The Sensitivity of Spaceborne Precipitation Radar GPMDPR Improved by Variable PRF Technique

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The Sensitivity of Spaceborne Precipitation Radar
(GPM/DPR) Improved by Variable PRF Technique
Shinsuke Satoh 1, Nobuhiro Takahashi 1, Toshio
Iguchi 1, Hiroshi Hanado 2 , Masahiro Kojima 2,
and Minoru Okumura 3   1 National Institute of
Information and Communications Technology (NICT),
2 JAXA, GPM/DPR Project Team 3 NEC TOSHIBA
Space Systems
2
Concept of Global Precipitation Measurement (GPM)

• Core Satellite
• Dual-frequency Precipitation Radar (DPR)
• Microwave Radiometer (GMI)
• High-sensitivity measurement
• Calibration for constellation radiometers

• Constellation Satellites
• Microwave Radio-meters installed on each
  countrys satellite
• Frequent precipitation measurement

Expected Partners NASA, NOAA (US), ESA
(EU), CNES-ISRO China, others
JAXA, NICT (Japan) DRP, H-IIA Launcher
NASA (US) Spacecraft Bus, GMI
3-hourly global rainfall map
Blue Inclination 65º (GPM core) Green
Inclination 35º (TRMM)
3
Concept of precipitation measurement by the GPM
core satellite
Dual-frequency precipitation radar (DPR) consists
of Ku-band (14GHz) radar KuPR and Ka-band
(35GHz) radar KaPR
Flight direction
407 km altitude, 65 deg inclination
GMI
DPR
Range resolution 250m and 500m
KuPR (13.6 GHz) swath width245 km
Microwave radiometer swath width 800km
KaPR (35.5 GHz) swath width120 km
5km
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Objectives of DPR

• Three-dimensional observation of precipitation
• High sensitivity measurement of light rainfall
  and snowfall in high latitude
• Accurate estimation of rainfall rate by combining
  the Ku- and Ka-band radar data.
• Improvement of MWRs precipitation estimation
  accuracy using the precipitation parameters (DSD,
  melting level, rain type, storm height, etc.)
  estimated by using DPR data.

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Precipitation measurement by DPR
Detectable range of KaPR (35 GHz)
Matched beam of KuPR and KaPR
Detectable range of KuPR (14 GHz)
Height
Sensitive observation by the KaPR
ICE
Discrimination of snow and rain using
differential attenuation method
SNOW
KuPR
MELTING LAYAR
Snowfall measure-ments in the frigid zones
KaPR
Accurate rainfall estimation using differential
attenuation method (DSD parameter estimation)
RAIN
Radar reflectivity
Accurate rainfall measurements in the tropics
and the temperate zones
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Main Characteristics of DPR
Minimum detectable rainfall rate is defined by
Ze200 R1.6 (TRMM/PR Ze372.4 R1.54 )
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Concept of the DPR antenna scanning method
49
37
49
38
13
12
1
1
KaPR 120 km (2425 beams)
KuPR 245 km (49 beams)
In the interlacing scan area ( ), the KaPR can
measure snow and light rain in a high-sensitivity
mode with a double pulse width.
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Variable PRF (VPRF)
As the inclination angle increases, the variation
of distance from the satellite to the surface
increases.We cannot collect enough samples with
a single PRF to achieve the required sensitivity.
VPRF is a solution to realize 0.2 mm/h
sensitivity. GPS altitude information is
available to realize VPRF.
380 km
390 km
Sample window at nadir
18 km
15 km (TRMM)
400 km
Distance from satellite to surface
ellipsoid surface (TRMM)
35 deg (TRMM)
410 km
65 deg (GPM)
ellipsoid surface (GPM)
420 km
5 km
Eq.
S
S
N
Eq
35S or 65S
35S or 65S
Latitude
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Variable PRF (VPRF) for High Sensitivity
The observation range of spaceborne radar is 23
km, while the distance between the satellite and
the surface is 398 418 km. The receiving range
window is located after the n-th transmitting
pulse. The Pulse Repetition Frequency (PRF) of
the DPR is varied to increase the sampling
number.
1/PRF
Distance between S/C and the surface target
The distance is changed by Antenna scan angle (?9
km)
Proper PRF (?250 Hz) is determined on board
Satellite altitude (407 km?10 km) is measured by
GPS data
Examples of VPRF
( by S. Kobayashi and T. Iguchi,2002 Variable
pulse repetition frequency for the GPM Project.
IEEE Trans. Geosci. Remote Sens )
1/PRF
Distance between S/C and a surface target
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Implementation of VPRF
Example of VPRF table
The VPRF table is written in a rewritable memory
in SCDP. The range of S/C altitude and its step
size in the table are TBD. The scan angle step
size is also TBD. The actual table will include
the start range bin and sampling number.
Input data
The S/C Altitude (from earth reference ellipsoid
WGS84) calculated using GPS data will be used.
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The increase of samples by VPRF
Sampling Window for GPM DPR (Variable PRF)
Sampling Window for TRMM PR (Fixed PRF)
-
17
deg
17
deg
0
deg
-
17
deg
17
deg
0
deg
A large sampling range window is reserved to
cover the fluctuation of satellite altitude
Use the altitude information by using a GPS
receiver.
50 km
400 range bin x 125 m
35km
140 range bin x 250m

20 km
Onboard sampling area (125 m resolution)
VPRF sampling window
Observation data sent to ground
Observation data sent to ground (250 m
resolution)
of samples for Pr 112 of samples for noise
P 720
Noise sampling area (4 range bins)
In GPM/DPR, we stop transmitting 4 pulses in
every 116 pulses and collect the noise data from
the receiving window that corresponds to these
missing transmission pulses.
of samples for Pr 64 of samples for noise P
256
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Definition of minimum detectable rainfall rate
(1) Radar Echo Power Signal Power Total
Power - Noise Power If a logarithmic
detection is used, the standard deviation of the
probability distribution of the averaged signal
due to fading becomes as follows SD of Total
Power ?t 5.57/SQRT(N) 0.526 dB
(N112) SD of Noise Power ?n
5.57/SQRT(M) 0.208 dB (M720) N and M are
number of samples for averaging (2) Average
noise power (hardware specification) in KaPR (500
m resolution) corresponds to 17.04 dBZ when
S/N1, Ta323 K (50ºC), Te290 K (17ºC),
Efficiency 0.95, Altitude 419 km, and no rain
attenuation. (3) If we set the rain/no-rain
threshold at 2s above the average of no-rain
signal power, then the total power at that point
(in terms of Z) is 17.04 dBZ 2 ?
SQRT(?t 2 ?n2) 18.17 dBZ, and the signal
power is 101.817 101.704 15.05
(linear scale) --gt 11.78 dBZ (4) If we use a
widely used Z-R relationship of Z200R1.6
11.78 dBZ --gt 0.198 mm/hr
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PDF of Z expected in rain measurements with DPR
KuPR KaPR in 250 m reso.
KaPR in 500 m reso.
Noise Total signal Signal Rain rate
Noise Total signal Signal Rain rate
N112, M720 s SQRT(st 2 sn 2) 0.566
dB TRMM PR (N64, M256) s 0.778 dB
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Summary

• GPM/DPR is expected to have a higher sensitivity
  than the TRMM PR in order to measure light rain
  and snow in high latitude regions.
• The VPRF technique which increases the number of
  samples at each IFOV is adopted to reduce the
  fading noise and thereby increase the S/N.
• The PRF of about 4200 Hz will be realized (cf.
  TRMM/PR 2776 Hz)
• With this PRF, the minimum detectable Z will be
  12 dBZ which corresponds to about 0.2 mm/h (if
  MP's Z-R relation is used) with Ka-band radar in
  500-m resolution mode.

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Backup Slides
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Variations in System Noise (TRMM PR)

• The system noise level is determined by the
  thermal noise and the background noise from the
  radiation of the earth surface, precipitation,
  etc.
• The variation of the thermal noise is less than
  0.15 dB, and is stable for the long period (Left
  Figure). The variation of the background noise is
  also small (lt 0.1 dB over ocean, lt 0.5 dB over
  land)
• The fading variation of the system noise is about
  1 dB (Right Figure).

Long-term change of the system noise and the
solar beta angle (top), and the FCIF temperature
(bottom)
An example of the system noise distribution
(no-rain, over ocean, 100 orbits data)
by Takahashi and IguchiIGARSS-2004
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TRMM PR Tx turn-off Experiments
Over Land (central Africa)
Over Ocean (off Peru)
1B21 special averaging 500 scans June 24, 2004
Over Sea
Over Land
0.7 dBup
- 0.1 dB / 100 range bin
- 0.1 dB / 100 range bin
The cause to appear the surface echo during
transmitting turn-off operations is that only the
seed signal in FCIF is turned-off, and a noise is
amplified by pulse driven SSPA and LNA.
Therefore, the system noise has to be sampled by
turned-off SSPA, LNA, FCIF in GPM DPR.
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Objectives of GPM
Mission Requirements - To observe the global
precipitation - To accurately measure the
precipitation - To frequently measure the
precipitation
Accurate 3-hourly global rain map - Climate
change assessment - Improvement in weather
forecasts - Flood forecasting (Flood Alert
System) - Water resource management -
Agricultural production forecasting
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Beam Matching
Both radars should have the same foot print
locations (requires good alignment and
synchronization between KuPR KaPR)
Concept for realizing matched beam - Cross-track
direction adjust the beam direction by
controlling the phase shifters. - Along-track
direction set delay of pulse transmitting
timing for one radar system.
Four kinds of mismatch - Dimensions and shape -
Cross-track direction - Along-track direction -
Scan direction
Post-launch checkout - Active radar calibrator
(ARC) experiment from ground to estimate the
alignment offset.
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GPM/DPR development schedule
Feb. 10, 2004 update
Reviews in JAXA
PRR2
DLR
PRR1
CDR (KaPR)
CDR (KuPR)
Launch
PIDR
PDR
PQR/PSR
Init.C/O
Normal Operation
Preliminary Design
Critical Design
Conceptual Design
Sustaining Design
Investigation
PFM (KuPR)
S/C-system Support
TR-comp BBM
Component Experiment
EM (KuPR)
Launch Operation (NASA/JAXA)
STM (KuPR,KaPR)
PFM (KaPR)
Comp. Exp
BBM (CRL)
EM (CRL)
DPR integ
15 months
Integration MST
Normal Operation
Ground System Design Fabrication
Conceptual Design
OperTrain
Init Oper
Cal/Val
Investigation
Algorithm Examination
Algorithm Test/Improvement, Application and
Validation
Algorithm Development
RA
RI
RI
Research and Utilization
Investigation
Critical Design
Preliminary Design
FM Fabrication Assembly
Install Test
PQR/PSR
CDR
PDR
SCR
PRR Project Readiness Review, PIDR Preliminary
Interface Design Review, PDR Preliminary Design
Review, CDR Critical Design Review, MST
Mission Simulation Test, PQR Post
Qualification-test Review, PSR Pre-Shipment
Review, DLR Delivery Review, T/R-unit
Transmitter/Receiver unit, RF-unit
Radio-Frequency unit (including antenna and T/R
component)
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Data Rate (KuPR)
Accuracy of GPS altitude
Inc. Error of S/C Roll, Pitch
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Data Rate (KaPR)
Total Data Rate (8bit A/D) 111.6 (KuPR) 78.0
(KaPR) 189.6 kbps
ltMax 190 kbpsgt