Earth-Centered, Earth-Fixed Coordinate System - Fundamentals of Global Positioning System Receivers: A Software Approach - page 56

FIGURE 4.3 Earth equator and orbit plane.

There are six different orbits for the GPS satellites; therefore, there are six
ascending points. It is desirable to use one x -axis to calculate all the satellite
positions instead of six. Thus, it is necessary to select one x -axis; this subject
will be discussed in the next section.

4.4 VERNAL EQUINOX ( 2 )

The vernal equinox is often used as an axis in astrophysics. The direction of the
vernal equinox is determined by the orbit plane of the earth around the sun (not
the satellite) and the equator plane. The line of intersection of the two planes,
the ecliptic plane (the plane of the earth's orbit) and the equator, is the direction
of the vernal equinox as shown in Figure 4.4.
On the first day of spring a line joining from the center of the sun to the center
of the earth points in the negative direction of the vernal equinox. On the first
day of autumn a line joining from the center of the sun to the center of the earth
points in the positive direction of the vernal equinox as shown in Figure 4.5.
The earth wobbles slightly and its axis of rotation shifts in direction slowly
over the centuries. This effect is known as precession and causes the line-of-
intersection of the earth's equator and the ecliptic plane to shift slowly. The
period of the precession is about 26,000 years, so the equinox direction shifts
westward about 50 (360 60/26,000) arc-seconds per year and this is a

very small value. Therefore, the vernal equinox can be considered as a fixed axis
in space.

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