Figure 5 4.  FRPA Ground Plane

5.3  CRPA  Equipment

CRPAs have been shown to be the only effective means of protecting GPS receivers

against multiple wideband jammers.  A CRPA has two components: an Antenna Control

Unit (ACU) and an antenna array.  Current aircraft CRPA s typically have seven

antenna elements in the array with seven associated processing channels.  CRPAs

under development for missile may use only four or five elements.

The antenna array is composed of antenna elements which may be of any of the above

FRPA types.  However the vehicle environment significantly limits the choice.  In the

case of  aircraft the array has to be conformal and is therefore usually made up of

patch or crossed dipole antennas.  The antenna elements are spaced at approximately

half wavelength separation, at the shortest operational wavelength.  It is essential for

optimum operation of the CRPA that all the antenna elements in the array have

omnidirectional performance with constant gain characteristics over as large a sector

as possible.

The ACU controls the array s radiation pattern by adjusting the gains and phase from

each antenna.  First generation ACU employed analogue electronics with some digital

control. Newer equipment digitizes the receiver signal in a similar manner to that used

in a GPS receiver.  The ACU contains a series of amplifiers and gain control systems

for each channel, a set of weights that make up a beam former and a microprocessor

and associated electronics that contains the control algorithm and drive the weights in

the beamformer.  Each weight is a phase shifter with gain control. The phase shift was

initially performed by analogue components but it is now cost effective to employ digital

multiplier circuits.  The receiver signal is downconverted to near baseband and

sampled into inphase and quadrature components.  By adjusting the gain and sign of

each component a 360 degree range is achieved.

As the GPS signal is below the thermal noise in the transmission bandwidth, any

signal detected above the thermal noise level can be considered to be harmful to

GPS operation. Initially the array s radiation pattern is set to omnidirectional, by

5 4