CHAPTER 5:  ANTENNA SUBSYSTEMS

5.1  INTRODUCTION

GPS users have different requirements for GPS system performance which demand a

variety of antennas and antenna subsystems.  There are three basic types of GPS

antennas, a passive Fixed Radiation Pattern Antenna (FRPA), a FRPA with an

integrated preamplifier, and a Controlled Radiation Pattern Antenna (CRPA).  The

requirement to drive a long cable run, with its associated signal loss between the

antenna and the GPS receiver  has resulted in  a FRPA with an integrated amplifier.  A

CRPA is required to reduce  the effects of RF interference which would otherwise jam

the receiver s operation.

5.2  FRPA

5.2.1  General Characteristics

A FRPA has a fixed antenna radiation pattern which is only affected by the size and

shape of  the ground plane on which it is installed.  As GPS antennas are typically

narrow band the radiation pattern does not change over either the L1 or L2 bandwidth

although due to the difference between the L1 and L2 wavelength there are significant

differences in the radiation at the L1 and L2 frequencies.

Typical specifications for FRPAs include parameters for operating frequencies,

impedance, Voltage Standing Wave Ratio (VSWR), radiation pattern, polarization, axial

ratio and gain. These specifications impact receiver performance.  The size, shape and

weight of the FRPAs will vary with the application.  A FRPA for an aircraft installation

has a different form than a FRPA for a hand held receiver.  A number of FRPAs are

discussed below.  FRPAs are generally non  repairable units which require no

adjustment over their lifetime.  Passive FRPAs require no power.  All FRPAs can be

fitted with an external low noise amplifier should this be needed to overcome losses

introduced by a long cable length.  The amplifier will probably require a low power DC

voltage.

An important parameter when selecting a GPS antenna is the gain.  Gain is defined

with respect to an isotropic radiator for circular polarization,  expressed as dBic, and

the sector of the sphere surrounding the antenna over which the gain can be

maintained, expressed as the angle from the antenna boresight.  (The boresight is the

central axis of the antenna usually the direction of maximum gain).

To receive the signals from GPS satellites, which may be at any angle in the upper

hemisphere, the gain must not drop below  5  dBic.  In the case of an aircraft there

is a significant problem of maintaining sufficient gain towards the satellites as the

aircraft maneuvers through high angles of pitch and roll.  Typically an aircraft s

GPS antenna gain falls to  15 dBic below the azimuth plane, although a worst case

gain of  20 dBic can be assumed.

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