Adaptive Interference Nulling and Direction of Arrival Estimation in GPS Dual-Polarized Antenna Receiver - Adaptive Signal Processing in Wireless Communications - page 407

which approach to undertake depends on various parameters, including the number of
available satellites in the field of view (FOV), the nature of the jamming environment,
the antenna polarization characteristics, and the ability to utilize the satellite direction
information. The four approaches, placed in broad categories, are:

Null-steering arrays : These antenna arrays adjust their reception pattern to have
low gain in the direction of the interferences, without considering the GPS
satellite direction of arrival (DOA). This is the case for the power inversion
technique, also known as minimum variance [1]. The power inversion (PI)
approach for jammer nulling may unintentionally allow satellite signals to be
suppressed along with the jammer.
Beam-steering arrays : These arrays make full use of the GPS satellite DOA infor-
mation in the adaptive algorithm. They adjust the array reception pattern to
have high gain in the direction of the GPS satellites and to form nulls in the
direction of the interferers. This is the case of the minimum-variance dis-
tortionless response (MVDR) beamforming approach [2]. The MVDR-based
interference nulling technique may inadvertently allow jammers to be passed
on, along with the GPS signals, to the correlation loops of the receiver.
GPS signal-dependent arrays : These arrays utilize the GPS coarse/acquisition (C/A)
temporal and periodic structure, and maximize the cross-correlation between
the received signal and its one code-length delayed version. By this action, beams
are formed toward the GPS satellites in the field of view and nulls are placed
toward the jammers. This approach is known as a self-coherence antijamming
GPS receiver [3]. It has a shortcoming of treating period jammers, which have
the same fundamental period as the GPS code period, as desired signals.
Dual-polarized arrays : These arrays replace the circularly polarized GPS antennas
by dual-polarized patch antennas. This approach requires a dual-feed dual-
polarized antenna array, and can implement either beam-steering or nulling
arrays. Including the polarization diversity in GPS adaptive antenna arrays
increases the maximum number of possible jammer cancellations and gives
more flexibility in the choice of the optimality criteria that can be applied to
reduce jammer contaminations [4, 5].

14.1.2 Interference Direction Estimation

Interference nulling can be pursued jointly with the equally important task of estimating
the interference directions of arrival. The latter can be achieved by implementing high-
resolution subspace methods operating on the data covariance matrix, as is typically
the case in commercial and military radar and communication receivers. Particular to
navigation GPS receivers is the capability of providing simultaneous multiple beams,
each oriented toward one satellite. For this reason, the sets of adaptive steering weights
corresponding to the different satellites embed the interference spatial information and,
as such, can be directly operated on by the eigndecomposition methods.
This chapter deals with interference nulling and DOA estimation for GPS receivers.
Both tasks are performed prior to GPS signal dispreading, which is executed via the