Module 2 : Global Positioning System
Lecture 6 : GPS Principles
GPS signal structure
Satellites have highly precise oscillators with a fundamental frequency of 10.23 MHz. Satellite signals basically consists of 3 componnts (Figure 6.2):
  1. Two micro wave L-band (also called Carrier) waves
    • L1 carrier: 1575.42 MHz
    • L2 carrier: 1227.60 MHz
  2. Ranging codes modulated on the carrier waves
    • C/A code, the clear/access or coarse/acquisition code modulated at 1.023 MHz, degraded code for civilian users, modulated on L1 only
    • P (Y) code, the private, protected, or precise code modulated at 10.23 MHz. It is modulated on both L1 and L2 carrier waves, for authorized military users
  3. Navigation message
    • Modulated on both L1 and L2 and contains satellite positions and constants
Two different frequencies are used to eliminate errors introduced by ionospheric refraction.


Figure 6.2 GPS signal structure

 
All frequencies derived from the fundamental frequency ( f = 10.23 MHz) with the following frequencies and wavelengths
L1                          = 154 f =  1575.42 MHz, wavelength of 19.0 cm,
L2                          = 120 f = 1227.60 MHz, wavelength of 24.4 cm
C/A code              = (1/10 f ) = 1.023 MHz (Mega bits per second Mbps), wavelength of 293.1 m, period of 1 millisecond
P code                  = 10.23 MHz, wavelength of 29.31 m, period of 266 days; 7 days/satellite
Navigation code = 50 bits per second (bps), data signal cycle length of 30 seconds
  • The structure of GPS carrier signals, codes and their combinations is quite complex. Since the carriers are pure sinusoids, they cannot be used easily for instantaneous positioning purposes and therefore two codes are modulated onto them: the C/A (coarse acquisition) code and P (precise) code. The codes (P and C/A) are nothing but binary sequence of information generated by a complicated algorithm.
  • For purposes of imposing these binary data onto the carriers, all of the codes are transferred from the 0 and 1 states to the -1 and 1 status respectively. Figure 6.3 shows how both code and carrier are combined in L1 signal (Sickle, 2001). The technique is called binary biphase modulation (alternatively binary phase shift keying BPSK). For more details on signal structure one can refer to Seeber (2003), Sickle (2001) and Parkinson et al. (1996).
 
 
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