1. Current Status and Lessons from TRMM/GPM XCAL Calibration Activities
Wesley Berg
Colorado State University
GPM: Global Precipitation Measurement
TRMM: Tropical Rainfall Measuring Mission
PMM: Precipitation Measurement Missions
XCAL: PMM intercalibration working group

2. Global Precipitation Measurement (GPM) Mission
GPM Core Satellite
Dual-Frequency radar (DPR)
Ku-band (13.6 GHz)
Ka-band (35.5 GHz)
Microwave Imager (GMI)
13 channels
GHz
GPM was designed to provide the next generation of global precipitation products characterized by
More accurate instantaneous precipitation estimates, particularly for light rainfall and cold season solid precipitation
Unified precipitation retrievals from a constellation of microwave radiometers through the use of intercalibrated brightness temperatures and a common observational hydrometeor database consistent with the combined radar/ radiometer measurements obtained by the GPM Core Observatory. Constellation needed to provide 3-hourly global sampling.

3. GPM Radiometer Constellation
Conical Imagers
Cross-Track Sounders

4. GPM/TRMM Radiometer Constellation
Satellite
(Sensor)
6-7 GHz
10 GHz
19 GHz
23 GHz
31-37 GHz
85-92 GHz
GHz
183 GHz
GPM GMI
10.65vh
18.7v/h
23.8v
36.64v/h
89.0v/h
166v/h
183.31±3, 7
TRMM TMI
10.65v/h
19.35v/h
21.3v
37.0v/h
85.5v/h
GCOM-W1
AMSR-2
6.925v/h
7.3v/h
23.8v/h
36.5v/h
89.0v/h (B)
DMSP F16, F17, F18, F19 SSMIS (4)
22.235v
91.655v/h
150h
183.31±1, 3, 6.6
METOP A/B, NOAA 18/19 MHS (4)
89qv
157qv
183.31±1, 3
190.31
Suomi NPP
ATMS
23.8qv
31.4qv
88.2 qv
165.5qh
183.31±1.0, 1.8, 3.0, 4.5, 7.0
Megha-
Tropiques SAPHIR
183.31±0.2, 1.1, 2.8, 4.2, 6.8, 11
GPM Era (Mar 2014 – Present)
GPM Imager Constellation (7)
GPM GMI (reference sensor)
TRMM TMI
GCOM-W1 AMSR2
DMSP F16, F17, F18 and F19 SSMIS
GPM Sounders (6)
Metop A and B MHS
NOAA 18 and 19 MHS
Suomi NPP ATMS
*Megha-Tropiques SAPHIR
TRMM Era (Dec 1997 – Apr 2015)
TRMM Imager Constellation (10)
TRMM TMI
EOS-AQUA AMSR-E
GCOM-W1 AMSR2
DMSP F11, F13, F14 and F15 SSM/I
DMSP F16, F17 and F18 SSMIS
TRMM Sounders (8)
NOAA 15, 16 and 17 AMSU-B
Metop A, NOAA 18 and 19 MHS
Suomi NPP ATMS
*Megha-Tropiques SAPHIR
*Experimental products currently

5. XCAL Responsibilities and Goals Primary Tasks
The XCAL team is responsible for the Level 1C intercalibrated brightness temperature files used as input to the operational precipitation retrieval algorithm for all of the GPM constellation radiometers (and TRMM constellation radiometers). Specific tasks include:
Identify sensor issues affecting the calibration and stability of the Tb for each of the constellation radiometers. This involves Investigating calibration errors across scan and/or along orbit (i.e. time-dependent)
Develop and apply corrections for sensor calibration issues.
Derive and deliver intercalibration tables to adjust for residual sensor calibration differences in a physically consistent manner.
Assess calibration of reference radiometer (GMI)
Estimate calibration differences using multiple approaches (e.g. double differences, vicarious, polar matchups)
Investigate both cold and warm-scene differences where applicable

6. XCAL Responsibilities and Goals Additional Tasks
Assess uncertainties
Investigate errors in RTM and geophysical parameters
Results from multiple approaches
Evaluate uncertainties in intercalibration techniques
Document results (full transparency)
Publications
Working with other space agencies, related efforts (i.e. GSICS) etc.
Work to improve intercalibration techniques
Improved retrieval algorithms
Identify and use best RTM models
Screening (precipitation, RFI, sun-glint, etc.)
Develop RTM model corrections?

7. Satellite/Instrument Characteristics Reference Sensor Attributes
GPM GMI: Calibration Reference Sensor
GMI Specs
10.65v/h
18.7v/h
23.8v
36.64v/h
89.0v/h
165.5v/h
183+3v
183+7v
DT x CT Res in km
32.1x19.4
18.1x10.9
16.0x9.7
15.6x9.4
7.2x4.4
6.3x4.4
5.8x3.8
Beamwidth (deg)
1.72
0.98
0.85
0.81
0.38
0.37
NEDT (K)
0.96
0.84
1.05
0.65
0.57
1.5
Beam Efficiency (%)
91.1
91.2
93.0
97.8
96.8
96.5
95.2
Uncorr Nonlinearity (K)
0.2
0.1
0.5
Band Width (MHz)
100
200
400
1000
6000
4000
3500
4500
Feedhorns 1
Integration Time (ms)
3.6
Satellite/Instrument Characteristics
Calibration Summary
Data from on-orbit calibration maneuvers were used to check for calibration anomalies and develop corrections for calibration issues (i.e. magnetic anomalies, spillover corrections etc.)
Significant changes were made to the spillover corrections based on the on-orbit maneuvers. Given limitations of pre-launch measurements this is a likely cause of significant calibration differences between sensors, particularly at lower frequencies.
Calibration corrections are based on data from on-orbit calibration maneuvers and are not dependent on radiative transfer models
Independent comparisons with by both Ball/RSS and XCAL indicate that the GMI calibration is very consistent with clear-sky ocean simulated Tb.
A conservative estimate for the absolute calibration errors of the GMI window channels are < 1K
Comparisons of the GMI 166 and 183 GHz channels with the MHS and SAPHIR cross-track sounders indicate differences of < 0.5K
Nominal EIA
52.8/49.2
Orbit Inclination
65.0 deg
Local Obs. Time
Variable (Precessing)
Altitude
407 km
Reflector Size
1.22 m
Sampling Interval
13.5 km
Reference Sensor Attributes
Has both imager and moisture sounding channels
65 degree orbit inclination provides regular daily coincident overpasses with other sensors
Very well calibrated and stable

8. TRMM TMI V8 Calibration Updates
Spacecraft attitude (updated: PR roll estimates, sun-sensor data, gyro data)
View-angle offsets (updated: roll, pitch, ½ cone, start angle, timing)
Updates to TMI along-scan correction (updated cold/warm scene)
RFI in cold reflector correction (new)
TMI antenna-pattern correction (updated)
TMI reflector emisivities (updated)
TMI hot-load correction (new)
TMI reflector temperature (updated)

9. TMI Along-Scan Correction Cold and Warm Scenes
*from Rachael Kroodsma, NASA GSFC Precipitation Processing System/University of Maryland

10. TMI Along-Scan Correction Cold and Warm Scenes
(pitch/roll offset applied)
*from Rachael Kroodsma, NASA GSFC Precipitation Processing System/University of Maryland

11. TRMM TMI V8 Geolocation/Pointing
Pitch/roll offset: -0.08/-0.08
Swath
Cone Angle
Start Angle
First pixel time
S1 (10v)
49.40
63.91
S1 (10h)
49.50
S2 (19, 21, 37)
49.28
64.36
0.1272
S3 (85)
Wentz 2015 numbers
Channel
Cone Angle
Start Angle 10
49.43
63.70
19-85
49.30
64.10
TMI forward – aft look Tb differences before (top) and after (bottom) adjustments for the 10 GHz v-pol and h-pol channels.
*from Rachael Kroodsma, NASA GSFC Precipitation Processing System/University of Maryland

12. TRMM TMI Emissivity and Spillover Values
(Derived from Deep Space Calibration Maneuvers)
Channel
Pre-launch spillover
RSS-Wentz spillover
UCF-CFRSL spillover
RSS-Wentz emissivity
UCF-CFRSL emissivity
10.65 V
10.65 H
19.35 V
19.35 H
21.30 V
37.00 V
37.00 H
85.50 V
85.50 H
*from Faisal Alquaied and Linwood Jones, University of Central Florida

13. TMI Hot Load Correction: Post-boost (2003-2013)
Sun beta angle K
Phase from orbit midnight (deg)
*from Faisal Alquaied and Linwood Jones, University of Central Florida

14. Main Reflector Physical Temperature (2003-2013, yaw 0)
Before Th correction
After Th correction
Sun beta angle
Time Since Eclipse (min)
*from Faisal Alquaied and Linwood Jones, University of Central Florida

15. 1D Physical Reflector Temperature Table (Beta Angle = 43)
Earth shadow (Eclipse)
Post-boost
array shadow
(post-boost)
Post-boost
( )
array shadow
(pre-boost)
Pre-boost
(2000-Jul 2001)
Tphy, K
Time since eclipse (min)
*from Faisal Alquaied and Linwood Jones, University of Central Florida

17. TMI 21.3 GHz V-Pol vs. GMI 23.8 GHz H-Pol
XCAL TMI/GMI Calibration Differences Radiative Transfer Model Dependency
TMI 21.3 GHz V-Pol vs. GMI 23.8 GHz H-Pol
MonoRTM
Absorption
Rosenkranz
Absorption
Raises the issue of consistency. If XCAL uses MonoRTM for calibration, but retrieval algorithm uses other model, inconsistency will lead to retrieval errors.

18. XCAL TMI/GMI Calibration Differences Radiative Transfer Model Dependency

19. Summary
GMI is extremely well calibrated and stable, providing an ideal calibration reference for the constellation radiometers. Potentially an excellent absolute calibration reference!
The current operational Level 1C Tb datasets for the radiometer constellation have been intercalibrated to GMI V4 and is publicly available from NASA PPS. (Berg et al. 2016: Intercalibration of the GPM Microwave Radiometer Constellation, J. Atmos. Oceanic Technol.)
GPM V5 reprocessing (April 2017)
Changes to the GMI calibration, primarily to low frequency channels (~1K max from V4)
Associated updates to the GPM constellation radiometers
TRMM V8 reprocessing (Late 2017)
TMI calibration updates
Intercalibrated to GMI V5
Goal is high-quality TMI/GMI reference dataset from December 1997 to present
Constellation sensors back to December 1997 (SSM/I, AMSR-E, AMSU-B, etc.)
Lessons
Importance of on-orbit calibration maneuvers
Value of multiple approaches for calibration analysis
Importance of transparency
Value of working with instrument developers to identify instrument issues
Important to identifying/correct errors based on root cause