a. Space Segment. The space segment consists of all GPS satellites in orbit. The first generation of
satellites was the Block I or developmental. Several of these are still operational. A full constellation of
Block II or production satellites is now in orbit. The full constellation consists of 24 Block II
operational satellites (21 primary with 3 active on-orbit spares). There are four satellites in each of six
orbital planes inclined at 55, to the equator. The satellites are at altitudes of 10,898 nautical miles and
have 11-hour, 56-minute orbital periods. The three active spares are transparent to the user on the
ground. The user is not able to tell which are operational satellites and which are spares. A
procurement action for Block IIR (replacement) satellites is underway to ensure full system performance
through the year 2025.
b. Control Segment. The control segment consists of five tracking stations located throughout the
world (Hawaii, Colorado, Ascension Island, Diego Garcia, and Kwajalein). The information obtained
from tracking the satellites is used in controlling the satellites and predicting their orbits. Three of the
stations (Ascension Island, Diego Garcia, and Kwajalein) are used for transmitting information back to
the satellites. The master control station is located at Colorado Springs, Colorado. All data from the
tracking stations are transmitted to the master control station where they are processed and analyzed.
Ephemerides, clock corrections, and other message data are then transmitted back to the three stations
responsible for subsequent transmittal back to the satellites. The master control station is also
responsible for the daily management and control of the GPS satellites and the overall control segment.
c. User Segment. The user segment represents the ground-based receiver units that process the
satellite signals and arrive at a user position. This segment consists of both military and civil activities
for an almost unlimited number of applications in a variety of air-, land-, or sea-based platforms.
5-5. Broadcast Frequencies and Codes. Each NAVSTAR satellite transmits signals on two L-band
frequencies, designated as L1 and L2. The L1 carrier frequency is 1575.42 megahertz and has a
wavelength of about 24 centimeters. The L1 signal is modulated with a precision code (P-code) and a
coarse-acquisition code (C/A-code). The L2 signal is modulated with only the P-code. Each satellite
carries precise atomic clocks to generate the timing information needed for precise positioning. A
navigation message is transmitted on both frequencies and contains ephemerides, clock corrections and
coefficients, satellite health and status information, almanacs of all GPS satellites, and other general
5-6. Pseudorandom Noise Codes. The modulated C/A- and P-codes are referred to as PRN codes.
These PRN codes are actually a sequence of very precise time marks that permit the ground receivers to
compare and compute the time of transmission between the satellite and the ground station. The range
to the satellite can be derived from this transmission time. This is the basis behind GPS range
measurements. The C/A-code pulse intervals are about every 300 meters. The more accurate P-code
pulse intervals are every 30 meters.