Global Positioning System Reference
In-Depth Information
when the narrowband interference is placed halfway between the spectral peak at
band center and the first spectral null at 10.23 MHz.
6.2.3 Interference Mitigation
The three primary means of interference mitigation are: (1) enhancements of the
receiver tracking threshold, especially by means of external velocity aiding from an
IMU, (2) frequency excision techniques to remove narrowband energy by hardware
means (transversal filters) or by signal processing means (FFT techniques), usually at
the digital IF stage, and (3) antenna null-steering toward jammers or gain steering
toward the SVs or preferably both. In addition to these controlled design features, a
number of factors help to reduce the RF interference effects. The RF interference can
only have the full effect of this analysis on the receiver if it is in the LOS of the
receiver antenna and unobstructed. For commercial aviation applications, the RF
interference sources will typically be at ground level, while the receiver antenna will
be elevated during en route navigation. This increases the LOS range, but because
the source of the interference will, in general, be from below the aircraft's horizon,
the body of the aircraft will help to block the interference. Also, the gain pattern of
the antenna rolls off significantly below the aircraft horizon, unless pseudolites are
being used to support local differential operation. In this case, the receiver antenna
used with gain toward the ground will be vulnerable to RF interference from the
ground. However, as the aircraft approaches for landing or for ground-based opera-
tion of receivers in general, the RF interference signals can be attenuated due to
Earth curvature, foliage, buildings, and so on.
6.2.3.1 Mitigating Narrowband and Pulse RF Interference
Narrowband interference can be mitigated by spectrum excision techniques. These
techniques essentially suppress the narrowband energy down to the thermal noise
level, but that also suppresses the signal in those frequency regions. This loss of
energy will have a small effect on the receiver's tracking performance if only a small
percentage of the spectrum is suppressed. C/A code receivers can still experience
acquisition problems with CW interference if suppressed to the thermal noise level
due to the strong spectral lines of the C/A code (see Section 6.2.2.6).
Pulsed interference can be easily mitigated by instant recovery analog design
techniques, such as front-end clipping, front-end saturation prevention, and fast
attack, fast recovery AGC design. Pulse blanking , or the zeroing of the received sig-
nal when strong pulsed interference is detected, is a particularly effective mitigation
technique [11]. The receiver cannot correlate with the signals during these bursts,
but the duty cycle of most burst jammers is usually so low that correlations take
place most of the time (unless saturation is permitted to take place in the front end).
Thus, a well-designed receiver front end renders the overall receiver immune to most
burst jammers [e.g., a pulse jammer with 50% duty cycle blanks out half the
received signal power, but that will degrade the ( C S / N 0 ) dB by only 3 dB]. It is rela-
tively inexpensive in terms of cost or size, weight, and power to build in pulse-jam-
ming mitigation features in a receiver, but most commercial receivers do not have
such protection.
 
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