Albers et al., FR1.T03 IGARSS 2011 Vancouver, BC July 29, 2011 1

Development, Fabrication, and Testing of 92 GHz Radiometer For Improved

Coastal Wet-tropospheric Correction on the SWOT Mission

Darrin Albers, Alexander Lee, and Steven C. ReisingMicrowave Systems Laboratory, Colorado State University,

Fort Collins, CO

Shannon T. Brown, Pekka Kangaslahti, Douglas E. Dawson, Todd C. Gaier, Oliver Montes, Daniel J. Hoppe, and Behrouz

Khayatian Jet Propulsion Laboratory, California Institute of Technology,

Pasadena, CA

Albers et al., FR1.T03 IGARSS 2011 Vancouver, BC July 29, 2011 2

Scientific Motivation• Current satellite ocean altimeters include a nadir-viewing, co-located

18-37 GHz multi-channel microwave radiometer to measure wet-tropospheric path delay. Due to the large diameters of the surface instantaneous fields of view (IFOV) at these frequencies, the accuracy of wet path retrievals begins to degrade at approximately 50 km from the coasts.

• Conventional altimeter-correcting microwave radiometers do not provide wet path delay over land.

Along track direction

High frequency footprint

Low frequency footprint

Along track directionAlong track direction

High frequency footprintHigh frequency footprint

Low frequency footprintLow frequency footprint

LandOcean• Advanced technology development

of high-frequency microwave radiometer channels to improve retrievals of wet-tropospheric delay in coastal areas, small inland bodies of water, and possibly over land such as for the Surface Water Ocean Topography (SWOT), a Tier-2 Decadal Survey mission.

Albers et al., FR1.T03 IGARSS 2011 Vancouver, BC July 29, 2011 3

SWOT Mission Concept Study

Low frequency-only algorithm

Low frequency-only algorithm

Low and High frequency algorithm

Low and High frequency algorithm

High-resolution WRF model results show reduced wet path-delay error using both low-frequency (18-37 GHz) and high-frequency (90-170 GHz) radiometer channels.

Albers et al., FR1.T03 IGARSS 2011 Vancouver, BC July 29, 2011 4

Objectives

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System Block Diagram

Waveguide Components

92-GHz multi-chip module

130-GHz multi-chip module

166-GHz multi-chip module

Common radiometerback end, thermal

control anddata

subsystem

Tri-Frequency Feed Horn

Coupler

Coupler

Noise Diode

Coupler

Noise Diode

Noise Diode

MMIC Components

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Tri-Frequency Horn Antenna

A single, tri-band feed horn and triplexer are required to maintain acceptable antenna performance, since separate feeds for each of the high-frequency channels would need to be moved further off the reflector focus, degrading this critical performance factor. The tri-frequency horn was custom designed and produced at JPL, with an electroform combiner from Custom Microwave, Inc. Measurements show good agreement with simulated results.

WR-5(166 GHz)

Feed Horn Rings

WR-8(130 GHz)

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Tri-Frequency Antenna (Detail)

WR-8 Waveguide

Port

WR-10 Waveguide

Port

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Example of a Ring to Produce Horn Corrugation

Largest Horn Ring

Pencil TipFor Scale

Ring Cross Section

Fin for theRing-Loaded Slot

Detail of Feed Horn Rings17

.9 m

m

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Antenna Return Loss

Bandwidth measurement with 15-dB return loss or better

Center Frequency (GHz)

Waveguide Band

Bandwidth (GHz)

92 WR-10 11

130 WR-08 18

166 WR-05 26

Albers et al., FR1.T03 IGARSS 2011 Vancouver, BC July 29, 2011 10

Antenna Pattern92 GHz 130 GHz

Center Frequency (GHz)

Waveguide Band

Half-power Beamwidth (°)

92 WR-10 22

130 WR-08 24

166 WR-05 32

166 GHz

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Noise Diodes for Internal Calibration

• Nadir-pointing radiometers are flown on altimetry missions with no moving parts, motivating two-point internal radiometric calibration, as on Jason-2. Highly stable noise diodes will be used to achieve one of these two points.

• Radiometric objectives

• Provide an electronically-switchable source for calibrating the radiometer over long time scales, i.e. hours to days.

• RF design objectives from radiometer requirements

• Noise diode output will be coupled into the radiometer using a commercially-available waveguide-based coupler.

• Stable excess noise ratio (ENR) of 10-dB or greater, yielding ~300 K of noise deflection after a 10-dB coupler.

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Noise Diode MeasurementsNoise Diode Measurements

*Noise diode manufactured for NASA/GSFC

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92-GHz Radiometer Design

• This direct-detection Dicke radiometer uses two LNAs and a single bandpass filter for band definition.

• Direct-detection architecture is the lowest power and mass solution for these high-frequency receivers. Keeping the radiometer power at a minimum is critical to fit within the overall SWOT mission constraints, including the power requirements of the radar interferometer.

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92-GHz Bandpass Filter: Modeled and Measured

4.6 mm

• 5-mil (125-µm) thick polished alumina substrate

• Measured using a probe station with WR-10 waveguides

• Modeled and measured in open air

Frequency (GHz)

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Matched Load for Calibration: Modeled and Measured Results

1.45 mm

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92-GHz Multi-Chip Module

92-GHz direct-detection radiometer with Dicke switching and integrated matched load

17.9 mm

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92-GHz Multi-chip Module (Close-up)

Matched Load

PIN-Diode Switch

Low-Noise Amplifier #1 Band Pass

Filter

Waveguide to Microstrip

Transition

Low-Noise Amplifier #2

DetectorAttenuator1 mm

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92-GHz Radiometer Prototype

Multi-Chip ModuleIsolator

WR-10 Horn Antenna

Coupler

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92-GHz Radiometer Noise Analysis

Component Gain (dB)Noise Figure

(dB)Cumulative Noise Temperature (K)

Noise Source - -  

Directional Coupler -0.75 0.75 55

Waveguide Through Line -0.20 0.20 71

Waveguide to Microstrip transition -0.50 0.50 115

Switch -2.75 2.75 473LNA 28.00 3.00 1232BPF -5.50 5.50 1235

Attenuator -4.25 4.25 1235LNA 28.00 3.00 1254

Attenuator -4.25 4.25 1254

Total Receiver Gain (dB) 37.80Receiver noise factor 5.32

Receiver noise figure (dB) 7.26Receiver noise temperature (K) 1253.53

Preliminary Noise Temperature Measurement of 1375K

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92-GHz Radiometer Performance Analysis

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• Conventional altimeters include a nadir-viewing 18-37 GHz microwave radiometer to measure wet-tropospheric path delay. However, they have reduced accuracy within 40 km of land.

• Addition of high-sensitivity mm-wave channels to Jason-class radiometer will improve wet-path delay retrievals in coastal regions and provide good potential over land.

• We have developed noise sources at 92 and 130 GHz and a tri-frequency feed horn for wide-band performance at center frequencies of 92, 130, and 166 GHz.

• To demonstrate these components, we have produced a millimeter-wave MMIC-based low-mass, low-power, small-volume radiometer with internal calibration sources integrated with the tri-frequency feed horn at 92 GHz.

Summary

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Backup Slides

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ENR Equation

Equation 10.6. Pozar. Microwave Engineering 3rd edition.

0 dB ENR with Tg = 580 K and To = 290K-2 dB ENR with Tg = 473 K and To = 290K

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Move to Higher Frequency• Supplement low-

frequency, low-spatial resolution channels with high-frequency, high-spatial resolution channels to retrieve PD near coast

• High-frequency window channels sensitive to water vapor continuum

• 183 GHz channels sensitive to water vapor at different layers in atmosphere

22.235 GHz (H2O)

55-60 GHz (O2)118 GHz (O2)

183.31 GHz (H2O)

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92-GHz Radiometer with Two LNAs

• Current MMIC detector from HRL has sensitivity of 15,000 V/W• One LNA– System Gain of 26.05 dB and cumulative noise temperature of 727.4 K

– Antenna Temperature of 600 K results in 550 µV, i.e. 417 nV/K

– Antenna Temperature of 77 K results in -46 dBm

• Two LNAs– System Gain of 54.55. dB and cumulative noise temperature of 727.6 K

– Antenna Temperature of 600 K results in 392 mV, i.e. 295 µV/K

– Antenna Temperature of 77 K results in -18 dBm

• TSS (Tangential Sensitivity) of these detectors is typically -44 dBm so might be measuring the noise at 77 K if more loss is in system than expected so two LNAs results in a more robust system