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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. Ka-band Antenna Design. Ka-band Planar Array Antenna: Test Model. (1) 8-elements are selected.

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ka band radar for gpm issues

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.

ka band planar array antenna test model
Ka-band Planar Array Antenna: Test Model

(1) 8-elements are selected

(2) 35.5, 35.55, 35.6 GHz

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

Determining Factors of Detectability
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

Requirements and Compromises
refractive index of water and ice
Refractive Index of Water and Ice

Refractive index: m

Permittivity: e

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.

Detectability of Rain
example of dual frequency radar x ka1
Example of Dual-Frequency Radar (X, Ka)

X-band radar reflectivity

Ka-band radar reflectivity

example of dual frequency radar x ka3
Example of Dual-Frequency Radar (X, Ka)

X-band radar reflectivity

X-band radar reflectivity

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?

What is the Ka-band radar for?
present status of ka band radar design
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)

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

Original Requirements
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:


phase state

storm structure (non-uniform beam filling)

Validation needed, but very difficult

PR Rain Retrieval Algorithm
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

Basic Design of Ka-band Radar
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)


Evaluation of measurement performance (FY2000, 2001)

Simulation Experiments

Dual-frequency algorithm development

CRL’s commitment
mass power consumption
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

scientific requirements
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
separation of ice from rain differences in ka ku echoes

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.

Separation of ice from rain(Differences in Ka & Ku echoes)
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?


Effects of beam mismatch?

needs simulations.

Non-Uniform Rain and Beam Matching
engineering issues in ka band radar development
Engineering Issues in Ka-band Radar Development


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.)


interfarence with TMI’s field of view?

Accuracy of beam matching

present status of ka band radar studies for atmos a1
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?)

intrinsic difficulties in rain estimation by trmm pr
Intrinsic Difficulties in Rain Estimation by TRMM PR

Sensitivity (0.5 mm/h)


Uncertainty in DSD and phase of hydrometeor

Attenuation correction & Z-R conversion

Low sampling frequency: 1/(3 days)

Observation coverage

  • Addition of 35GHz radar
    • Dual-freq. algorithm
  • GPM
  • Core satellite.
  • (Atmos-A1)
  • 35 deg => 70 deg
  • (>95% of precipitation)
Hardware Specifications


Power Consumption



Science issues

Dual-Frequency Algorithm

Combining DPR and TMI Information



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

need for 35ghz radar
Need for 35GHz Radar

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.


mid and high latitude rain

tropical rain

(weak rain)

Rain Rate

(strong rain)

New measurable range by the

addition of 35GHz radar

merits of dual frequency radar measurement
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 RadarMeasurement


radar beam


radar beam

strong scattering

weak scattering

small attenuation

in snow

small attenuation in snow

small attenuation in rain

large attenuation in rain

scattered wave with large attenuation

scattered wave with small attenuation

possible scan patterns
Possible Scan Patterns

Ku footprint

Ka footprint (Matched with Ka)

Ka footprint (Interlaced)

Ka-scan (interlaced)


Ka-scan (Matched with Ku)

125 km (25 beams)

245 km (49 beams)