1 16-18/5/2001 GPM Planning Workship Ka-band Radar for GPM: Issues Toshio Iguchi Communications Research Laboratory The Global Precipitation Mission Planning

1

16-18/5/2001

GPM Planning Workship

Ka-band Radar for GPM:Issues

Toshio Iguchi

Communications Research Laboratory

The Global Precipitation Mission Planning WorkshopThe Global Precipitation Mission Planning Workshop

University of MarylandUniversity of Maryland

College Park, Maryland, U.S.A.College Park, Maryland, U.S.A.

2

GPM Planning Workship16-18/5/2001

Ka-band Antenna Design

Antenna Type

Planar Array, 128-element Slotted Waveguide

Edge Slot Array(=Slot in E-plane), Nonresonant Type

Weighting Taylor Distribution (SL = -35 dB, N = 6)

Array Element

Along-Track 5.69 mm 142 807.98 mm

Cross-Track 6.33 mm 128 810.24 mm

Beam WidthAlong-Track 0.71 deg

Cross-Track 0.71 deg (nadir) – 0.74 deg (scan edge)

Gain 47.4 dBi

Sidelobe < -27 dB

VSWR < 1.2Waveguide

Loss0.71 dB

3


Ka-band Planar Array Antenna: Test Model

Rot Number

Resonant Frequency

Element Number

VSWR(2)

G1(1) 35.67 6 1.08-1.09

G1(2) 35.63 2 1.04

G1(3) 35.95 - -

G2(1) 35.67 8 1.04

G2(2) 35.77 4 1.05-1.06

G2(3) 35.90 - -

G3(1) 35.40 5 1.04-1.07

G3(2) 35.87 1 1.03-1.05

G3(3) 36.07 - -

G4(1) 35.52 7 1.04-1.05

G4(2) 35.60 3 1.03

G4(3) 35.67 - -

(2) 35.5, 35.55, 35.6 GHz

Antenna Type

8-element Slotted Waveguide(1)

Weighting Taylor Distribution (SL = -35 dB, N = 6)

Array Element

Along-Track 5.69 mm 142

Cross-Track 6.33 mm 8

Beam Width

Along-Track 0.71 deg

Gain 35.8 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


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 m

atched beam

5


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


mr mi er ei Ki Ki/ lambda abs. ratio13.8GHz 10C 6.93 2.78 40.30 38.53 0.0353 0.4873 0.16135.5GHz 10C 4.61 2.64 14.28 24.34 0.0851 3.0228 6.20313.8GHz Ice 0C 1.781 0.002 3.171 0.007 0.0007 0.0101 0.76135.5GHz Ice 0C 1.780 0.001 3.169 0.003 0.0004 0.0133 1.315

Refractive Index of Water and Ice

• Refractive index: m

• Permittivity:

7


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, a

ttenuation 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


Example of Dual-Frequency Radar (X, Ka)

9


Example of Dual-Frequency Radar (X, Ka)X-band radar reflectivity

Ka-band radar reflectivity

10



X-band radar reflectivity

11





12


Example of Dual-Frequency Radar (X, Ka)X-band radar reflectivity

13



X Ka

14


15


Phase Shifter and SSPA

16


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


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


Original Requirements

• Frequency = 35.5GHz• Sensitivity 11dBZ (S/N_e = 3 dB) or better• Resolutions 4 km (horizontal), 250 m (vert

ical)• Beams matched with Ku-band beams• Swath 20 ~ 40 km• Weight < 100 kg, Power < 100 W

19


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


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


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


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


• 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


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


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


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


Present Status of Ka-band Radar Studies for Atmos-A1

• Designing with a phased-array system– Increasing # of array elements increases total power consu

mption 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


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

GPMCore satellite. (Atmos-A1)35 deg => 70 deg (>95% of precipitation)

Addition of 35GHz radarDual-freq. algorithm

29


Issues

• Hardware Specifications– Mass

– Power Consumption

– Sensitivity

– Accuracy

• Science issues– Dual-Frequency Algorithm

– Combining DPR and TMI Information

30


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


Fre

quen

cy

tropical rainmid 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


３５ GHzradar beam

14GHzradar 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


Ku-scanKa-scan (Matched with Ku)

Ka-scan (interlaced)

Ku footprintKa footprint (Matched with Ka)Ka footprint (Interlaced)

245 km (49 beams)125 km (25 beams)

Possible Scan Patterns

Documents

1 16-18/5/2001 GPM Planning Workship Ka-band Radar for GPM: Issues Toshio Iguchi Communications Research Laboratory The Global Precipitation Mission Planning