Global Positioning System Reference
In-Depth Information
Next, we solve for the range, d , to the jammer to attenuate the 4-W transmitted
jamming signal to the threshold level at the receiver input. This corresponds to the
range from the jammer for the receiver to reach its tolerable J level:
(
)
L p
dB
λ
10
4 000
20
(
)
j
d
=
=
83
.
km
45
.
nmi
,
π
Table 6.7 illustrates this distance computation for all three jammer types and L1
C/A code, L1 P(Y) code, and L1 M code receivers using the same assumptions for
EIRP j and the receiver antenna gain toward the jammer, G j . Note that the
narrowband jammer is the most effective (lowest Q ) for a given power, but it is also
the easiest to mitigate.
Using the same equations and assumptions in the previous example, Figure
6.6(a-c) are plots of the range to the jammer as a function of the EIRP of a BLWN
null-to-null(s) jammer with ( J / S ) dB as a running parameter for L1 C/A code, L1 P(Y)
code, and M code receivers, respectively. Using the ( C S / N 0 ) eff obtained from the rule
of thumb for carrier tracking threshold analysis or by using Monte Carlo simula-
tions, the actual J / S performance of a receiver can be determined from Figure 6.4.
Using the actual J / S performance, the LOS range to the jammer can be determined
from Figure 6.6 for a wideband jammer, given the EIRP of the jammer. Alterna-
tively, for a given range to the jammer, the tolerable EIRP can be determined.
6.2.2.6 Vulnerability of C/A Code to CW Interference
The GPS C/A code is a Gold code with a short 1-ms period (i.e., the PRN sequence
repeats every 1 ms). Therefore, the C/A code (neglecting the navigation data) has a
line spectrum with lines that are 1 kHz apart. Although it is typical for each line in
the C/A code power spectrum to be down 24 dB or more with respect to the total
power, there are some lines in every C/A code that are stronger. The C/A code line
spectrum characteristic is inferior to a maximum-length PRN sequence with the
same number of shift register bits [8]. As a result, a CW jammer can mix with a
strong C/A code line and leak through the correlator. Table 6.8 summarizes the
worst line frequency and the worst line (strongest) amplitude for every C/A code [9].
These phenomena cause more of a problem during C/A code search and acquisition
modes than in tracking modes. If the receiver has a CW jammer detector such as the
Table 6.7 Tolerable J Distance to 4-W Jammer, Assuming Free-Space
Propagation
Jammer Type
Distance, km (nmi)
L1 C/A Code
L1 P(Y) Code
M Code (Normal)
Wideband
null to null
18.2 (10.0)
8.3 (4.5)
4.9 (2.7)
Wideband matched
spectrum
22.1 (12.1)
10.1 (5.5)
5.7 (3.1)
Narrowband at
spectral peak(s)
27.2 (14.4)
12.5 (6.8)
7.4 (4.1)
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