Global Positioning System Reference
In-Depth Information
Table 5.1
Scale Factors for Carrier Aided Code
Carrier
Frequency (Hz)
Code
Rate (chips/s)
Scale
Factor
R 0 /10 for C/A
L1
=
154 R 0
1/1540
=
0.00064935
R 0 for P(Y)
L1
=
154 R 0
1/154
=
0.00649350
L2
=
120 R 0
R 0 for P(Y)
1/120
=
0.00833333
code and carrier loops must maintain track, nothing is lost in tracking performance
by using carrier aiding for an unaided GPS receiver, even though the carrier loop is
the weakest link.
5.2.5 External Aiding
As shown in Figure 5.3, external velocity aiding, say from an inertial measurement
unit (IMU), can be provided to the receiver channel in closed carrier loop operation.
The switch, shown in the unaided position, must be closed when external velocity
aiding is applied. At the instant that external aiding is injected, the loop filter state
must be set to the time bias rate if known; otherwise, it is zeroed. The external rate
aiding must be converted into LOS velocity aiding with respect to the GPS satellite.
The lever arm effects on the aiding must be computed with respect to the GPS
antenna phase center, which requires knowledge of the vehicle attitude and the loca-
tion of the antenna phase center with respect to the navigation center of the external
source of velocity aiding. For closed carrier loop operation, the aiding must be very
precise and have little or no latency or the tracking loop must be delay-compensated
for the latency. If open carrier loop aiding is implemented, less precise external
velocity aiding is required, but there are no meaningful delta range measurements
available. Also, it is not likely that the SV navigation message data can be demodu-
lated in this mode, so it is a short-term, weak signal hold-on strategy. In this
open-loop weak signal hold-on case, the output of the carrier loop filter is not com-
bined with the external velocity aiding to control the carrier NCO, but the
open-loop output of the filter can be used to provide a SNR computation. (External
aiding using IMU and other sensor measurements is discussed further in Chapter 9.)
5.3
Carrier Tracking Loops
Figure 5.8 presents a block diagram of a GPS receiver carrier tracking loop. The pro-
grammable designs of the carrier predetection integrators, the carrier loop
discriminators, and the carrier loop filters characterize the receiver carrier tracking
loop. These three functions determine the two most important performance charac-
teristics of the receiver carrier loop design: the carrier loop thermal noise error and
the maximum LOS dynamic stress threshold. Since the carrier tracking loop is
always the weak link in a stand-alone GPS receiver, its threshold characterizes the
unaided GPS receiver performance.
The carrier loop discriminator defines the type of tracking loop as a PLL, a
Costas PLL (which is a PLL-type discriminator that tolerates the presence of data
 
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