Interference, Multipath, and Scintillation - Understanding GPS: Principles and Applications - page 247

Table 6.2 (Continued)
International Table U.S. Table Notes

1,610-1,626.5 MHz
MOBILE-SATELLITE
(Earth-to-space)
AERONAUTICAL
RADIONAVIGATION
RADIO ASTRONOMY*
RADIODETERMINATION-
SATELLITE*

MOBILE-SATELLITE
(Earth-to-space)
AERONAUTICAL
RADIONAVIGATION
RADIO ASTRONOMY*
RADIODETERMINATION-
SATELLITE

Uplink frequencies for commercial satellite
communications services. Portions of the band
are protected for radio astronomy sensors.

*Only in some nations or portions of the band.

throughout the 960-1,215-MHz band. Fortunately, DME/TACAN and Link 16 are
pulsed.
It is inevitable that some out-of-band energy from the signals in adjacent bands
will at times fall within the range of frequencies processed by GNSS receivers. This
energy can originate from the spillover of energy from bands immediately above
or below one of the GNSS carrier frequencies, from harmonics, or from
intermodulation products . Harmonics are signals at integer multiples of the carrier
frequency of a transmitter that are caused by nonlinearities (e.g., saturation of an
amplifier that leads to clipping) upon transmission. Intermodulation products occur
when two or more signals at different frequencies are passed through a nonlinearity.
Even if interfering signals are out of the nominal band processed by a GNSS
receiver, strong RF signals can still deteriorate GNSS receiver performance (e.g., by
saturating the low-noise amplifiers used in the receiver front end). Although regula-
tions are in place within the United States and internationally to protect GNSS spec-
trum, there are occasionally instances of equipment malfunctions or equipment
misuse that can lead to intolerable levels of interference. Nonlinear effects (e.g.,
amplifier saturation) may accidentally occur in high-powered transmitters causing
lower power harmonics that become in-band RF interference to GNSS receivers.
The offending transmitter source has to be located and corrected before normal
GNSS operation in that vicinity can resume. Some regions of the world experience
more frequent problems with interference to GNSS than others. For example, in the
Mediterranean, there have been a number of reports of GPS L1 C/A code receivers
failing to operate properly because of strong in-band harmonics from television
transmitters in the region.

6.2.2 Effects of RF Interference on Receiver Performance
6.2.2.1 Front-End Considerations
Since the GNSS signals of all of the SVs in view of the antenna are below the thermal
noise level, the receiver front-end (analog) hardware requires considerable gain
(approximately 100 dB) and very little dynamic range if the design excludes the
effects of RF interference on the receiver hardware. It is essential that the receiver
front-end hardware have a dynamic range that will tolerate the front-end gain vari-
ations due to component tolerance and aging plus temperature variations. A pru-
dent commercial GNSS receiver front-end design should also accommodate the RF