GPS satellites transmit low-power radio signals on two carrier frequencies designated as L1 and L2. The L1 carrier is 1575.42 MHz, and the L2 carrier is 1227.60 MHz in the ultrahigh frequency (UHF) band. GPS is a line-of-sight (LOS) system, and thus the signals can penetrate clouds, glass, and plastic but will not go through most solid objects, such as buildings and dense foliage. A GPS signal contains three different types of information: a Pseudo-Random Noise (PRN) code, ephem-eris data (navigation message), and almanac data. The coarse-acquisition (C/A) code is available on the L1 frequency and the precise (P) code is available on both L1 and L2 signal. Due to the spread spectrum characteristic of the signals, the system provides a large margin of resistance to interference. All signals transmitted by GPS satellites are coherently derived from a fundamental frequency of 10.23 MHz as shown in Table 9.1.
Each satellite transmits a C/A-code and a unique segment of a P-code, which is the satellite's designated identification (ID), ranging from 1 to 32. This carrier modulation enables the measurement of the signal travel time between the satellite and the receiver (user), which is converted to pseudorange (i.e., raw distance between the satellite and the receiver, see Sections 9.6 and 9.7). The travel time, and thus the pseudorange, measured using the P code is up to ten times more accurate
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