Global Positioning System Reference
In-Depth Information
Carrier
wipeoff
Scale
factor
Carrier aiding
to code loop
Digital IF
Integrate
and
dump
I
I
PS
SIN
replica carrier
P
Carrier
loop
discriminator
Carrier
loop
filter
Numerical
controlled
oscillator
Integrate
and
dump
Q
Q
P
PS
Clock f
COS
replica
carrier
c
Carrier
NCO
bias
Prompt replica code
External
velocity
aiding
COS
map
SIN
map
Figure 5.8
Generic GPS receiver carrier tracking loop block diagram.
modulation on the baseband signal), or a frequency lock loop (FLL). The PLL and
the Costas loop are the most accurate, but they are more sensitive to dynamic stress
than the FLL. The PLL and Costas loop discriminators produce phase error esti-
mates at their outputs. The FLL discriminator produces a frequency error estimate.
Because of this, there is also a difference in the architecture of the loop filter,
described later.
There is a paradox that the GPS receiver designer must solve in the design of the
predetection integration time and the discriminator and loop filter functions of the
carrier tracking loop. To tolerate dynamic stress, the predetection integration time
should be short, the discriminator should be an FLL, and the carrier loop filter
bandwidth should be wide. However, for the carrier measurements to be accurate
(have low noise), the predetection integration time should be long, the discriminator
should be a PLL, and the carrier loop filter noise bandwidth should be narrow. In
practice, some compromise must be made to resolve this paradox. A well-designed
GPS receiver should close its carrier tracking loops with short predetection integra-
tion times, using an FLL and a wideband carrier loop filter. Assuming there is data
modulation on the carrier, it should then systematically transition into a Costas
PLL, gradually adjusting the predetection integration time equal to the period of the
data transitions while also gradually adjusting the carrier tracking loop band-
width as narrow as the maximum anticipated dynamics permits. Later, an
FLL-assisted-PLL carrier tracking loop will be described that automatically adjusts
to dynamic stress.
5.3.1 Phase Lock Loops
If there was no 50-Hz data modulation on the GPS signal, the carrier tracking loop
discriminator could use a pure PLL discriminator. For example, a P(Y) code receiver
could implement a pure PLL discriminator for use in the L2 carrier tracking mode if
the control segment turns off data modulation. Although this mode is specified as a
possibility, it is unlikely to be activated. This mode is specified in IS-GPS-200 [2]
because pure PLL operation enables an improved signal tracking threshold by up to
6 dB. All modernized GPS signals make provisions for dataless carrier tracking in
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