action, a small bias signal is
added to the signals of the solar sensors that amounts to a yaw of about 0.5°. As a
result, during the time when the sun can be observed, the yaw will be in error by
that amount. During eclipse times, the biased noise will yaw the satellite in the di-
rection of the bias, thus avoiding larger and erratic yaw motions. When the satellite
leaves the shadow region, the solar sensors provide the information to determine the
correct yaw angle. The yaw maneuvers carried out by the satellite from the time it
enters the shadow region to the time it leaves it are collectively called “the midnight
maneuvers.” When the satellite is on the sun-earth axis and between the sun and the
earth, the ACS encounters a singularity because any yaw angle represents an optimal
orientation of the solar panels for this particular geometry. Any maneuvers that deal
with this situation are called “the noon maneuver.”
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[72
Lin
—
0.0
——
Nor
PgE
[72
3.2 GPS GLOBAL POSITIONING SYSTEM
Satellite-based positioning has been pursued since the 1960s. An early and very
successful satellite positioning system was the Navy Navigation Satellite System
(TRANSIT). Since its release for commercial use in 1967, the TRANSIT positioning
system was often used to determine widely spaced networks covering large regions—
even the globe. It was instrumental in establishing modern geocentric datums and in
connecting various national datums to a geocentric reference frame. The TRANSIT
satellites were orbiting in polar plane at about 1100 km altitude. The TRANSIT
satellites were affected more by gravity field variations than the much higher-orbiting
GPS satellites. In addition, their transmissions at 150 and 400 MHz were more
susceptible to ionospheric delays and disturbances than the higher GPS frequencies.
The TRANSIT system was discontinued at the end of 1996 and replaced by GPS.
3.
2.1 General Description
The Navigation Satellite Timing and Ranging (NAVSTAR) GPS provides position-
ing and timing 24 hours per day, anywhere in the world, and under any weather
