Geodetic Data Collection Techniques - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 37

Fig. 2.10 GPS satellite
constellation
located in Hawaii, Ascension Island, Diego Garcia, and Kwajalein. They are
designed to receive signals from the satellites and monitor the satellite working
status. The three up-link stations are located at Ascension Island, Diego Garcia, and
Kwajalein. These stations upload to the satellites the correction parameters of
ephemeris and clocks of the satellites computed by the master control station.
The User Segment
The user segment consists of the GPS receivers, the data processing software, and
corresponding auxiliary equipment for users, etc. It is intended to receive signals
sent by GPS satellites and to use these signals for navigation, positioning, and so on.
Signals From GPS Satellites
GPS satellites transmit carrier signals for civilian use at three frequencies:
1,575.42 MHz (L 1 carrier wave), 1,227.60 MHz (L 2 carrier wave), and
1,176.45 MHz (L 5 carrier wave). Their wavelengths are 19.03, 24.42, and
25.48 cm, respectively. Many signals, chiefly the C/A, P, and D codes, are modu-
lated on carrier waves L 1 ,L 2 , and L 5 .
The C/A code, also known as coarse acquisition ranging code, is a pseudo-
random noise code (PRN code) with a frequency of 1.023 MHz, The total code
period contains 1,023 chips and lasts 1 ms. Different satellites can be distinguished
by their PRN names because each satellite differs in its C/A code.
The P code, known as precision ranging code, is a PRN code at a frequency of
10.23 MHz.