1. 1 16-18/5/2001 GPM Planning Workship Ka-band Radar for GPM: Issues Toshio Iguchi Communications Research Laboratory The Global Precipitation Mission Planning Workshop University of Maryland College Park, Maryland, U.S.A.

2. 2 GPM Planning Workship 16-18/5/2001 Ka-band Antenna Design Antenna Type Planar Array, 128-element Slotted Waveguide Edge Slot Array(=Slot in E-plane), Nonresonant Type WeightingTaylor Distribution (SL = -35 dB, N = 6) Array Element Along-Track5.69 mm142807.98 mm Cross-Track6.33 mm128810.24 mm Beam Width Along-Track0.71 deg Cross-Track0.71 deg (nadir) – 0.74 deg (scan edge) Gain47.4 dBi Sidelobe< -27 dB VSWR< 1.2 Waveguide Loss 0.71 dB

3. 3 GPM Planning Workship 16-18/5/2001 Ka-band Planar Array Antenna: Test Model Rot Number Resonant Frequency Element Number VSWR ( 2) G1(1)35.6761.08-1.09 G1(2)35.6321.04 G1(3)35.95-- G2(1)35.6781.04 G2(2)35.7741.05-1.06 G2(3)35.90-- G3(1)35.4051.04-1.07 G3(2)35.8711.03-1.05 G3(3)36.07-- G4(1)35.5271.04-1.05 G4(2)35.6031.03 G4(3)35.67-- (2) 35.5, 35.55, 35.6 GHz Antenna Type 8-element Slotted Waveguide (1) WeightingTaylor Distribution (SL = -35 dB, N = 6) Array Element Along-Track5.69 mm142 Cross-Track6.33 mm8 Beam Width Along-Track0.71 deg Gain35.8 dBi Sidelobe -27.03 dB(@-54.4 deg) -27.43 dB(@+42.4 deg) -28.55 dB(@-1.13 deg) Cross Pol -24.29 dB(@+43.0 deg) -26.89 dB(@-53.8 deg) VSWR< 1.1 (1) 8-elements are selected

4. 4 GPM Planning Workship 16-18/5/2001 Determining Factors of Detectability Assume constat Tx peak power & constant antenna gain –S/N ∝ sqrt(number of samples)  freq. agility –S ∝ Tx pulse width –1/N ∝ 1/(band width) ∝ Tx pulse width –Range resolution ∝ Tx pulse width –# of samples in Ka = # of samples in Ku for a matched beam

5. 5 GPM Planning Workship 16-18/5/2001 Requirements and Compromises Sensitivity (detectability) Horizontal resolutions (Averaging horizontally) Vertical resolution –Lowest observable height Matched beams (How many? All or partial?) Swath width (245 km or 100 km or less) Oversamples (125 m?) – data rate Range of observation (0-15 km?) – data rate

6. 6 GPM Planning Workship 16-18/5/2001 Refractive Index of Water and Ice Refractive index: m Permittivity: 

7. 7 GPM Planning Workship 16-18/5/2001 Detectability of Rain Is Z=270 R^1.27 valid for weak rain? If k=0.23 R^1.05, R=10mm/h, and H=5km, attenuation is about 26 dB. This is the maximum R we can measure near surface. If R=1mm/h, attenuation =2.3dB. No problem to see to the surface. As long as rain is uniform, attenuation is not a limiting factor of detection of weak rasin.

8. 8 GPM Planning Workship 16-18/5/2001 Example of Dual-Frequency Radar (X, Ka)

9. 9 GPM Planning Workship 16-18/5/2001 Example of Dual-Frequency Radar (X, Ka) X-band radar reflectivity Ka-band radar reflectivity

10. 10 GPM Planning Workship 16-18/5/2001 Example of Dual-Frequency Radar (X, Ka) X-band radar reflectivity

11. 11 GPM Planning Workship 16-18/5/2001 Example of Dual-Frequency Radar (X, Ka) X-band radar reflectivity

12. 12 GPM Planning Workship 16-18/5/2001 Example of Dual-Frequency Radar (X, Ka) X-band radar reflectivity

13. 13 GPM Planning Workship 16-18/5/2001 Example of Dual-Frequency Radar (X, Ka) X Ka

14. 14 GPM Planning Workship 16-18/5/2001

15. 15 GPM Planning Workship 16-18/5/2001 Phase Shifter and SSPA

16. 16 GPM Planning Workship 16-18/5/2001 What is the Ka-band radar for? High sensitivity –to measure weak rain and snow ． High precision –Increase of information by the combination of two channels Attenaution and rain rate are nearly proportional at 35GHz. –Rain estimation independent of DSD. Separation of snow from rain. Vertical structure  microwave radiometer algorithm To what extent can we realize high sensitivity and high precision? –What kind of science can we do with DPR data?

17. 17 GPM Planning Workship 16-18/5/2001 Present Status of Ka-band Radar Design Phased Array System –Increase in power consumption and mass –Heat release –Pulse compression too risky Doppler broadening → range sidelobe Increase in power consumption –Matched beam realizable Sensitivity vs. swath width and vertical resolution What are the scientific requirements? Priority? (sensitivity, accuracy, resolution, swath)

18. 18 GPM Planning Workship 16-18/5/2001 Original Requirements Frequency = 35.5GHz Sensitivity 11dBZ (S/N_e = 3 dB) or better Resolutions 4 km (horizontal), 250 m (vertical) Beams matched with Ku-band beams Swath 20 ~ 40 km Weight < 100 kg, Power < 100 W

19. 19 GPM Planning Workship 16-18/5/2001 PR Rain Retrieval Algorithm Attenuation correction essential –needs k-Ze relationship –utilizes the surface reference technique Conversion from Ze to R –Needs Ze-R relationship Both relationships depend on: –DSD –phase state –storm structure (non-uniform beam filling) Validation needed, but very difficult

20. 20 GPM Planning Workship 16-18/5/2001 Basic Design of Ka-band Radar Phased-Array system –Matched beams –No need for pulse compression –Flexibility in scanning Independent unit –Easy in test and inspection

21. 21 GPM Planning Workship 16-18/5/2001 CRL’s commitment Ka-band Radar Development (Designing and testing the key components of the 35GHz radar)  Examination of basic performance of hardware  Overall configuration  Pulse compression (FY2000)  Designing of critical components and testing (FY2000)  SSPA (2.5 W)  Phase shifter (5 bits)  Antenna (90 cm)  Examination of basic performance of hardware (FY2001)  BBM  Evaluation of measurement performance (FY2000, 2001)  Simulation Experiments Dual-frequency algorithm development

22. 22 GPM Planning Workship 16-18/5/2001 Mass & Power Consumption Total Mass: 290 kg Phased-array system is heavy Heat sink Power consumption: 250 W Efficiency of SSPA is limited Dimensions: 1.0 ×1.0 ×0.5 m

23. 23 GPM Planning Workship 16-18/5/2001 DF algorithm is essential for DSD estimation and liquid-ice separation DF algorithm requires a matched beam –How well do two beams need matched? –Matched beam requirement restricts # of pulses per beam for Ka-band Sensitivity or DF information? Scientific Requirements

24. 24 GPM Planning Workship 16-18/5/2001 Separation of ice from rain (Differences in Ka & Ku echoes) Rain –Effective Z （ Ze) is nearly idential up to 2 mm/h –Attenuation (Ka) is about 10 times of attenuation (Ku) Detection of melting height Snow (ice) –Ze of snow is different from Ze of rain –Ze is nearly identical when particles are small –Ze is different when particles are large (hail) –Attenuation by absorption are negligible at both Ka, and Ku. At 35.5GHz, 1/22 of rain ． At 13.8GHz, 1/48 of rain ． –Difference in scattering by large ice particles (hail). Difference in attenuation. Difference in Ze. –Can we distinguish hail from rain? Interdependence of phase judgement and DSD estimation.

25. 25 GPM Planning Workship 16-18/5/2001 Non-Uniform Rain and Beam Matching DPR algorithm uses attenuation difference. Non-uniform rain decreases apparent attenuation. –underestimates rain rate. –overestimates large drops in DSD. Non-uniformity of rain and beam mismatching may overturn the basic assumptions in dual- frequency algorithms. –How well can we match beams? 0.2°(1400m) ？ –Effects of beam mismatch? needs simulations.

26. 26 GPM Planning Workship 16-18/5/2001 Engineering Issues in Ka-band Radar Development Sensitivity –pulse compression –Vertical resolution (Is 500m res. acceptable?) Mass and power consumption (heat release) Data rate –Sampling interval 125 m oversample? –On-board processing surface detection data compression –No. of bits for each echo datum (TRMM uses 8 bits, 0.38 dB res.) Mount: –interfarence with TMI’s field of view? –Accuracy of beam matching

27. 27 GPM Planning Workship 16-18/5/2001 Present Status of Ka-band Radar Studies for Atmos-A1 Designing with a phased-array system –Increasing # of array elements increases total power consumption and mass –Mass and power consumption (heat release) are the issues Possibility of 500-m vertical resolution –To increase sensitivity by 6 dB –Almost no degradation of V resolution except near nadir –Power consumption and mass will increase Matched beam requirement Trade-off between sensitivity and swath width –Needs scientific compromise –Provides multiple observation modes? (Confusing?)

28. 28 GPM Planning Workship 16-18/5/2001 Intrinsic Difficulties in Rain Estimation by TRMM PR Sensitivity (0.5 mm/h) Accuracy –Uncertainty in DSD and phase of hydrometeor Attenuation correction & Z-R conversion Low sampling frequency: 1/(3 days) Observation coverage GPM Core satellite. (Atmos-A1) 35 deg => 70 deg (>95% of precipitation) Addition of 35GHz radar Dual-freq. algorithm

29. 29 GPM Planning Workship 16-18/5/2001 Issues Hardware Specifications –Mass –Power Consumption –Sensitivity –Accuracy Science issues –Dual-Frequency Algorithm –Combining DPR and TMI Information

30. 30 GPM Planning Workship 16-18/5/2001 Issues Sensitivity –Pulse compression –Vertical resolution (500 m acceptable?) Mass and power consumption (& heat release) Data rate –Sampling interval -- 125 m over sample (?) –On-board processing –Quantization of data Mount -- Interference with TMI field of view

31. 31 GPM Planning Workship 16-18/5/2001 Frequency tropical rain mid and high latitude rain ｝ Rain Rate Measurable range by 35GHz radar Measurable range by 14GHz radar 35 GHz-band radar is needed to measure weak rain in mid and high latitude regions. New measurable range by the addition of 35GHz radar （ strong rain ） （ weak rain ） Need for 35GHz Radar

32. 32 GPM Planning Workship 16-18/5/2001 ３５ GHz radar beam 14GHz radar beam strong scattering small attenuation in snow small attenuation in snow small attenuation in rain scattered wave with small attenuation weak scattering large attenuation in rain scattered wave with large attenuation High sensitivity by the use of high frequency (11 dBZ (target) or RR=0.2 mm/h) Discrimination between rain and snow by attenuation difference Accurate estimation of rainfall rate from attenuation difference in common range (2-15 mm/h) Merits of Dual-Frequency Radar Measurement

33. 33 GPM Planning Workship 16-18/5/2001 Ku-scan Ka-scan (Matched with Ku) Ka-scan (interlaced) Ku footprint Ka footprint (Matched with Ka) Ka footprint (Interlaced) 245 km (49 beams) 125 km (25 beams) Possible Scan Patterns