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.
Communications Research Laboratory
The Global Precipitation Mission Planning Workshop
University of Maryland
College Park, Maryland, U.S.A.
(1) 8-elements are selected
(2) 35.5, 35.55, 35.6 GHz
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 beamDetermining Factors of Detectability
Horizontal resolutions (Averaging horizontally)
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 rateRequirements and Compromises
Refractive index: m
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
X-band radar reflectivity
Ka-band radar reflectivity
X-band radar reflectivity
X-band radar reflectivity
to measure weak rain and snow．
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?
Phased Array System
Increase in power consumption and mass
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)
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 WOriginal Requirements
needs k-Ze relationship
utilizes the surface reference technique
Conversion from Ze to R
Needs Ze-R relationship
Both relationships depend on:
storm structure (non-uniform beam filling)
Validation needed, but very difficultPR Rain Retrieval Algorithm
No need for pulse compression
Flexibility in scanning
Easy in test and inspectionBasic Design of Ka-band Radar
Ka-band Radar Development (Designing and testing the key components of the 35GHz radar)
Examination of basic performance of hardware
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)
Dual-frequency algorithm developmentCRL’s commitment
Total Mass: 290 kg
Phased-array system is heavy
Power consumption: 250 W
Efficiency of SSPA is limited
Dimensions: 1.0 ×1.0 ×0.5 m
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
Effective Z（Ze) is nearly idential up to 2 mm/h
Attenuation (Ka) is about 10 times of attenuation (Ku)
Detection of melting height
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)
DPR algorithm uses attenuation difference. separation
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
Vertical resolution (Is 500m res. acceptable?)
Mass and power consumption (heat release)
125 m oversample?
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
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?)
Sensitivity (0.5 mm/h)
Uncertainty in DSD and phase of hydrometeor
Attenuation correction & Z-R conversion
Low sampling frequency: 1/(3 days)
Hardware Specifications separation
Combining DPR and TMI InformationIssues
Vertical resolution (500 m acceptable?)
Mass and power consumption (& heat release)
Sampling interval -- 125 m over sample (?)
Quantization of data
Mount -- Interference with TMI field of view
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
New measurable range by the
addition of 35GHz radar
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
small attenuation in snow
small attenuation in rain
large attenuation in rain
scattered wave with large attenuation
scattered wave with small attenuation
Ka footprint (Matched with Ka)
Ka footprint (Interlaced)
Ka-scan (Matched with Ku)
125 km (25 beams)
245 km (49 beams)