Pattern measurements of POD and RO antennas for COSMIC-2

Pa#ern Measurements of POD and RO Antennas for COSMIC-­‐2 George Purcell, Larry Young, Luis Zuniga, and Neil Chamberlain Jet Propulsion Laboratory, California Ins?tute of Technology Pasadena, California © 2014 California Ins?tute of Technology. Government sponsorship acknowledged. This document has been approved for unlimited release. Acknowledgements We recognize the contribu?ons of the following people. It wouldn’t have been possible without you!   JPL planar near-­‐field range: •  Jefferson Harrell   Construc?on of antenna moun?ng brackets for Goddard at JPL: •  Bryce Peters •  Joshua Ravich   Goddard Electromagne?c Anechoic Chamber: •  Steve Seufert •  Ken Hersey •  Victor Marrero Fontanez •  Shannon Rodriguez 2 Outline I.  Introduc?on: Configura?on of COSMIC-­‐2 spacecra] and antennas II.  Tes?ng issues A.  Test with or without spacecra] mockup 1.  Pros and cons 2.  Simula?ons and conclusion B.  Type of test chamber to use 1.  Pros and cons of near-­‐field chamber 2.  Decision to use far-­‐field chamber at Goddard III.  Tes?ng set-­‐up and procedures IV.  Test results A.  POD antennas B.  RO antennas V.  Discussion and conclusions: Performance in rela?on to requirements 3 Configura@on of SpacecraB and Antennas (Simplified)   POD and RO antennas fore and a]   POD antennas conven?onal design, Exelis model C126-­‐24-­‐1 mounted on lightweight 2-­‐ring choke ring, boresight ?lted 15° up from horizontal   RO antennas a JPL design, 12 two-­‐turn tapered helices arranged in a 3x4 array   3 1x4 subarrays side by side on a 40 cm x 60 cm moun?ng plate ( a single 3-­‐D printer product)   Three subarray outputs digitally phase-­‐shi]ed by the TriG receiver, genera?ng as many as six independent beams pointed to arbitrary azimuths   Mounted in a ver?cal plane with each subarray phased to op?mize the gain at the nominal earth limb at eleva?on –25° RO antenna volume RO antenna 2 Nadir deck objects Bus RO antenna 1 Star tracker (1 of 2) nadir
Ion velocimeter POD antenna (1 of 2) Solar panel RO antenna 4 Issue: Test with or without SpacecraB Mockup?   Reason to test RO antenna with mockup behind it: o  More realis?c result   Reasons not to test with mockup: o  Mockup expensive and ?me-­‐consuming to build o  Mockup heavy and hard to mount, may tax test fixture o  Mockup possibly not realis?c a]er all   Decision: Simulate test using High Frequency Structure Simulator (HFSS) o  Simulate antennas alone and antennas with spacecra] o  Pick a solar-­‐panel configura?on to maximize mul?path o  Judge whether the difference is small enough to ignore   Result: Effect of spacecra] is well below required accuracy. We tested the antennas without a mockup of the spacecra]. 5 Results of Simula@on at L1 Frequency: Azimuth Cuts at Eleva@on -­‐25° Within an azimuth range of ±60°, the perturba?ons between accommodated antenna pajerns and unaccommodated antenna pajerns are within 3dB for amplitude and 10 degrees for phase. 6 Antenna 1 Unaccommodated Sub-­‐Array Pa#erns: L1 Unaccommodated Ant Sub-array 1 Gain
20
elevation
10
10
20
0
30
-10
-20
40
-80
-60
-40
-20
0
20
azimuth
Unaccommodated Ant Sub-array 2 Gain
40
60
80
-30
20
elevation
10
10
20
0
30
-10
-20
40
-80
-60
-40
-20
0
20
azimuth
Unaccommodated Ant Sub-array 3 Gain
40
60
80
-30
20
elevation
10
10
20
0
30
-10
-20
40
-80
-60
-40
-20
0
azimuth
20
40
60
80
-30
7 Antenna 1 Accommodated Sub-­‐Array Pa#erns: L1 Accommodated Ant 1 Sub-array 1 Gain
20
elevation
10
10
20
0
30
-10
-20
40
-80
-60
-40
-20
0
20
azimuth
Accommodated Ant 1 Sub-array 2 Gain
40
60
80
-30
20
elevation
10
10
20
0
30
-10
-20
40
-80
-60
-40
-20
0
20
azimuth
Accommodated Ant 1 Sub-array 3 Gain
40
60
80
-30
20
elevation
10
10
20
0
30
-10
-20
40
-80
-60
-40
-20
0
azimuth
20
40
60
80
-30
8
Issue: Where to Test Antenna   Many types of antenna ranges available: far-­‐field, compact far-­‐field, near-­‐
field spherical, cylindrical, and planar.   Experimental measurements on planar near-­‐field range unsa?sfactory: o  Limited sampling at edges of reference antenna’s range of mo?on induces ripples in phase. o  Inexperience with near-­‐field ranges made it difficult to es?mate errors.   Decision: Use far-­‐field range, Electromagne?c Anechoic Chamber at Goddard Space Flight Center. o  We have extensive previous experience there and think we understand the error sources. o  Personnel are competent and coopera?ve. o  Test fixture could accommodate a moderate-­‐sized spacecra] mockup if necessary. o  Previous results have been excellent . 9 Test Set-­‐Up and Procedures   We measured 2 engineering models of the POD antennas, and 2 of the RO antennas.   Antenna under test is ajached to a moun?ng bracket that interfaces with the test fixture, providing access to the back of the antenna and posi?oning it as desired. o  POD bracket is a cylindrical cage; beam axis of antenna is horizontal. o  RO bracket is ?lted up 25° so that the center of the downward-­‐
pointed beam is horizontal. (See figure.)   Measurements are made at 1° intervals in azimuth from -­‐176° to +176° and 10° (for the wide-­‐beam POD antennas) or 5° (for the RO antennas) in head angle from 0° to 180°.   Measured RCP and LCP, processed only RCP.   Measured frequencies GPS L1, L2, and L5, GLONASS L1 and L2, and frequencies from 1100 to 1700 MHz at 50-­‐MHz intervals. Processed only the 5 GNSS frequencies. 10 COSMIC-­‐2 Precision Orbit Determina?on antenna mounted on test fixture at GSFC 11 COSMIC-­‐2 Radio Occulta?on Antenna Mounted on the Test Fixture at GSFC 12 Results: POD Antennas: Gain Gain%at%Seven%Head%Angles,%POD=1,%L1%Frequency%
10$
Head$angle$0°$
Head$angle$30°$
Head$angle$60°$
Head$angle$90°$
Head$angle$120°$
Head$angle$150°$
Head$angle$180°$
0$
Gain%(dBiC)%
!10$
!20$
!30$
!40$
!50$
!180$
!150$
!120$
!90$
!60$
!30$
0$
30$
60$
90$
120$
150$
180$
Azimuth%angle%(degrees)%
13 Results: POD Antennas: Gain, Con@nued Two antennas virtually iden?cal; parameters of antenna 1 below: 14 Results: POD Antennas: Phase Fi#ed for Beam Angles ≤75° Loca?ons of POD phase centers along boresight axis To calculate distance from antenna reference point (at the center of the rear surface of the choke ring) forward to the phase center, add 89.96 mm to the values of d above. 15 Results: RO Antennas: Gain 16 Results: RO Antennas: Gain, Con@nued 17 Results: RO Antennas: Gain, Con@nued 18 Results: RO Antennas: Gain, Con@nued 19 Results: RO Antennas: Phase   Phase center computed for each of the three subarrays.   Phase center fijed to data between azimuths ±45° and in a band 20° wide in eleva?on centered on the nominal eleva?on of the boresight.   Phase center solu?on calculated in the chamber frame and rotated into a frame aligned with the backing plate of the antenna.   Unlike the POD antennas, the loca?ons of the subarrays’ phase centers differ significantly between the two RO antennas. o  Differences in the boresight and horizontal direc?ons mostly <2 mm. o  Difference in the ver?cal direc?on reaches ~26 mm at the RL1 frequency. o  Phase centers move out along the boresight direc?on as frequency increases. 20 Results: RO Antennas: Phase, Con@nued y
z
x
Coordinate frame for phase centers Origin at center of back surface of moun?ng plate Units: millimeters 21 Discussion: Key POD Antenna Requirements Requirements and Performance L1 L2 L5 Requirement: Unknown phase vs. angle <30 mm <30 mm <30 mm Es?mated phase errors <3 mm <3 mm <3 mm Required gain at boresight Required gain at 40° boresight angle >+4.9 dBiC >+1.0 dBiC >+3.6 dBiC >+0.0 dBiC >+2.7 dBiC >+0.0 dBiC Measured at boresight Measured at 40° boresight angle
7.4 dBiC 3.6 dBiC 7.1 dBiC 3.4 dBiC 5.9 dBiC 3.5 dBiC 22 Discussion: Key RO Antenna Requirements (Apply Along Limb) L1 L2 Requirement: Unknown phase varia?on vs angle <1.9 mm/deg el. <4.7 mm/deg az. <1.9 mm/deg el. <4.7 mm/deg az. Es?mated phase varia?on errors from not including satellite mockup, varia?on among antennas, and range error 0.7 mm/deg el. 0.6 mm/deg az. (worst error from no mockup) 0.5 mm/deg el. 0.7 mm/deg az. (worst error from no mockup) Required gain at 0° azimuth Required gain at 55° azimuth >+15 dBiC @ 0° az. >+15 dBiC @ 0° az. >+8.1 dBiC @ 55° az. >+8.1 dBiC @ 55° az. Measured gain at –25° eleva?on 16.4 dBiC @ 0° az. 15.6 dBiC @ 0° az. 7.4 dBiC at 55° az. 10.9 dBiC @ 55° az. (10.0 dBiC at –55°) 23 Conclusions   The 2 RO and 2 POD antennas meet all requirements except L1 gain at +55° azimuth for the RO antenna.   These antennas will provide the best GNSS RO data the world has seen. 24