RECEIVING ANTENNAS FOR THE NAVSTAR GLOBAL POSITIONING SYSTEM

R.M.T. MILNE

ABSTRACT

Two new types of shaped beam antennas have been designed and developed forreception of signals from satellites operating in the NAVSTAR GlobalPositioning System. The antennas are required to provide a circularlypolarized 2wr steradian coverage. To achieve the required radiationpattern, arrays of scattering elements are used to shape the pattern ofa cavity-backed planar spiral.

This paper describes the antennas and their principles of operation, andpresents their measured radiation patterns.

INTRODUCTION

In the NAVSTAR Global Positioning System, a number of satellites travel in12 hr. orbits inclined at 630 to the equatorial plane. The satellitestransmit information to aircraft, land vehicles, and ships which use theinformation to locate their position. Receiving systems, which areintended to provide high positional accuracy (±7 meters), operate atboth 1227 MHz and 1575 MHz with instantaneous bandwidths of 3%. Systemsproviding a lower positional accuracy (±70 meters) have a single frequencyof operation of 1575 MHz.

Ideally the receiving antennas should be capable of receiving signals over2wr steradians i.e. above the local horizon.: Low ellipticity ratios arerequired to minimize the effect of spurious signals reflected anddiffracted by objects in close proximity to the antenna. Low sidelobelevels are required below the horizon to discriminate against multipathsignals and the effect of the antenna support structure.

Two antennas designated "Hybrid Spiral" and "Composite Array" have beendesigned and developed. In both cases, use is made of the scatteringcharacteristics of linear elements (Fig. 1) to enhance the gain of acavity-backed planar spiral in a direction normal to its axis. Theco-ordinate system used is shown in Fig. 2.

HYBRID SPIRAL ANTENNA

This antenna is shown in Fig. 3. The crossed dipole elements are 0.6X longand located 0.5\ above the planar spiral. The signal received by thespiral is the sum of the direct signal and the signal scattered by thedipole elements. These signals subtract on axis and the E components ofthe signals add in a direction normal to the axis. The 8 hadial monopolesaround the periphery of the spiral are 0.3X long. The monopoles preventreflections from the ground plane interfering with the E0 component of thereceived signal. It can be seen from Fig. 1 that the scattering

Department of Communications, Communications Research Centre,Shirley Bay, P.O. Box 11490, Station H, Ottawa, Ontario, CanadaK2H 8S2

132

characteristics of a dipole are insensitive to changes in its electricallength in the vicinity of 0.6\ (0.3\ in the case of a monopole). Ashaped pattern is achieved over a 25% bandwidth. Antenna patterns at1227 MHz and 1575 MHz, with the antenna mounted on a 6X diameter groundplane, are shown in Figs. 4 and 5. The measurements were made usinglinear polaSization, the polarization vector being rotated in discretesteps of 30 . The patterns are symmetrical about the vertical Z axisand almost independent of azimuth angle 4. The gain above the horizonis insensitive to ground plane size for diameters greater than 2.5X. AV.S.W.R. of less than 1.5 was measured between 1227 MHz and 1575 MHz.

The antenna, without the radial monopoles, has a height of 7.0 inches anda diameter of 6.0 inches. The 4 dielectric pillars supporting thecrossed dipoles can be replaced by a thin dielectric cylinder withoutany change in electrical performance.

COMPOSITE ARRAY ANTENNA

The composite array antenna components are shown in Fig. 6. The geometryof the assembled antenna is shown in Fig. 7. The antenna consists of acavity-backed planar spiral surrounded by an array of short verticaldipoles and an array of short horizontal dipoles. The dipole length isless than 0.25X. The arrays are supported by thin dielectric shellswhich play no part in the pattern shaping process. The components aremounted on a 2.5X diameter ground plane.

As the incident wavefront passes through the convex surface of an arraythere is little or no attenuation. It can be seen from Fig. 1 that thesignal scattered by a 0.25\ dipole is almost in phase with the incidentwavefront. The spiral antenna receives a direct signal and a signalreflected from the concave surface of the array. By adjusting the positionof the array relative to the spiral, the direct and reflected signals canbe made to add in phase. If the reflected signals from all of the dipoleelements are to add, the contour of the array in the vertical plane hasto take the form of parabola. To shape both the E0 and E components ofthe antenna pattern two parabolic arrays are required wits the orientationof the dipoles aligned to their respective polarization vectors. Botharrays have a common focal point and are circularly symmetric about acommon Z axis. The symmetry of the structures and the orthogonality ofthe dipoles permits each array to function almost independently of theother. The Ee and E4 array focal lengths are 0.75X and 1.0X respectively.

To illustrate the pattern shaping process the array pattern has beensuperimposed on a cardioid antenna pattern in Fig. 8. The direct andreflected signals are in phase at O=a, resulting in an increase in gainin the composite pattern. The direct signal lags the reflected signalfor angles of incidence less than cx and leads for angles greater than a.At some angle on either side of O=a, i.e. at O=ac, the direct and reflectedsignals will be in antiphase. At O=-a$ the relative difference in gainbetween the cardioid pattern antenna and the array is appreciableresulting in a small reduction in gain in the composite antenna pattern.At O=ct+f3 the gains are comparable resulting in an appreciable reductionin gain in the composite pattern and an improvement in sidelobe level.The pattern shaping process has a bandwidth of 5%.

The pattern of the composite array antenna, at 1575 MHz, is shown inFig. 9. The measurements were made using linear polarization, thepolarization vector being rotated in discrete steDs of 30 . The natterns

133

are symmetrical about the vertical Z axis, almost independent of azimuthangle 4 and are insensitive to ground plane size for diameters greaterthan 2.5X.

The prototype antenna has an angle ca=75 , a height of 12 inches, amaximum diameter of 15 inches and sits on a 18 inch diameter ground plane.

CONCLUS TON

The Hybrid Spiral antenna is a compact broadband antenna that provides ashaped antenna pattern with almost 2w steradian coverage at both theupper and lower NAVSTAR frequencies. The components are simple and easyto manufacture. It would appear to have wide application. The CompositeArray is a relatively narrow band antenna operating at the upper NAVSTARfrequency. It provides a shaped pattern over almost 2w steradians withlow ellipticity ratios and low sidelobe levels. It should be particularlysuitable for land vehicle and ship application.

ACKNOWLEDGEMENTS

This work was carried out for the Canadian Department of National Defence.

PHASE(deg.) 0 0MAPLITUDE(db.)

/ \\/45 / -

-135 \

_ /_-15

/ AMPLITUDE (NORMAL IZED AT 0.45 A,)

I

-180 ------ DHASE (RELATIVE TO THE INCIDENT PLANE WAVE)

-20

DIPOLE LENGTH(x) 0/3 0M 4 O Z5 0.6 0.7 0.8

FIG. 1 DIPOLE SCAD)ERING C 0.R0CTERISTICS

134

I (ZENITH)

FIG.2 CO-ORDINATE SYSTEM FIG.3 HYBRID SPIRAL ANTENNA

FIG.4 SHAPED PATTERN AT UPPER NAVSTAR FREQUENCY (1575MHz)

FIG.5 SHAPED PATTERN AT LOWER NAVSTAR FREQUENCY (1227MHz)

I -75

FIG.6 COMPOSITE ARRAY COMPONENTS

FIG.7 ANTENNA GEOMETRY FIG.8 PATTERN SHAPING PROCESS

136

FIG.9 SHAPED PATTERN AT UPPER NAVSTAR FREQUENCY (1575MHz)