NASAnyndn B. Johnson Space Center. ABSTRACT. In this paper, the magnitude and phase of the Global Positioning System (GPS) signals with multipath ...
SPACE STATION GPS ANTENNAS MULTIPATH ANALYSIS Shim U. Hwu, Ba P.Lu Lockheed fiigiirreeriiig & Sciences Company'
Robert J. Panneton, Brian A. Bourgeois* NASAnyndn B. Johnson Space Center ABSTRACT In this paper, the magnitude and phase of the Global PositioningSystem (GPS) signals with multipathinterference from Space Station stnrdures are computed using the Geometrical Theory of Diffraction (GTD). Obtained results indicate that reflections from and diffractions around the Space Station structuresin the vicinity of the GPS antennas can produce larger than 10 mm phase shift. The multipath effects are more evident in the phase shift pattems than in the gain pattems. This demonstrates that the phase variations are more sensitive to multipath than the gain variations. Results also show that significant phase shifts of greater than 10 mm, due to multipath from other objects, are possible in the unobscured antenna boresight regionwhere gain levels are high. In general, the phase shift caused by multipath is a fundion of the indirect-to-direct signal strength ratio and the path length difference betweenthe indirect and direct signals. To minimize multipath, the GPS antennas should be placedso that the blockagesand reflections from the Space Station major structural elements are minimized.
1. INTRODUCTION The Global Positioning System (GPS) sob;/stern on the International Space Station Alpha (ISSA) will be used for both position and attiitude determination. For attitude determination, the GPS receiver can determine a GPS satellite signal's arrival angle with respect to a baseline by measuring the phase difference betweenthe signals received by a pair of antennas at either end of the baseline. The signal's arrival angle can be related to d e phase difference by a simple relation if the received signals are undisturbed. The Space Station GPS antennas are surrounded by many large metallic structures. Multipath due to specular reflection and diffractionfrom structure in the vicinity of the receivingantennas has been identified as a significant error source for the GPS, especially in an interferometric mode for attitude determination. This study analyzesthe multipath characteristicsand induced phase errors for the GPS antenna in the Flight 12A ( F I 2 A ) ISSA environment. It is anticipatedthat the CPS antennas will encounter significant interference due to blockagesand strong reflected fields ffom the near-by ZTower and two major structure elements the solar panel and thermal radiator on the 2-Tower. The multipath effects were measured as the difference between the phase with and without the ISSA structures In place.
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II. COMPUTER SIMULATIONS Space Station GPS antenna pattemswith multipath interferencefrom Space Station Flight 12A structures are computed using the GeometricalTheory of Diffraction(GTD) modeling technique. It is assumedthat the electromagnetic properties of the Space Station structure are such that the surfaces are highly reflective at L-band. Thus, the Space Station structure elements are modeledas perfect electrical conductors.
'This work was supported by NASNJohnsonSpace Cenler under contract NAS9-19100. 0.78w.2719-5/95/s4.CiJ0 1995 IEEE
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An equivalent magnetic current model 121was used to simulate a proposed right hand circular polarized GPS antenna. The radiation pattem of this equivalent radiation source exhibits the same polarizationsense and similar characteristicsas the proposed GPS antenna in both gain and phase . All GPS antennas are modeled as transmitting sources in this analysis. By reciprocity, the transmitting and the receiving patterns of an antenna are identical. The Space Station coordinate system is shown in Fig. 1, the 0 is defined as the angle measured from the -Z axis (zenith) to the +Z axis (earth). The 4 is defined as the angle measured from the +X axis to +Y axis. The locations of the GPS antennas used in this (0.62, 0.88. -2.16) meters, Ant.2: (0.62. -0.88, -2.16) meters, Ant.3: analysis are: -1: (1.66,4.10, -1.29) meters, and Ant.4: (1.66, -4.10, -1.29) meters. All antenna locations are in meters and referencedto the origin of the Space Station coordinate system. The Flight 12A Configurationfor the ISSA, as shown in Fig. 2, is considered in this analysis because of the severe multipath environment caused by the close proximity of the 2-Tower and other major structwal elements. The solar panels and thermal radiators were oriented as follows: Case 1: Solar panel on the Z-1 truss horizontally positioned in X-Y plane: Case 2: Solar panel on the 2-1 truss vertically positionedin Y-2 plane; Station solar panel on the port side truss positionedin X-2 plane; Russian solar panels positioned in Y-2 plane; Station thermal radiator on P i truss segment oriented horizontally in X-Y plane. 111. RESULTS In this section, selected data for Ant. 1with the Case 1 Configurationare shown and discussed. A complete data packagewith all 4 antennas and the 2 configurations investigated in this study can be found in the report 131. The obtained results indicate that the GPS antennas will encounter significant interference due to blockages and strong reflected fields from the near-by 2-Tower and two major structure elements the solar panel and thermal radiator on the Z-Tower. The spherical gain Radiation DistributionPattern (RDP) plot, as shown in Figure 3.is for Ant.1 with multipath effeds from the Case 1 Flight 12A Station structures. The solar panels on the 2-Tower were oriented horizontally in the X-Y plane. The large white region around the center of the plot is caused by the blockages of the Z-Tower and two major structure elements the solar panel and thermal radiator on the Z-Tower. The gain levels drop below -15 d6 here.
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Figure 4 shows the phase shifl RDP for Ant.1 due to multipath effects from the Case 1 Flight 12A Station structures. This phase shifl RDP is the difference of the phase RDPs with and without Station structurespresent. The Z-Tower and the two major structure elements solar panel and thermal radiator on the &Tower can be easily identified by dark regions around the center of the plol. Also. it is interestingto note the dark spots around the boresight region which are due to the reflections off the Z-Tower and the thermal radiators on the 2-Tower.
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Table 1 summarizes the percent coverage of phase shifl RDPs for all 4 GPS antennas and the 2 configurations investigatedin this study. As indicated in the previous plots, the GPS RDP covem the GPS satellite field-of-view given by O o ~ s l O O o ,O 0 ~ 6 O 0 .As expected, poor percent coverages were obtained for all GPS antennas due to severe reflection and blockage from the near-by 2-Tower and associated structures. The
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average percent coverages of phase shifl RDPs in the GPS satellite field-of-view for the F12A Space Station Configurationare 24% for phase shift greater than 20 mm, 28% for phase shifl greater than 15 mm. 36% for phase shifl greater than 10 mm, 50% for phase shift greater than 5 mm,and 61% for phase shift greater than 3 mm. The Flight 12A Configurationis expected to be the worst configurationfor GPS antennas in terms of multipath since the Z-Tower is located so close to the antennas. The following is a summary of the observations supported by the data presented in this section: 1. Structure blockages cause more than 10 dB gain decrease. 2. Structure blockages cause more than 10 mm (18.94 phase shifl. 3. Reflections from and diffractions around the Space Station major structures cause more than 5 mm (9.454 phase shift. 4. The average percent coverage of phase shift RDPs in the GPS satellite field-of-view due to multipath effeds for the Flight 1% ISSA is about 50% for phase shifl greater than 5 mm (9.454.
REFERENCES
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[I] R. J. Marhefka, and J. W. Silvestro, "Near Zone Basic Scattering Code User's Manualwith Space Station Applications," NASA Contmdor Report 181944, ElectroScienceLaboratory, Ohio State University. December 1989.
[2]W. F. Richards, Y. T. Lo, and D. D.Harrison, "An ImprovedTheory for Microstrip Antennas and Applications," IEEE Trans. on Antennas and Propagation, pp. 38-46, January 1981. [3]S. U. Hwu. B. P. Lu, and R. J. Panneton, 'GPS Antennas Multipath Analysis For Flight 12% Space Station," NASNJSCTechnical Report EV3-9414357, Johnson Space Center, NASA, November 1994.
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Fig.1. Space Station Coordinate System. Fig.2. Space Station Flight 12A Configuration.
GPS Antenna 1
GPS Antenna 1 Phase Shift
Gain 0 20
40
00 80 100
Fig.3. Gain Radiation Distribution Pattern.
Fig.4. Phase Shifl Radiation Distribution Pattern.
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