Global Positioning System Reference
In-Depth Information
N bits
<<
Frequency selection
digital input
value = M
COS map
COS
N bits
N bits
N bits
Holding register
SIN map
SIN
Adder
Clock = f s
Numerical controlled oscillator (NCO)
fM
s
2 N
f s
2 N
N = Length of holding register
= Output frequency
N
2
= Count length
= Frequency resolution
Figure 5.5
Digital frequency synthesizer block diagram.
Overflow
Π
M
Π /2
0
1/f
s
−Π /2
−Π
t 0
t 3
t 6
t 9
t 12
t 15
(a)
t 0
t 6
t 12
(b)
t 3
t 9
t 15
(c)
Figure 5.6 Digital frequency synthesizer waveforms: (a) NCO phase state, (b) COS map output,
and (c) SIN map output.
5.2.4 Carrier Aiding of Code Loop
In Figure 5.3, the carrier loop filter output is adjusted by a scale factor and added to
the code loop filter output as aiding. This is called a carrier-aided code loop. The
scale factor is required because the Doppler effect on the signal is inversely propor-
tional to the wavelength of the signal. Therefore, for the same relative velocity
between the SV and the GPS receiver, the Doppler on the spreading code chip rate is
much smaller than the Doppler on the L-band carrier. (Keep in mind that even
 
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