GPS C/A Code Signal Structure - Fundamentals of Global Positioning System Receivers: A Software Approach - page 69

data carried by the signals will be presented. The applications of the data will be
briefly discussed.

5.2 TRANSMITTING FREQUENCY ( 1-4 )

The GPS signal contains two frequency components: link 1 (L1) and link 2 (L2).
The center frequency of L1 is at 1575.42 MHz and L2 is at 1227.6 MHz. These
frequencies are coherent with a 10.23 MHz clock. These two frequencies can be
related to the clock frequency as

L1 = 1575 . 42 MHz = 154 × 10 . 23 MHz

L2 = 1227 . 6MHz = 120 × 10 . 23 MHz

These frequencies are very accurate as their reference is an atomic frequency
standard. When the clock frequency is generated, it is slightly lower than
10.23 MHz

to take the relativistic effect into consideration. The reference

by ( 3 )

− 4 . 567 × 10 − 3

frequency is off Hz, which corresponds to a fraction

− 4 . 4647 × 10 − 10 − 4 . 567 × 10 − 3 / 10 . 23 × 10 6 ) .

of ( Therefore, the reference

frequency used by the satellite is 10.229999995433 MHz ( 10 . 23 × 10 6 −

10 − 3 ) rather than 10.23 MHz. When a GPS receiver receives the signals,
they are at the desired frequencies. However, the satellite and receiver motions
can produce a Doppler effect as discussed in Section 3.5. The Doppler frequency
shift produced by the satellite motion at L1 frequency is approximately

4 . 567 ×

5kHz.
The signal structure of the satellite may be modified in the future. However,
at the present time, the L1 frequency contains the C/A and P(Y) signals, while
the L2 frequency contains only the P(Y) signal. The C/A and P(Y) signals in the
L1 frequency are in quadrant phase of each other and they can be written as:

±

S L 1 = A p P(t)D(t) cos ( 2 πf 1 t + φ) + A c C(t)D(t) sin ( 2 πf 1 t + φ) ( 5 . 1 )

where S L 1 is the signal at L1 frequency, A p is the amplitude of the P code, P(t) =

1 represents the data code, f 1 is the
L1 frequency, φ is the initial phase, A c is the amplitude of the C/A code, C(t) =± 1
represents the phase of the C/A code. These terms will be further discussed in the
following sections. In this equation the P code is used instead of the P(Y) code.
The P(Y), C/A, and the carrier frequencies are all phase locked together.
The minimum power levels of the signals must fulfill the values listed in
Table 5.1 at the receiver. These power levels are very weak and the spectrum
is spread, therefore they cannot be directly observed from a spectrum analyzer.
Even when the signal is amplified to a reasonable power level, the spectrum of
the C/A code cannot be observed because the noise is stronger than the signal.
When a high gain antenna is used to track a certain satellite, its signal structure
can be displayed clearly on a scope or a spectrum analyzer.
As discussed in Section 3.3, the received power levels at various points on
the earth are different. The maximum difference is about 2.1 dB between a point
just under the satellite and a point tangential to the surface of the earth. In order

± 1 represents the phase of the P code, D(t) =±