Interference, Multipath, and Scintillation - Understanding GPS: Principles and Applications - page 279

6.2.3.2 Mitigating Gaussian and Spectrum Matching Wideband Interference
Encrypted military signals such as Y code or M code provide no intrinsic advantage
against enemy jamming owing to encryption. The most difficult military jamming
threats to mitigate are bandlimited Gaussian noise jammers whose bandwidths are
matched to the null-to-null signal spectrum of the target signal and jammers whose
spectrum characteristics exactly match the target signal. For these wideband
jammers, there are only two mitigation techniques available, receiver tracking
threshold enhancements and antenna directional gain control. The latter consists of
directional null steering toward the jammers and gain steering toward the SVs. The
most advanced controlled reception pattern antenna (CRPA) technology that com-
bines both techniques is STAP. It is possible to steer nulls toward the jammers with-
out assistance from the receiver, but to steer gain toward the SVs requires direction
cosines. This, in turn, requires an attitude and heading reference system for the
CRPA, preferably from an inertial measurement unit. The typical military CRPA
uses seven elements and can therefore steer up to six nulls toward enemy jammers. It
is possible to obtain 30 dB or more null steering with a high-performance CRPA,
but the null depth decreases as the number of jammers increase. With so few
antenna elements, STAP cannot provide much, if any, beam steering or gain toward
each SV, but it helps to minimize the adverse effects of the jammer null-steering pro-
cess. Obviously, if the jammer direction is colocated with the SV direction, that SV
will be lost. But it is much better to lose that SV than all of the SVs, as would be the
case with a FRPA. Also, the most advanced STAP will remove all narrowband jam-
ming energy by frequency excision techniques from each antenna element so as not
to lose wideband nulls to narrowband jammers.
There are also low-cost antenna null-steering techniques that depend on the
jammers to be ground located (or some a priori known location). One example is
the analog cancellation technique that senses the jamming energy from a “bow-tie”
antenna element that has a sector antenna gain coverage at and slightly above the
horizon and then subtracts this energy from the FRPA portion of the GPS antenna.
Another technique assumes and senses nonpolarized jammer energy and removes it
from the RHCP GPS signals. All of these low-cost antenna techniques require that
the enemy cooperate with the a priori restrictions of the antenna design.

6.3 Multipath

Improvements due to GNSS augmentations and GNSS modernization are reducing
many sources of error, leaving multipath and shadowing as significant and some-
times dominant contributors to error. This section discusses these sources of error,
their effects, and ways to mitigate their effects.
Multipath is the reception of reflected or diffracted replicas of the desired signal.
Since the path traveled by a reflection is always longer than the direct path, multipath
arrivals are delayed relative to the direct path. When the multipath delay is large (e.g.,
greater than twice the spreading code symbol period for a BPSK-R modulation), a
receiver can readily resolve the multipath. As long as the receiver tracks the direct
path (which always arrives earlier than any multipath), such resolvable multipaths
have little effect on performance. However, multipath reflections from nearby