Global Positioning System Reference
In-Depth Information
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Thus, it is possible to assign weekly portions of this code to the various satellites.
As a result, all satellites can transmit on the same carrier frequency and yet can be
distinguished because of the mutually exclusive code sequences being transmitted.
All codes are initialized once per GPS week at midnight from Saturday to Sunday,
thus creating, in effect, the GPS week as a major unit of time. The L1 and L2 carriers
are both modulated with the same P(Y)-code.
The period of the coarse/acquisition (C/A) code is merely 1 ms and consists of
1023 bits. Each satellite transmits a different set of C/A-codes. These codes are cur-
rently transmitted only on L1. The C/A-codes belong to the family of Gold codes,
which characteristically have low cross-correlation between all members. This prop-
erty makes it possible to distinguish the signals received simultaneously from differ-
ent satellites rapidly.
One of the satellite identification systems makes use of the PRN weekly number.
For example, if one refers to satellite PRN 13, one refers to the satellite that transmits
the thirteenth weekly portion of the PRN-code. The short version of PRN 13 is SV
13 (SV
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space vehicle). Another identification system uses the space vehicle launch
number (SVN). For example, the identification of PRN 13 in terms of launch number
is NAVSTAR 9, or SVN 9.
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3.2.2.1 Signal Structure
The carrier is modulated by several codes and the
navigation (data) message. There are at least three commonly used digital modulation
methods: amplitude shift keying (ASK), frequency shift keying (FSK), and phase
shift keying (PSK). GPS uses PSK. Figure 3.8 briefly demonstrates some of the
principles involved. The figure shows an arbitrary digital data stream consisting of
binary digits 0 and 1. These binary digits are also called chips, bits, codes, or pulses.
In the case of GPS, the digital data stream contains the navigation message or the
pseudorandom sequences of the codes. The code sequences look random but actually
follow a mathematical formula. ASK corresponds to an on/off operation. The digit 1
might represent turning the carrier on and 0 might mean turning it off. FSK implies
transmission on one or the other frequency. The transmitting oscillator is required
to switch back and forth between two distinct frequencies. In the case of PSK, the
same carrier frequency is used, but the phase changes abruptly. With binary phase
shift keying (BPSK), the phase shifts 0° and 180°. The BPSK method is used with
GPS signals.
Figure 3.9 shows two data streams. The sequence (a) could represent the naviga-
tion data chipped rate of 50 bits per seconds (bps), and (b) could be the C/A-code or
the P(Y)-code chipped at the 1.023 MHz or 10.23 MHz, respectively. The times of
bit transition are aligned. The navigation message and the code streams have signif-
icantly different chipping rates. A chipping rate of 50 bps implies 50 opportunities
per second for the digital stream to change from 1 to 0 and vice versa. Within the
time of a telemetry chip there are 31,508,400 L1 cycles, 20,460 C/A-code chips, and
204,600 P(Y)-code chips. Looking at this in the distance domain, one telemetry chip
is 5950 km long, whereas the lengths of the C/A and P(Y)-codes are 293 m and 29.3
m, respectively. Thus, the P(Y)-code can change the carrier by 180° every 29.3 m,
the C/A-code every 293 m, and the telemetry every 5950 km.
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