The Development of an Anechoic GPS Test Facility (2002)

GPS provides position, velocity and time information
measurements with unparalleled accuracy to an ever
increasing number of world-wide users in many civil
and military applications. However, due to the low
powered intensity of GPS signals, it is extremely
vulnerable to interference or jamming.
Controlled Radiation Pattern Antenna (CRPA)
technologies have been under development for a
number of years. These multi-element phased
antenna arrays allow the gain pattern of the user’s
receive antenna to be electronically manipulated
such that interference source(s) are placed within a
‘null’, whilst at the same time allowing the wanted
GPS signals into the receiver – increasing the operational capabilities of the GPS user when
functioning in such a hostile electronic environment.
The testing of such an antenna system could be
carried out in real-world trials involving, for example,
a number of static interference sources directed
towards a dynamic airborne target CRPA system.
However, these tests are not highly repeatable. The
GPS constellation is constantly changing and flight
paths would undoubtedly differ from sortie to sortie.
Also, in a world where GPS is used in such a wide
range of activities, from the automatic location of
emergency vehicles to mountaineering, trials
involving GPS jamming have to be carefully planned
and RF power constrained such that the minimum
amount of disruption is caused. Interference
sources from outside the trials control can also
cause concern over the accuracy of the results
obtained.
The anechoic test facility under development by
QinetiQ Telematic Solutions provides a repeatable
and accurate setting in which the performance of
GPS receiver and antenna technologies can be
evaluated. The spherical shaped chamber, certified
up to 18GHz, is based within a hardened aircraft
shelter at Boscombe Down, Wiltshire, UK and
provides a high integrity, low noise environment with
exceptionally low signal reflectivity performance,
beyond that of a conventional anechoic chamber.
The test facility’s noteworthy features include:
• A GPS signal simulator, comprising 12 discrete
antenna outputs which is used to feed a
constellation of source antennae, placed around
the periphery of the chamber to replicate a real
GPS constellation.
• Jamming source antennae, which are moveable
anywhere within the chamber.
• A variety of RF interference sources (Broad /
narrow band, pseudo random noise, frequency
swept etc.)
• A dynamic platform to facilitate real time attitude
control of the receive antenna in sympathy with
the simulated host vehicle trajectory.
• A STANAG 4572 compliant interface is available
which injects real time inertial aiding data into
the unit under test to enable testing of integrated
GPS and Inertial navigation systems.
It would be true to say that this facility is stretching
the limits of anechoic chamber capabilities. This
paper explores the difficulties faced when designing
such a test facility, from choosing the correct
antenna types to ensuring that the reflectivity at GPS
frequencies is low enough to avoid reflections being
seen by the unit under test as another jamming
source. Also discussed are some of the initial
results obtained to date and suggestions of other
uses for the chamber.