Interference, Multipath, and Scintillation - Understanding GPS: Principles and Applications - page 267

( (

) ) JS + S r

dB dB

J r = 10

10

(6.27)

Using the minimum specified received signal power level for C/A code at normal
power, ( S r ) dB =− 158.5 dBW and the jamming performance for the C/A code receiver

in the previous section where ( C S / N 0 ) eff,dB

39.9 dB for a
BLWN null-to-null white noise jammer, the incident jammer power is determined
from (6.27) as follows:

= 28 dB-Hz and ( J / S ) dB =

( ) 39 9 . +− 158 5 .

− 12

J r = 10 = 138 . × 10 W

10

This demonstrates that for the FRPA design assumed in the previous section,
less than 1.5 pW of incident RF interference power is required to disable a C/A code
receiver with a moderate ( J / S ) dB performance of 39.9 dB.
Table 6.6 depicts the disabling incident power (along with the tolerable J and
J / S performance for easy comparison). Note that even though the J / S performance
of M code is about the same as that for P(Y) code, the disabling incident power and
tolerable J are significantly larger for M code. Table 6.6 also shows clearly how
much more robust P(Y) code and M code are than C/A code in the presence of jam-
ming. Disabling incident power and tolerable J both provide insight into receiver
antijam performance when there are different received signal powers and different
values of R c and Q involved. For example, one might think that M code would have
a 3-dB lower J / S performance than P(Y) code since its spreading code rate is half
that of P(Y) code. Since the Q for M code is more than double that of P(Y) code and
its power is 3.5 dB stronger, the M code J / S performance is higher, as shown in
brackets in Table 6.6. In fact, Table 6.6 shows that M code is more robust in the
presence of jamming than P(Y) code for all three jammer types.

Computing Range to RF Interference
Usually, the receiver operating range from the source of the RF interference is
desired given the effective isotropic radiated power (EIRP) of the interference
source. (Refer to Appendix C for greater insight into propagation loss.)
The formula for the link budget for the transmitted jammer power is given by:

( ) ( ) ( ( )

)

()

EIRP = J − G + L + L dBW (6.28)

j r j p f

dB

dB dB dB dB

Table 6.6 Disabling Incident Power, Tolerable J, and J / S for 28-dB-Hz Carrier Tracking

Threshold

Incident Power in pW
(Tolerable J dB (dBW)) [(J/S) dB (dB)]
L1 C/A Code

Jammer Type

L1 P(Y) Code M Code (Normal)

Wideband
null to null

1.38 (-118.6) [39.9] 6.61 (-111.8)[49.7] 18.62 (-107.3) [50.7]

Wideband matched
spectrum

0.93 (-120.3) [38.2] 4.47 (-113.5) [48.0] 14.13 (-108.5) [49.5]

Narrowband at
spectral peak(s)

0.62 (-122.1) [36.4] 2.95 (-115.3) [46.2] 8.13 (-110.9) [47.1]