A Large Aperture 650 GHz Near-Field Measurement System for the EarthObserving System Microwave Limb Sounder

Dan SlaterNearfield Systems Inc.,1330 E. 223rd Bldg. 524

Carson, CA 90745

Paul Stek, Rick Cofield, Robert Dengler,Jack Hardy, Robert Jarnot, Ray Swindlehurst,

Jet Propulsion Laboratory,California Institute of Technology,

4800 Oak Grove Drive Pasadena, CA 91109

ABSTRACT

This paper describes a large aperture, 650 GHz, planarnear-field measurement system developed for field ofview characterization of the Earth Observing SystemMicrowave Limb Sounder (EOS MLS). Scheduled forlaunch in 2003 on the NASA EOS Aura spacecraft, EOSMLS is being developed by the Jet PropulsionLaboratory to study stratospheric chemistry usingradiometers from 118 to 2500 GHz. The combination ofa very high operating frequency and a 1.6-meteraperture, coupled with significant cost and weightrestrictions, required a new look at near-field scannerdesign approaches. Nearfield Systems Inc. (NSI)developed a planar scanner that provides a planaraccuracy of 4 microns RMS over the entire 2.4 x 2.4meter scan area. This paper presents an overview ofthis system including the sub-millimeter wave RFsubsystem and the ultrahigh precision scanner.Representative measurement results will be shown.

Keywords: Near-Field, Antenna, JPL, Microwave LimbSounder, MLS, EOS, Microwave Inter-ferometry, THz,Sub-millimeter

1. IntroductionAn emerging technology is the design, production andtesting of large aperture sub-millimeter wave antennas.These antennas are often used in space research as part ofradiometer systems. One such antenna is the EarthObserving System Microwave Limb Sounder (EOS MLS).The MLS GHZ antenna has a 1.6 meter aperture andoperates at frequencies as high as 640 GHz. There is aseparate THz antenna operating at 2500 GHz.

The MLS instrument consists of an off-axis ellipticalCassegrain antenna coupled to a multi-band sub-millimeterwave radiometric receiver operating at a number offrequencies between 118 and 640 GHz. The receiveroutputs are simultaneously processed in spectrometers withthe resultant spectra being transmitted back to earth . TheMLS antenna is electrically quite large, providing abeamwidth only a few arcminutes at 640 GHz.

Testing a large aperture sub millimeter wave antenna suchas the EOS MLS is a technical challenge. Wave front errorsin the measurement system must be low over the entireantenna aperture. A previous sub-millimeter wave near-field measurement system developed by NSI providedsuccessful sub-millimeter wave antenna measurements forseveral NASA projects including the NASA SWAS radio

astronomy satellite (Slater 1994) and Rosetta comet orbitersatellites (Koch 2001). This system, while having thenecessary accuracy, did not have a sufficiently large scanarea. The technology used in this scanner was not readilyadaptable to the cost and weight requirements necessary forthe EOS MLS project. As a result, a new second generationsystem called the SubMillimeter Wave Scanner (SMWS)was designed and fabricated for JPL by NSI.

2. Submillimeter wave near- field systemrequirements

The EOS MLS sub-millimeter wave near-fieldmeasurement system had a system wide axial wave frontmeasurement uncertainty limit of 16 microns RMS . Thefinal system exceeded this significantly with a deliveredaccuracy of less than 10 microns RMS. This paper willconcentrate on the technology needed to provide high phasemeasurement accuracy in this application.

JPL MLS near-field antenna measurement system designconstraints were:

1. The scan area was to be 2.8 by 2.8 meters.

2. The system would require a system wide Z planarityof 16 microns RMS. The error budget was allocatedas follows:

Contractrequirement

As delivered

Scanner Zplanarity

10 microns rms 4 microns rms (measured)

Phase referencecable

12 microns rms 8 microns rms (measured)

Phase referencethermal

3 microns rms 3 microns rms (estimated)

Receiver phasenoise

1 microns rms 1 microns rms (estimated)

RSS total 16 microns rms 9.5 microns rms(calculated)

3. A vertical scan plane orientation was required forcompatibility with the EOS MLS flight article.

4. The entire near-field measurement system wouldneed to be operationally and thermally compatiblewith a class 1000 clean room spacecraft assemblyarea.

5. The scanner weight was limited to 10,000 poundsand the system would need be moveable for storagebetween projects.

3. Submillimeter wave near-fieldmeasurement system design

3.1 Thermal design

Thermally induced phasefront errors become an importantissue when the near-field scanner is electrically large, that iswhen the scanner dimensions are large when measured inwavelengths. Because of the high operating frequency ofthe EOS MLS instrument, the near-field measurementsystem has a system wide 16 micron RMS phasemeasurement accuracy requirement. This specificationrequires that the RMS phase error from all sourcesincluding mechanical scanner errors, phase reference cablesystematic errors, receiver errors, test article supportfixtures and thermal drift will result in less than a 16 micronRMS change in RF path length. Temperature changes in anear-field test facility will cause the following thermaleffects:

1. Thermally induced time varying changes in theelectrical length of the probe antenna phasereference cable, test antenna cable and receiverphase reference cable will cause phase changesthat appear to be along the antenna boresightdirection. A one meter long RF cable with a 10ppm/deg C temperature coefficient and 1 deg Ctemperature change will have an electrical pathlength change of 10 microns.

2. The EOS MLS instrument is mounted onto analuminum handling fixture which willdifferentially expand due to facility thermalvariations. In general, any Z axis motioncomponent of the MLS telescope has the sameeffect and magnitude as any Z error in the planarnear-field scanner.

3. Thermal changes in the test antenna mount cancause the antenna location (X, Y, Z) andorientation (yaw, pitch, roll) to drift unpredictably.The MLS antenna near-field region is similarlyquite sensitive to unmodeled azimuth andelevation changes as these include a differential Zmotion of reflector. A Z component of motion ispresent for relative Z, azimuth and elevationmotion. A 10 microradian (2 arcsecond) pointingchange of the antenna mount would move one sideof the projected 1.6 meter MLS antenna aperture16 microns closer to the scanner than the otherside of the antenna. This would use up the entire Zerror budget.

4. Temperature changes can warp both the scannerand antenna under test.

A number of techniques can be used to reduce the effects ofthermally induced phase errors. These techniques include:

1. Thermally stabilizing the environment minimizesall error terms. Electrically large systems generallyneed to be thermally stabilized to a +/- 1 deg Frange or better. Both the JPL spacecraft assemblyclean room and the NSI fabrication facility werethermally stabilized to this value.

2. Low temperature coefficient materials are usedwhere possible. Granite has a low temperaturecoefficient (5 ppm/deg C) and a high thermalinertia.

3. Thermally induced cable phase errors are reducedby minimizing the length of the RF cables andother mechanical elements. The phase referencecables have a relatively high thermal coefficient.Errors here are minimized by a combination offacility temperature control, cable thermal inertiaand MTI processing.

4. As the RF subsystem of the near-field range is aform of a two arm interferometer, a differentialthermal drift cancellation technique can be used. Inthis technique, the receiver phase reference cableis made equal in length to the sum of the probe andAUT cable lengths. The assumption here is that allcables have the same temperature environment,coefficient and time constants. This wouldminimize the first error term only. Because muchof the receiver phase reference path was internal tothe EOS MLS instrument, this method could notbe used.

5. Motion Tracking Inter-ferometry (MTI) uses sub-millimeter wave phase tracking in conjunctionwith least squares computations to estimate thetime varying solid body azimuth, elevation, and Zmotion between the near-field scanner and theMLS antenna. MTI also compensates for phasereference cable thermal errors.

4. MotionTracking Interferometery

The Motion Tracking Interferometer (MTI) system is anextension of the tie scan concept sometimes used forthermal drift compensation of near-field measurements.Unlike the tie scan, MTI provides a multiple degree offreedom of measure of the relative rigid body motion

between the scanner and test antenna during the test.Additionally, the MTI system provides an estimate of thedifferential motion measurement uncertainty.

The MTI processor provides a measure of the relativeazimuth, elevation and Z motion between the scanner andMLS instrument antenna. Measurements of other degrees offreedom (X, Y, roll) are not needed because the significantMLS antenna energy is aligned with the scanner bore sightaxis. The MTI data is acquired by periodically interruptingthe normal data acquisition process and then measuring thecomplex (S21) transmission at four spatially separatedpoints. The MTI scan is performed at a single frequencywith a co- polarized unsteered beam, even in the case ofmulti-frequency and multi-polarization measurements. TheMTI measurements are phase unwrapped and distancenormalized to remove frequency dependence. A series ofleast squares solutions to the plane orientation provides atime history of the relative Z, azimuth and elevation motionbetween the scanner and MLS instrument antenna. Eventhough the MTI measurements are made at a singlefrequency, polarization and beam steering, the results applyto all polarizations and frequencies.

Because the solution is over determined, the MTImeasurement uncertainty can be readily estimated. Themeasurement uncertainty is a function of the RF signal tonoise ratio, scanner repeatability and unmodeled nonrigidbody motion. For example, if the scanner or antennabecame thermally warped, the MTI measurementuncertainty would increase. Thermal drift in the phasereference cable appears as a solid body motion aligned withthe MTI reference antenna beam direction, in this case,aligned with the Z axis.

The MTI measurements can be used to correct for theunwanted solid body relative motion in two ways. The MTImeasurements can be nulled by periodically rotating andtranslating the scan plane, so as to maintain a constantorientation relative to the drifting test article. Alternately,the MTI measurements can be interpolated over theduration of the scan to provide an estimate of the timevarying relative solid body motion history. A postprocessoris used to perform a time varying derotation and translationof the phase front that effectively nulls the drift. The secondtechnique is used in the EOS MLS near-field measurementsystem as the SMWS scanner did not include a motorized Zaxis.

MTI, as used in the SMWS near-field measurement systemdoes not measure X, Y or roll motion nor does it explicitlyseparate out the cable thermal term. MLS antenna near-fieldmeasurements are relatively insensitive to these motionsbecause the antenna beam is directed along the scanner Zaxis.

5. RF subsystem

The RF subsystem design was primarily driven by the needto use the internal receiver within the MLS payload. Forthis reason, the near-field scanner probe was configured asa transmitter. A microwave reference signal is passedthrough a phase reference cable to a probe mountedvaractor multiplier.

The MLS receiver IF output is passed to an Agilent 8530Amicrowave instrumentation receiver, which produces ameasure of the complex (S21) transmission between theprobe and MLS payload.

6. Scanner design

Six basic scanner concepts were used to provide theextreme precision needed for the MLS antenna test. Theseelements are:

1. T scanner design – The scanner is built in a Tconfiguration with two horizontal granite beamsand one vertical column. This approach minimizesthe weight of the scanner, improves the portabilityand simplifies the scanner interface to the facilityfoundation.

2. Granite beams – Granite beams are used to providea structure with deformations having predominantlow spatial frequencies. Granite provides severaladvantages in this application, including excellentlong term mechanical stability and compatibilitywith precision surface lapping methods. Lappedsurfaces are necessary to produce the requiredplanarity. Center column support – The Y(vertical) axis granite column is supported at itscenter of mass. This reduces the magnitude ofcertain structural deformation errors that increaseas the square of the support distance.

3. Triple vertical carriage – The scanner Y axis uses3 separate carriages, the probe carriage, thecounterweight carriage and a drive carriage. Thiscombination provides a minimum set ofdisturbance forces to the probe carriage.

4. Active heat control system – Heat produced by themotors and sub-millimeter wave up-converter isextracted in a manner that does not allow thermalplumes to impinge upon the scanner structure ortest article.

5. Clean room compatibility – Constant attention waspaid to the need to operate in a class 1000 cleanroom with significant out gassing restrictions.

The SMWS scanner on the left is set up totransmit RF between 118 and 640 GHz to thebreadboard EOS MLS antenna on the right.

7. Submillimeter wave scanner (SMWS)validation

The very high precision of this scanner required a new lookat the construction and validation technologies. NSIconstructed a thermally controlled facility that was usedboth during the construction and validation of this scanner.Temperature within this facility was stabilized to within +/-1 deg F. At JPL, the SMWS scanner is operated in a cleanroom spacecraft assembly area that is also thermallystabilized to +/- 1 deg F.

Conventional optical tooling and laser planaritymeasurement techniques do not provide sufficient accuracyfor this application. Planarity was instead determined by

integrating a series of auto collimation and biaxial tilt metermeasurements. The measured planarity of the deliveredsystem was less than 4 microns RMS over the entire2.4x2.4 meter scan area.

8. Representative measurement results

This section shows a few of the early measurement resultsof the JPL MLS breadboard antenna made with the NSISMWS near-field measurement system.

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9. Conclusions

The SMWS near-field measurement system has met allproject requirements. The scanner Z planarity was betterthan 4 microns RMS and the system planarity was less than10 microns RMS over the full scan plane. The operation ofan electrically large sub-millimeter wave near-field antennameasurement system required several extensions to normalnear-field test methods. These extensions included a very

high precision granite scanner mechanism, a highly stablephase reference system, and the RF based Motion Tracking(MTI) system. The MTI system provides a simple andhighly accurate alternative to autocollimator, tiltmeter, andother mechanical referencing methods. The need for thecustomer to provide a thermally stable test article mount isreduced. Elements of this system are patented or patentpending.

10. References

Koch, T., P. Stek, D., Slater, “A 190 and 560 GHz Near-field Range for Beam Pattern Testing of the MIRO CometOrbiting Radiometer”, AMTA conference proceedings,2001

Slater, D., “A 550 GHz Near-field Measurement System forthe NASA Submillimeter Wave Astronomy Satellite”,AMTA conference, CA, 1994.

Slater, D., US Patent 5,419,631, Three axis motion trackinginterferometer

Slater, D., "Nearfield Antenna Measurements", ArtechHouse, Norwood, MA, 1991

Waters, J.W., “An Overview of the EOS MLSExperiment”, JPL report D-15745, Jet PropulsionLaboratory, Pasadena