Integration of GPS with Other Sensors and Network Assistance - Understanding GPS: Principles and Applications - page 489

Another technique that is used is a function of the GPS receiver. Most bad mea-
surements are caused by disruptions in the GPS signal. These disruptions result in
changes in the power levels of the GPS signal being received. By calculating the
received power level during each GPS sub dwell (1 ms) and comparing these levels
to set thresholds, the receiver can recognize significant fluctuations. When thresh-
olds are exceeded, the measurement is declared bad and not utilized by the filter.

9.2.6 Integration with CRPA
This section discusses the integration of a CRPA antenna, originally discussed in
Chapter 6, with a GPS/inertial system. The gain pattern of the CRPA antenna as
compared to a standard FRPA when a source of interference is present is illustrated
in Figure 9.15.
The CRPA antenna minimizes gain toward the interference source adaptively
by utilizing an array of N antenna elements, as shown in Figure 9.16. The signal
from each element is weighted and combined in such a fashion that signals coming
from the direction of jamming source are greatly attenuated (or nulled, using the
terminology prevalent in adaptive antenna literature) to minimize the effects of the
interference on the GPS receiver. Another aspect of this antenna is that the gain
toward the GPS satellites can be increased, improving the GPS signal strength. This
technique is generally referred to as beam steering . Null steering and beam steering
antennas have been successfully used to mitigate the effects of interference and
multipath for GPS applications for a number of years. Null steering antennas are
currently used on a number of military platforms. Drawbacks of the use of CRPAs
include high cost (relative to FRPAs), and weight/size issues. A number of programs
have been trying to address these concerns by reducing the size of the antenna from
14 inches to 5.5 inches [20-22]. The majority of current CRPA applications imple-
ment null steering without beam steering. The reason is that beam steering requires
knowledge of the platform attitude, which is not always available or easily accessed.
A diagram of the antenna electronics (AE), which in airborne installations is
usually housed within the aircraft rather than in the antenna, is shown in Figure
9.17. The electronics consists of circuitry to control the weighing of the signal from

SV

Noise

FRPA
CRPA

Figure 9.15 Antenna pattern of a FRPA and CRPA antenna.