Interference, Multipath, and Scintillation - Understanding GPS: Principles and Applications - page 245

Table 6.1 Types of RF Interference and Potential Sources
Class—Type

Potential Sources

Wideband—band-limited Gaussian Intentional matched bandwidth noise jammers

Wideband—phase/frequency modulation Television transmitters' harmonics or near-band microwave link

transmitters overcoming the front end filter of a GNSS receiver

Wideband—matched spectrum Intentional matched-spectrum jammers, spoofers, or nearby

pseudolites

Wideband—pulse Any type of burst transmitters such as radar or ultrawideband

(UWB)

Narrowband—phase/frequency modulation Intentional chirp jammers or harmonics from an amplitude

modulation (AM) radio station, citizens band (CB) radio, or
amateur radio transmitter

Narrowband—swept continuous wave Intentional swept CW jammers or frequency modulation (FM)

stations transmitters' harmonics

Narrowband—continuous wave Intentional CW jammers or near-band unmodulated transmit-

ter's carriers

potential for spoofing military GPS receivers. The encrypted M code is even more
secure.

6.2.1.2 Unintentional Interference
Unintentional RF interference can be expected at low levels for a GNSS receiver
operating practically anywhere in the world. There are a large number of systems
that we depend on in daily life that rely on the transmission of RF energy within L-
band. Table 6.2 shows abridged versions of U.S. and international tables of alloca-
tions for frequencies near those used by the GPS signals. The services shown in all
capital letters are primary , and the ones shown with initial capitals are secondary .
Secondary services are permitted to operate in their designated bands but are not
generally provided protection from the primary services and further are not allowed
to provide harmful levels of interference to the primary services.
GPS L1 is in a band that is designated for use only by other satellite navigation
signals in most regions of the world. The GPS L2 and L5 signals are in less pris-
tine spectrum. L2 is in the 1,215-1,240-MHz band, which has a coprimary
radiolocation allocation worldwide. Radiolocation services that operate in the band
include a large number of radars that are used for air traffic control, military sur-
veillance, and drug interdiction. Although some of these radars operate with very
high transmit power (kilowatts to megawatts), fortunately they are pulsed systems,
and, as will be discussed in Section 6.2.2, GPS receivers can be very robust against
pulsed interference due to the clipping that occurs in their A/D converters if their
front-end design does not saturate and their automatic gain control has a very fast
attack and recovery rate. Additionally, a number of countries permit fixed and
mobile services to operate in the 1,215-1,240-MHz band.
L5 is located within a portion of the 960-1,215-MHz band that is used world-
wide for electronic aids to air navigation. DME and TACAN ground beacons trans-
mit at power levels up to 10 kW on frequencies that fall within a GPS L5 receiver
passband. Some nations also permit the use of Link 16, a tactical military communi-
cations system with radios that nominally transmit 200W over 51 frequencies