GPS System Segments - Understanding GPS: Principles and Applications - page 69

3.2.1 GPS Satellite Constellation Description
The U.S. government baseline configuration for the constellation consists of 24 sat-
ellites. Within this configuration, the satellites are positioned in six Earth-centered
orbital planes with four satellites in each plane. The nominal orbital period of a GPS
satellite is one-half of a sidereal day or 11 hours, 58 minutes [1]. The orbits are
nearly circular and equally spaced around the equator at a 60° separation with a
nominal inclination relative to the equatorial plane of 55°. Figure 3.1 depicts the
GPS constellation. The orbital radius (i.e., nominal distance from the center of mass
of the Earth to the satellite) is approximately 26,600 km. This satellite constellation
provides a 24-hour global user navigation and time determination capability. Fig-
ure 3.2 presents the satellite orbits in a planar projection referenced to the epoch
time of 0000h July 1, 1993 UTC (USNO). Thinking of an orbit as a ring, this figure
opens each orbit and lays it flat on a plane. Similarly, for the Earth's equator, it is
like a ring that has been opened and laid on a flat surface. The slope of each orbit
represents its inclination with respect to the Earth's equatorial plane, which is
nominally 55°.
The orbital plane locations with respect to the Earth are defined by the longi-
tude of the ascending node, while the location of the satellite within the orbital
plane is defined by the mean anomaly. The longitude of the ascending node is the
point of intersection of each orbital plane with the equatorial plane. The Greenwich
meridian is the reference point where the longitude of the ascending node has the
value of zero. Mean anomaly is the angular position of each satellite within the
orbit, with the Earth's equator being the reference or point with a zero value of
mean anomaly. It can be observed that the relative phasing between most satellites
in adjoining orbits is approximately 40°. The Keplerian parameters for the 24-SV
constellation are defined in Section 2.3.1.
The orbital slot assignments of this baseline design are contained in [2] and are
provided in Table 3.1. (Note that RAAN is the Right Ascension of the Ascending
Node, as defined in Section 2.3.1.)
The remaining reference orbit values (with tolerances) are:

Figure 3.1 GPS satellite constellation. ( Source: Lockheed Martin Corp. Reprinted with permission.)