RESEARCH ON GPS ANTENNAS AT MITRE

B. Rama Rao, M. N. Solomon, M. D. Rhines, L. J. Teig, R. J. Davis, E. N. Rosario The MITRE Corporation

Bedford, Massachusetts 01730

ABSTRACT

This paper presents the results of various investigations related to GPS adaptive nulling antennas that were conducted in 1997 under a research program sponsored by The MITRE Corporation. Under this program, MITRE has demonstrated the feasibility of using fundamental and higher order modes within a single microstrip patch radiator to create a relatively simple, low cost adaptive antenna capable of forming a null in the vicinity of the horizon where the jamming threat is the greatest. Resistivity tapered ground planes have been built and tested to reduce the high back lobes of GPS microstrip antennas caused by fields diffracted from the outer edges of the ground plane on which the antenna is mounted; a 10 to 20 dB reduction of these back lobes has been measured, thereby providing the antenna with greater immunity against jammers and external interference. Investigations have been performed to determine the performance limitations imposed by an airborne platform on jamming suppression of GPS adaptive antennas through scale model measurements and analysis. Antenna pattern measurements have been made on a scaled, right hand circularly polarized GPS antenna placed on a 1/8 scale model of a typical tactical aircraft. An extensive electromagnetic analysis has also been conducted to investigate the effects of platform interaction of a GPS microstrip antenna installed on a cruise missile. The accuracy of the simulation has been validated by comparison with measurements. These investigations will help to quantify the effect on the nulling performance of these airborne arrays by electromagnetic effects such as finite size and curvature of the platform, multipath, creeping waves and mutual coupling between the main and the auxiliary elements in the adaptive array.

Introduction

This paper describes the results of analysis and measurements on GPS antennas conducted during 1997 under a research program sponsored by The MITRE Corporation. The objective of this program was to explore new antenna concepts for improving the anti-

jam performance and reducing the cost of adaptive GPS antennas on airborne platforms, ground vehicles and other portable systems. Outlines of the various topics that were investigated under this project along with important results that were achieved are described below.

Miniature Multimode Adaptive Antenna

Hand-held GPS receivers have revolutionized navigation for the dismounted soldier. However, current military hand-held receivers are vulnerable to jamming (both intentional and unintentional). For hand-held applications, the GPS receiving antenna pattern is necessarily hemispherical, further increasing its vulnerability to jamming. Adaptive antennas and associated receive electronics do exist although they rely on antenna arrays which are physically large for practical hand-held use. Small arrays of two elements can be used to steer a single null in azimuth and elevation by combining their received signals with suitable amplitude and phase weighting.

MITRE investigated the feasibility of using fundamental and higher order modes within one microstrip patch radiator to create a compact antenna element capable of forming a null. The result is a relatively simple, miniature, low-cost multimode rectangular microstrip antenna.

In this antenna, the fundamental TMolo and the TM,, modes are phase shifted by 90 degrees to form a typical right hand circularly polarized boresight antenna pattern for receiving GPS signals. The higher order TMO2,, and the TM,, modes are also created simultaneously in the same patch to generate a monopole antenna type pattern with a null at boresight. The higher order modes are below cut-off because of the dimensions of the patch, but can be weakly excited by matching the large, higher-order mode impedance at the center of the patch. By properly weighting the amplitude and phase between the fundamental and higher order modes, a null can be formed in the desired direction in the vicinity of the horizon where the jamming threat is greatest. A miniature adaptive

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0-7803-4330-1/98/$10.00 01998 IEEE

nulling antenna of this type, when integrated with a low cost receiver, can be used for portable GPS applications.

Figures l(a) and l(b) show the antenna designed and fabricated to validate this concept. The antenna uses an air dielectric and has provisions for five probes used for different excitations, one pair for each fundamental mode excitation (along the two principal axes), and one in the center of the patch to excite the higher order mode. The center port was impedance matched using a coaxial double-stub tuner.

The fabricated phenomenology model antenna was designed to operate at the L l GPS frequency band. Unmatched, the fundamental mode excitation ports had a return loss of better than 10 dB. The unmatched higher-order mode excitation port has a very high input impedance (return loss of less than 1 dB). Using the double-stub tuner to match the port, a return loss of better than 10 dB was measured. Isolation between the ports exciting the fundamental modes and the matched higher order modes was measured to be greater than 20 dB.

Figure 2(a) is an example of a measured antenna pattern taken in MITRE’S near field antenna range. During this experiment, the antenna was excited in a linear polarization. The figure shows an elevation cut where 0 degrees (zenith) is normal to the patch while the horizon is located at 90 and 270 degrees. The dashed line shows the quiescent antenna pattern while the solid curve shows the formation of a spatial null of greater than 20 dB at the horizon. This prototype antenna is capable of steering a null in elevation by amplitude weighting of the two antenna modes and in azimuth by proper phase weighting of the modes.

Figure l(b). Close-up oFpatch showing location of five feed points.

....----. Quiescent -Nulled

0

150 \-io 180

Figure 2(a). Measured patterns of multimode adaptive antenna element.

Impedance and antenna pattern simulations were performed on this antenna using Microstripes [l] (transmission line matrix (TLM) method three dimensional electromagnetic simulator). Figure 2(b) is a comparison of calculated and measured linear polarized antenna gain. Note the good agreement above the ground plane:; simulations assumed an infinite ground plane which explains the differences in the side lobe at low elevation angles. To verify that

Figure 1 (a). Photograph of multimode air-dielectric phenomenology model.

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-Measured Gain (dBi) -. . . Simulated Gain

0

180

Figure 2(b). Comparison of calculated and measured linear polarized antenna pattern.

the results presented are applicable to a right hand circularly polarized (RCP) antenna, Figure 3 is a rear projection of the simulated RCP antenna gain pattern and combination of the higher order mode pattern. The simulated null depth is greater than 40 dB. RCP gain pattern measurements are planned in the near future.

Figure 3 . Rear projection of simulated RCP pattern and higher order pattern demonstrating null capability.

Resistivity Tapered Ground Planes for Reducing Back Lobes of GPS Antennas

Radiation patterns of GPS antennas mounted on small airborne and missile platforms are affected by

diffraction and reflection from the edges of the finite size ground plane or the aircraft fuselage. Edge diffraction from the finite dimensions of the ground plane causes high antenna back lobes and poor pattern cut off below the horizon making the antenna vulnerable to ground based jammers and interference. Ground plane edge diffraction effects can be reduced by the use of resistivity tapered ground planes whose surface resistance gradually increase from the center to the outer edge of the ground plane. Attachment of resistive cards to the edges of a scatterer is a well known technique for reducing the scattering cross section of the target by reducing the edge diffraction fields; similar techniques can also be applied for reducing antenna backlobes caused by fields diffracted from the outer edges of the ground plane.

To demonstrate this concept, antenna pattern measurements were taken on a right hand circularly polarized square GPS patch antenna placed at the center of a 26 inch square resistivity tapered ground plane. The ground plane was formed by using resistivity tapered thin films sputtered on a plastic substrate; surface resistivity increases from 0 (perfect conductor) at the center to about 2000 Ohms per square at the outer edge. Figure 4 shows the resistivity profile of the 26 inch square ground plane.

Resistivity Profile of 26” Square Ground Plane

0 2 4 6 8 1 0 1 2 1 4 Distance from Center (Inches)

Figure 4. Resistivity profile of “Resistivity Tapered Ground Plane” for GPS antennas.

Pattern measurements were also made for the same antenna placed at the center of a conventional aluminum ground plane (or a perfectly conducting ground plane with no resistivity taper) for comparison with the resistivity ground plane pattern measurements. The GPS microstrip antenna used in these measurements was a 1.172 inch by 1.18 1 inch “nearly square” patch antenna on a 0.100 inch thick Rogers

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Duroid 6010 LM substrate of dielectric constant 10.2; right hand circular polarization was obtained with a single, diagonally placed coaxial feed probe. The location of the probe relative to the center of the patch was optimized to obtain the best VSWR and CP axial ratio at the GPS L1 frequency of 1 .575 GHz.

Figure 5 shows the comparison of the sidelobe antenna patterns for the E theta component of the microstrip antenna when measured on the resistivity tapered ground plane and on a conventional metallic ground plane.

Comparison of GPS Microstrip Antenna on Resistivity Tapered .& Metallic Ground Planes

-150 -100 -50 0 50 100 150 Azimuth (Deg)

Figure 5. Comparison of measured antenna sidelobes for GPS microstrip antenna on resistivity tapered and metallic ground planes.

These results indicate that the resistivity tapered ground plane decreases the amplitude of the back lobes of the antenna by 10 to 20 dB relative to the metallic ground plane.

Comparison of GPS Antenna on Resistivity Tapered & Metallic Ground Planes

5 L ' " ! " " ! ' " ' ! " ' ! " " ! ' " ' 1

-150 -100 -50 0 50 100 150 Azimuth (Deg)

Figure 6. Comparison of measured main beam of GPS microstrip antenna on resistivity tapered and metallic ground planes.

shows the effects of resi:stivity tapering on the main beam of the antenna rela.tive to that for the aluminum ground plane. Note that the ripples in the antenna beam at broadside in the measurements with the aluminum ground plane are suppressed and the phase center of the antenna becomes more stable when the edge diffracted fields arc: reduced by resistivity tapering. More experimental and analytical results on this topic will be presented at the conference.

Scale Model Investigations on Tactical Aircraft

Investigations have been performed to determine the performance limitations imposed by an airborne platform on jamming suppression of GPS adaptive nulling antennas through measurements on scale models of typical aircraft and cruise missiles. Independent analysis arid verification of the measured results have also been performed through the use of numerical electromagnetic codes that use either GTD (Geometric Theory of Diffraction) or MOM (Method of Moments). These investigations will help to quantify the amplitude and phase perturbations at the elements of the nulling array by electromagnetic effects such as the finite size and curvature of the airborne platform, shadowing by aircraft structures, multipath, creeping waves, and mutual coupling effects between antenna elements in the array.

Antenna pattern measurements have been made on a scaled, right hand circularly polarized GPS microstrip patch antenna placed on a 118 scale model of a typical tactical aircraft. The plhysical location of the patch antenna on the scale model corresponds to its equivalent position on the full scale aircraft. Antenna patterns in the roll, azimuth and pitch planes were measured at a frequency of 12.6 GHz (8 times the GPS L1 frequency) in the Near Field Antenna Range at MITRE using a 6 foot high cylindrical scanner.

Figure 7 shows the 1/8 scale model aircraft installed inside the microwave anechoic chamber for antenna pattern measurements in the azimuth plane. Near field phase and amplitude information is collected by a calibrated probe that is robotically scanned parallel to the rotational axis of tlhe aircraft; for the azimuth plane measurements this rot,ationaI axis is perpendicular to the longitudinal axis o'f the aircraft. The calibrated probe and scanner can be seen in Figure 7 behind the aircraft. The scale model aircraft was installed on a motor driven pedestal and rotated through 360 degrees in 0.8 degree increments. For each angle, the calibrated probe collected near field amplitude and

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phase information as it was moved in half wavelength intervals over a distance of 6 feet parallel to the rotational axis. The data collected by the probe, which is in the near field aperture space at the probe scan plane, is then transformed to the angular space of the far field pattern of the antenna. For antenna measurements in the roll plane, the aircraft is oriented orthogonal to the position shown in Figure 7 , i.e., with the longitudinal axis of the aircraft vertical with respect to ground. The aircraft is again rotated through 360 degrees in 1 degree intervals. For each angle, the probe is scanned through a distance of 6 feet in half wavelength steps and the near field amplitude and phase information gathered by the probe is stored for generating the far field pattern.

Figure 7. Near field azimuth plane measurements of a GPS microstrip antenna on a 1/8 scale model tactical aircraft.

The GPS antenna was a square patch antenna, 0.2889 inch by 0.2889 inch, on a 0.020 inch thick substrate made from Rogers Duroid 5870 (dielectric constant = 2.33); right hand circular polarization was obtained by exciting the patch with two microcoax probes that were fed by a 90 degree hybrid. Each probe was offset by 0.037 inch from the center of the patch to obtain an input impedance close to 50 Ohms. Figure 8 shows the measured roll plane pattern for the microstrip antenna when mounted on the scale model aircraft.

1/8 Scale GPS Antenna on Fighter: Roll Plane

0

210 150 180

Figure 8. Measured RHCP roll plane pattern of scaled GPS microstrip patch antenna on the 118 scale model of tactical aircraft.

Figure 9 shows the measured RHCP azimuth plane pattern at an elevation angle of 0 degrees.

F16; RHCP; Az. Cut; 0 deg. Elev 0

0 -5

-10 -15

-25 tz -20

Ei -35

.- w

= 2

E 5 -30

& -40

-

E

180

Figure 9. Measured RHCP azimuth plane pattern of scaled GPS microstrip patch antenna on 1/8 scale model of tactical aircraft at an elevation angle of 0 degrees.

Notice the sharp nulls that occur near the nose and tail due to the interaction of the antenna with the aircraft fuselage. Antenna patterns for the scaled GPS antenna mounted on the aircraft are being simulated using the NEW-AIR code [4] developed by Ohio State University. The electromagnetic model of the tactical fighter was generated using a composite ellipsoid for the main body and flat plates for the wing and tail section. The microstrip antenna has been simulated by four magnetic lines sources excited in their proper

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relative phases. Results of these simulations and their comparison with measured results shown in Figures 8 and 9 will be presented at the conference.

Computer Simulation of Missile Effects

One of the main objectives of the study was to model the platform effects on a microstrip patch antenna appropriate for GPS signal reception when the antenna is installed on a cruise missile. PATCH, a method-of-moments (MOM) computer code for electromagnetic analysis [5,6], was used to model the combined antenndmissile configuration and to evaluate antenna performance as a function of installation location. This code, with MITRE additions, has been used for a number of years at M I T S for both scattering (radar cross section) and antenna radiation calculations. An out-of-core capability was added to the code to permit storage of the MOM matrix on fast striped disks rather than in memory, thus allowing a much larger problem to be solved. Model size for the out-of-core version of the code was limited to 21000 current elements at the time of this study. For comparison, the largest size problem that can be solved in memory is 5000 current elements, far smaller than needed for the missile at GPS frequencies. The simulation also required the use of a symmetry plane (discussed briefly later in this section) to construct an accurate missile model.

As a first step in the computer simulations, a PATCH model of the phenomenology antenna was generated and patterns calculated. The phenomenology antenna, fabricated for proof-of-concept testing, consists of a square plate, 3.386 inches by 3.386 inches, suspended above an 11.811-inch-by-11.811-inch ground plane. Patch and ground plane are separated by an air gap of 0.197 inch. In a fielded configuration, the air gap would be replaced by a dielectric to obtain a substantially smaller form factor. Figures 10 and 11 compare calculated and measured elevation plane patterns for the principal linear polarizations. (The phenomenology antenna had been configured only for linear polarization at the time of the measurements.) In these figures zenith (the overhead direction) corresponds to 0 degrees and the horizon to 90 and 270 degrees. Principal planes are orthogonal to the patch edges and to each other. The agreement is seen

to be good in theta component and quite good for the phi component. Cross-polarization results were not good and further investigation is needed to resolve this discrepancy.

-Calculated main polarization ........- Measured main polarization

0

150 --*lo 180

Figure 10. Comparisoin of calculated and measured E-theta plane elevation patterns for the isolated antenna.

-Calculated main polarization ......-.. Measured main polarization

0

1 5 c b ' ' 210 180

Figure 11. Comparison of calculated and measured E-phi plane elevation patterns for the isolated antenna.

An intermediate simulation consisting of the antenna installed on a small section of missile fuselage without wings and tail structures was then run to check the circular polarization operation of the antenna model. Results verified the required polarization performance.

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Figure 12. Simulation model of the antenna integrated with the missile.

The antenna model was finally integrated into a model of a cruise missile and radiation patterns calculated for three locations of the antenna on the missile. The entire model is assumed to be a perfect electric conductor (PEC). The missile model is not fully accurate since it includes no engine air intake or outlet. Antenna locations investigated were forward of the wings, about half-way between the wings and the nose; over the wings; and aft of the wings, about half-way between the wings and the tail structure.

Figure 12 shows the simulation model with the antenna on the missile in the forward location. Note that the antenna is mounted with its diagonal along the spine of the missile. This orientation is imposed by the use of symmetry to perform the calculations. Simulation of the antenna on a cruise missile at GPS frequencies could not be accomplished without taking advantage of the symmetry in the longitudinal plane. Without symmetry, the number of elements needed to model the missile would be twice as large as could be accommodated at the time of this study.

Figures 13 through 15 show calculated elevation plane patterns for the three antenna locations for both hands of circular polarization. In all plots the patterns are in an elevation (vertical) plane at an azimuth angle of -45 degrees. (Positive azimuth angles are to the right of the missile flight direction. Negative azimuth angles

are to the left.) The zenith direction is 0 degrees while 90 degrees is towards the front of the missile and 270 degrees is towards the rear of the missile.

Inspection of Figures 13 through 15 reveals that for all antenna locations, right circular polarization dominates at zenith while left circular polarization dominates around the horizon. Below the horizon the patterns are independent of polarization. Patterns for antennas located either forward or aft of the wings are quite similar in all respects. Antennas located over the wings are subject to increased blockage and the response to signals from beneath the missile is reduced, typically 10 dB compared to both forward and aft locations. Corresponding patterns for an azimuth angle of 45 degrees are similar with the cross polarization exhibiting more lobing in this plane.

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--RHCP ......... LHCP 0

--RHCP ......... LHCP

0

180

Figure 13. Calculated elevation plane patterns for the antenna on the missile at the forward location.

--RHCP ......... LHCP

0

21 0 -1 50 180

Figure 14. Calculated elevation plane patterns for the antenna on the missile at the wing location.

In the GPS application, the specific angles of interest are those near and slightly below the horizon where intentional jamming or unintentional interference signal sources are most likely to originate. Our results suggest that any of the antenna locations considered affords the same level of performance in this region. These results also suggest that the antenna beam should be broadened for coverage approaching the horizon from above. Techniques to provide increased coverage are being investigated.

210 ‘- 150 180

Figure 15. Calculated elevation plane patterns for the antenna on the missile al. the aft location.

References

[ 11 KCC Microstripes TLM Electromagnetic Simulator; distributed by Sonnet Software, Liverpool, NY.

[ 2 ] C. A. Balanis, Antenna Theory Analysis and Design, New York, NY: John Wiley & Sons, 1997.

[3] R. A. Sainati, CAD of Microstrip Antennas for Wireless Applications, Norwood, MA: Artech House, 1996.

[4] W. D. Burnside et ad, “Airborne Antenna Radiation Pattern Code User’s Manual,” Report Number 716199-4; Ohio State University, September 1985.

[5] W. A. Johnson, D, R. Wilton, and R. M. Sharpe, “Modeling Scattering from and Radiation by Arbitrary Shaped Objects with the Electric Field Integral Equation Triangular Surface Patch Code,” Electromagnetics; Vol. 10; 1990; pp. 41 - 63.

[6] W. A. Johnson, D. R. Wilton, and R. M. Sharpe, “The Patch Code IJsers’ Manual,” Sandia Report SAND87-2991, May 1988.

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