Darrin Albers, Alexander Lee, and Steven C. Reising

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. Reising Microwave Systems Laboratory, Colorado State University, Fort Collins, CO Shannon T. Brown, PekkaKangaslahti, Douglas E. Dawson, Todd C. Gaier, Oliver Montes, Daniel J. Hoppe, and BehrouzKhayatian Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA

2. Ocean Land 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. • 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.

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.

4. Objectives • Develop low-power, low-mass and small-volume direct-detection high-frequency microwave radiometers with integrated calibration sources at the center frequencies of 90-170 GHz • Design and fabricate a tri-frequency feed horn with integrated triplexerat the center frequencies of 92, 130, and 166 GHz • Develop and test sufficient Excess Noise Ratio (ENR) noise sources at the center frequencies of 90-170 GHz • Integrate components into MMIC-based radiometers at 92, 130 and 166 GHz with the tri-frequency feed horn and test at a system level

5. System Block Diagram 92-GHz multi-chip module Waveguide Components MMIC Components Coupler Tri-Frequency Feed Horn Noise Diode Common radiometer back end, thermal control and data subsystem 130-GHz multi-chip module Coupler Noise Diode Coupler 166-GHz multi-chip module Noise Diode

6. Tri-Frequency Horn Antenna Feed Horn Rings WR-8 (130 GHz) WR-5 (166 GHz) 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.

7. Tri-Frequency Antenna (Detail) WR-8 Waveguide Port WR-10 Waveguide Port

8. Detail of Feed Horn Rings Ring Cross Section Pencil Tip For Scale 17.9 mm Fin for the Ring-Loaded Slot Largest Horn Ring Example of a Ring to Produce Horn Corrugation

9. Antenna Return Loss Bandwidth measurement with 15-dB return loss or better

10. Antenna Pattern 92 GHz 130 GHz 166 GHz

11. 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.

12. Noise Diode Measurements *Noise diode manufactured for NASA/GSFC

13. 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.

14. 92-GHz Bandpass Filter: Modeled and Measured 4.6 mm Frequency (GHz) • 5-mil (125-µm) thick polished alumina substrate • Measured using a probe station with WR-10 waveguides • Modeled and measured in open air 35 mils (0.89mm) wide

15. Matched Load for Calibration: Modeled and Measured Results 1.45 mm

16. 92-GHz Multi-Chip Module 92-GHz direct-detection radiometer with Dicke switching and integrated matched load 17.9 mm

17. 92-GHz Multi-chip Module (Close-up) Matched Load Attenuator Detector PIN-Diode Switch 1 mm Low-Noise Amplifier #1 Low-Noise Amplifier #2 Band Pass Filter Waveguide to Microstrip Transition

18. 92-GHz Radiometer Prototype Coupler Multi-Chip Module Isolator WR-10 Horn Antenna

19. 92-GHz Radiometer Noise Analysis Preliminary Noise Temperature Measurement of 1375K Measured noise temperature of 2073 K (lossy coupler and isolator) and 964 K (without waveguide components) .

20. 92-GHz Radiometer Performance Analysis

21. Summary • 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.

22. Backup Slides

23. 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

24. 22.235 GHz (H2O) 118 GHz (O2) 55-60 GHz (O2) 183.31 GHz (H2O) 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

25. 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