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Modern Geomatics Technologies and Applications

time from the satellite to the receiver and the satellite coordinates at any time. The main signal components, including: Carrier
wave: Radio frequency sinusoidal signal at a given frequency [7, 8]. Ranging code: Sequences of zeroes and ones which allow
the receiver to determine the travel time of the radio signal from the satellite to the receiver. They are called pseudo random
noise (PRN) sequences or PRN codes[9]. PRN codes have random noise characteristics, but are precisely defined. They are a
sequence of zeroes and ones, each zero or one referred to as a “chip”. Navigation data: A binary-coded message providing
information on the satellite ephemeris, clock bias parameters, almanac, satellite health status and other complementary
information. Figure 3 shows three main signal components of GNSS satellites.

Figure 3. Three main signal components of GNSS satellites [10].

The PRN codes have special mathematical properties which allow all satellites to transmit at the same frequency without
interfering with each other. These codes also allow precise range measurements between satellite and user receivers. Figure 4
shows a short repeating PRN code sample.

Figure 4. A short repeating PRN code sample [11].

2. The Research Method

GPS, is one of the famous four GNSS systems. The four global GNSS systems are – GPS (US), GLONASS (Russia),
Galileo (EU), BeiDou (China) [12]. To the all GNSS systems, the structure of the PRN code generation is conceptually similar,
while, the form of using mathematical equations in the production of these codes is slightly different. GPS is the first of a new
generation of GNSS systems. There are two different PRN code strings used by the GPS. They are the Coarse Acquisition Code
(C/A-code), sometimes called the “Civilian Code,” and the Precise, or Protected Code (P-Code) [6]. The C/A code is a particular
series of ones and zeroes, but the rate at which it is generated is 10 times slower than the P-Code. The C/A code rate is 1.023
million bits per second. Here, satellite identification is quite straightforward. Not only does each GPS satellite broadcast its own
completely unique 1023 bit C/A code, it repeats its C/A code every millisecond. The PRN code sequences are used as part of
the Code-division multiple access (CDMA) scheme in GPS [13]. (CDMA is a channel access method used by
various radio communication technologies). The PRN code don’t just use any random number. In fact, it’s not random, it’s a
kind of pseudo-random number. A signal that’s statistically random doesn’t repeat itself at any point in its sequence and so it has
very good autocorrelation properties. We also want the different codes for each satellite that are nearly orthogonal so the cross
correlation of one code will be very high even when many satellite codes are mixed in. This is the scheme used to create pseudo-
random sequences with all these properties for GPS and other GNSS satellites.

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