Radio Signal

Radio Signal

GPS L1 CA-­signal

The GPS radio signal is modulated through Biphase shift key (BPSK) modulation, with the amplitude of the signal given by:

\[ s(t) = \sqrt{2P} C(t) D(t) \cos(2\pi f_{\text{ca}} t + \phi_0) \]

with:

• \(P\): the signal power.
• \(C(t)\): the spreading code (±1). It is also called the Pseudo Random Noise (PRN) and is unique to each satellite, publicly available.
• \(D(t)\): the navigation data (±1). It contains the satellite orbit and clock information.
• \(f_{\text{ca}}\): the carrier frequency. It is in the L-band between 1 and 2 GHz.
• \(t\): time.
• \(\phi_0\): phase offset.

Figure 1: GPS L1 CA-­signal (scale is not accurate)

Message Format

There are two main formats of navigation data message:

• Fixed frame: data always transmitted in same order with same repetition intervals
• Variable frame: a serie of fixed-length messages may be transmitted in any order