matt sapiano1 wes berg - cnripwg/meetings/saojose-2012/training/sa... · 2016. 6. 7. · • xcal...

41
Satellite intercalibration activities and methods for window channel radiometers Matt Sapiano 1 and Wes Berg 2 1 University of Maryland 2 Colorado State University 1 [email protected] 1

Upload: others

Post on 18-Mar-2021

1 views

Category:

Documents


0 download

TRANSCRIPT

Page 1: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

Satellite inter‐calibration activities and methods for 

window channel radiometers

Matt Sapiano1 and Wes Berg21University of Maryland

2Colorado State University

1

[email protected]

1

Page 2: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

The Global Precipitation Mission

CONSTELLATION SATELLITES

• ~8 satellites• Radiometer only• Rely on existing radiometers• Multifrequency radiometer• ~3 hour average revisit time

CORE SATELLITE

•Dual frequency radar•Multifrequency radiometer•Non‐sun synchronous orbit• ~70 deg inclination•~400km altitude•~4km horizontal resolution•250m vertical resolution

MISSION: Understand the horizontal andvertical structure of rainfall and itsmicrophysical elements. Provide trainingfor constellation radiometers

MISSION: Provide enough sampling to reduceuncertainty in short‐term rainfall accumulations.Extend scientific and societal applications.

Page 3: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

Outline

• Introduction• Overview of NOAA CDR Project• XCAL and other intercalibrationactivities

• Cross‐track biases• Ephemeris and geolocation errors• Diurnal variability• Monitoring of changes in TBs over time

• Approaches to intercalibration• Summary

Page 4: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

Sample products from passive microwave radiometers

Page 5: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

Currently Available Radiometer Data

Page 6: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

Differences in Sensor Characteristics

Sensors for GPMSensor Channel Frequency (GHz) Inc Ang

TMI 10.7 19.4 21.3a 37 85.5 53.4°

AMSR‐E 6.9 10.7 18.7 23.8 36.5 89 55.0°

SSM/I 19.4 22.2a 37 85.5 53.1°

SSMIS 19.4 22.2a 37 91.7 150b 183.3 ±1,3,7b

53.1°

WINDSAT 6.8 10.7c 18.7c 23.8 37c 49.9°‐55.3°a V‐Pol only;   b H‐Pol only;   c V, H, +/45º, L, R

Page 7: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

Effect on precip: SSM/I Calibration

Page 8: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

FCDR for SSM/I & SSMIS at CSU

• Produce two sets of NetCDF4  files– Basefiles include granularized raw data 

with duplicates removed – FCDR files include intercalibrated TBs 

with new geolocation, QC, etc.• Produce well documented software 

package (“Stewardship code”) that ingests basefiles, applies corrections and outputs the FCDR

• Generate a transparent and documented FCDR of SSM/I and SSMIS brightness temperatures (Tb) from 1987 – Present

• Tb will be Intercalibrated for SSM/I and SSMIS– Tb will be physically consistent: time‐of‐day bias and EIA dependence not removed

Page 9: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

SSMI(S) FCDR Intercalibration• Sensor data must be physically consistent

– Sensor still different due to differences in frequencies, view angles, resolution, observation time etc.

– Geophysical retrieval algorithms must take into account sensor differences in order to create TCDR (eg: precipitation, wind, humidity, etc).

• Investigate calibration differences using multiple approaches [described later].– Consistency/inconsistency between approaches will provide insight into methods as well as estimate of the uncertainties.

• Goal is to understand any differences and use sensor information to select correct solution

Page 10: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

Known issues/risks• Inconsistency between Calibration Methods

– Using multiple approaches will help to identify problems and estimate uncertainties.

– Ultimately stewardship must allow for future investigators to look into discrepancies and develop new/improved techniques/approaches.

• Geolocation Errors– Difficulty in separating effects of spacecraft orientation from other 

issues.– Limitations in determining spacecraft attitude

• Other sensor Issues– Warm versus cold end calibration, nonlinearities etc.– Unknown error sources that can be difficult/impossible to 

quantify/correct.

Page 11: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

Xcal and other intercalibration activities

• XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose goal is to intercalibrate available radiometers for GPM.– Meets ~2‐3 times per year– Comprised of multiple institutions/groups who have developed 

different techniques for intercalibration.– Working group consists of experts in both engineering and science 

application aspects of radiometers– Responsible for developing intercalibration adjustments for Level 1C 

Data (Intercalibrated TB dataset for GPM).• GSICS – Global Space‐based Inter‐Calibration System

– Effort is focused on operational weather satellites and funded by operational agencies

– http://www.wmo.int/pages/prog/sat/GSICS/

Page 12: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

Cross Track Biases

• Caused by partial beam blockage or other (unknown?) issues

Page 13: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

Spacecraft ephemeris and pixel geolocation: SSMI/SSMIS FCDR

• Many operational satellites such as SSM/I on board the DMSP spacecraft use predicted ephemeris.

• Spacecraft ephemeris are recomputed using 2‐line element data and SGDP4 code.

• Pixel geolocation depends on accurate spacecraft ephemeris and pointing information (spacecraft attitude and sensor alignment).

• Retrieval algorithms are often sensitive to the EIA or view angle of the sensor.

Page 14: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

Predicted vs computed spacecraft ephemeris

Computed values derived from SGP4 NASA/NORAD code.

Page 15: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

Estimation of Satellite Attitude• Satellite attitude offsets required to accurately calculate geolocation– Roll, Pitch, Yaw

• Earth Incidence Angle (EIA) was not included in original data– EIA is needed for intercalibration and geophysical retrievals

– Estimates of EIA are grossly inaccurate without satellite attitude estimates

Change in Calibration Using Calculated EIA with zero roll/pitch/yaw vs. Nominal

100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280Brightness Temperature (K)

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

Cha

nge

(K)

19.35 GHz V-Pol19.35 GHz H-Pol22.235 GHz V-Pol37.0 GHz V-Pol37.0 GHz H-Pol85.5 GHz V-Pol85.5 GHz H-Pol

Page 16: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

Geolocation and solar angles• Original data source (TDR files) did not 

include Earth Incidence Angle (EIA) or Sun glint

• Used Two‐Line Elements (TLEs) with NORAD SGP4 code to add Spacecraft Position and Velocity vectors to Basefiles

• Stewardship code uses SCpos and SCvel to calculate ephemeris (SClat, SClon, SCaltitude) for every scan, as well as pixel lat/lon, EIA and sun glint angle for every pixel– Stewardship code can also be set to output 

satellite azimuth, solar zenith/azimuth and time since eclipse

Page 17: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

Methodology: Coastline analysis• Used mean of ascending TBs minus mean 

descending to assess Geolocation error – Used 11 months of 85H TBs mapped onto 

1/20th degree grid– Before corrections: coastlines are visible 

as red and blue bands because A&D do not line up

• Differences in A‐D due meteorological events unaffected by small changes in geolocation

• Root mean square error (RMSE) of the A‐D used as a metric for geolocationaccuracy

• Fast iterative technique developed to find optimal roll, pitch and yaw– Involves estimating RMSE associated with 

many RPY combinations

Page 18: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

SSM/I Satellite Attitude Estimates

• Attitude is relatively stable over the life of each spacecraft– F08 and F10 are less stable than 

F11‐F15• Technique works well

– Yields Time varying attitude with estimated accuracy within ~0.1 degrees.

• Determination of high‐frequency changes not possible with this approach– Expected impact for climate is 

generally negligible

Page 19: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

Impact on Geolocation• Trends in EIA or differences 

between sensors can introduce biases if not properly accounted for

• EIA variability over orbit (driven by altitude) may also be important– Particularly true for F10 due 

to its more eccentric orbit• Significant differences in EIA 

for each sensor will impact intercalibration– Probably also important for 

some geophysical retrieval algorithms

Time Series of Mean Earth Incidence Angle for SSM/I Sensors

52.8

52.9

53.0

53.1

53.2

53.3

53.4

53.5E

arth

Inci

denc

e A

ngle

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

Year

Time Series of Equatorial Crossing Spacecraft Altitude for SSM/I Sensors

725

750

775

800

825

850

875

Alti

tude

(km

)

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

F08F10F11F13F14F15

Page 20: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

EIA effect on retrievals

SSM/I Precip (GPROF2010) SSM/I TPW (OE)

Constant nominal EIA 

(53.1)

Calculated EIA 

Page 21: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

TRMM V7 Correction and Angle IssuesPresentation material for GPM inter‐calibration working group meeting, May 19, 2009

Sayak Biswas, Dr. Linwood JonesUCF, CFRSL

Dr. Kaushik GopalanUMD, ESSICSteve BilanowWyle, PPS

Page 22: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

Time Since Eclipse Start(min)

Sola

r Bet

a A

ngle

(deg

)TMI Bias Table (post boost data)

10 20 30 40 50 60 70 80 90

60

50

40

30

20

10

0

-10

-20

-30

-40

-50

-60

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3

Earth Shadow Exit

Space Craft Aft‐Structure Shadowing

TB Bias (K)

Page 23: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

0 20 40 60 80 100-0.5

0

0.5

1

1.5

Orbit Time Since Eclipse Start (min)M

ean

Bia

s (K

)

450 < < 460

0 20 40 60 80 100-1

0

1

2

Orbit Time Since Eclipse Start (min)

Mea

n B

ias

(K)

-570 < < -560Time Since Eclipse Start(min)

Sola

r Bet

a A

ngle

(deg

)

Filtered Table (PostBoost)

10 20 30 40 50 60 70 80 90

60

50

40

30

20

10

0

-10

-20

-30

-40

-50

-60

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3

Eclipse = 31min

Eclipse = 26.4 min

Page 24: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

Time of Day Dependence

0 5 10 15 20-1

-0.5

0

0.5

1

1.5

Local Time of The Day at Observation Point (hour)

Mea

n B

ias

(K)

Time of Day Dependence (2006)

BEFORE CORRECTIONAFTER CORRECTION

Page 25: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

Diurnal Impacts

• It is important to properly account for these differences when comparing TBs from different satellites

• The intercalibrated brightness temperatures (i.e. FCDR) should retain these differences.

Page 26: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

Window Channel Radiometer Record

Page 27: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

Monitoring TBsSSM/I F15 RADCAL issue: beacon was switched on in 2006, lead to 

severe interference– Important to monitor satellites for calibration issues

RadCal Beacons Turned on

22 V

85 H

19 V

Page 28: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

Intercalibration• Satellite providers deal with the absolute calibration of satellite observations

– Errors can be large (1‐2K)– Providers do not use same technique or standard so absolute cals differ

• Projects such as NOAA SDS and NASA GPM need consistent estimates from several satellites– Seek to intercalibrate the satellites to within ~.1‐.2K– Goal is to estimate adjustments to TBs for each channel so as to make observations 

consistent with some calibration standard– Calibration standard choice not easy and might be arbitrary/political– Aim to make sensors physically consistent (not correct channel diffs, etc)

• Intercalibration often takes the form of simple offsets, although TB dependent adjustments may be more appropriate– How do we calculate TB‐dependent adjustment?  Need to use techniques that cover a 

large portion of TB space– Need at least 2 points: low and high TB 

Page 29: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

Intercalibration techniques• Different satellites have different sampling characteristics, so intercalibration has 

to use a common target– May need to account for Incidence angle, channel differences

• The following is not an exhaustive list, but does encompass the techniques used by XCal and for the CSU SDS project

1. Direct comparison of satellites being intercompared− For polar orbiters, this takes the form of polar matchups− Matchups are available elsewhere if one of the satellites is not in a non‐Sun‐

synchronous orbit (TRMM, GPM core)2. Comparison of each satellite with a non‐Sun synchronous satellite3. Comparison with a fixed target

− Target could be an in situ observation related to TBs by a geophysical retrieval− Target could be a strictly homogeneous area (not many of these exist)

4. Calculation of some fixed reference point with known properties− Only example of this is from Vicarious calibration

Page 30: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

SSM/I Intercalibration

• As part of creation of SSM/I FCDR, require intercalibration estimates for F8‐F15

• Applied 5 different techniques to SSM/I– Model double difference– Polar Crossovers– TMI matchups– Vicarious Cold Calibration– Amazon Warm Calibration

• Strategy is to seek agreement between methods and use to estimate uncertainty of intercalibration

Page 31: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

1. Model Double Difference• Simulate TBs using model (reanalysis) data at 

1 degree as an input to radiative transfer model– Match with gridded observed TBs– Screening used to remove land/coast– Also screen rain and clouds based on model 

(liquid water) and satellite (SD85: standard deviation of 85Ghz in 1 degree box)

• Two implementations based on– NASA MERRA – ECMWF ERA‐I

• In this case, the model is used as a transfer standard from satellite to satellite– Satellites are not co‐located in time or space 

in this technique

Page 32: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

2. Polar Crossovers• Gridded SSM/I data onto equal area polar grid (~100km grid box size)• Extracted coincident overpasses (within 30 mins) for two SSM/I

– Matchups occur only at the poles• Used standard RT model with MERRA atmosphere to correct EIA bias 

over open ocean

F13 F15 Open Ocean (1day)

Page 33: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

3. TMI Matchups• Gridded to 1 degree and extracted 

coincident overpasses of SSMI and TRMM TMI– Occur only in the tropics

• Estimated sensor differences between TMI and SSM/I (EIA, channel diffs) using three methods– Optimal Estimation (OE): estimate 

geophysical profile from TMI; simulate TBs from TMI and SSMI; use this to estimate sensor diffs

– MERRA: use model profile to simulate TBs from TMI and SSMI; use this to estimate sensor diffs

– ERA‐I: use ERA‐I instead of MERRA• All three implementations use the 

same RT model

SSMI and TMI Ground tracks for  2‐3am on 20 Nov 2004

Page 34: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

4. Vicarious Cold Cal• For each frequency and polarization, 

there is a fixed surface temperature at which the minimum TB occurs– TB minimum is affected by water vapor, 

so finding minimum can be challenging in practice

• Vicarious Cold Cal technique involves finding this minimum– Build histogram of TBs from large amount 

of data (~1 year)– Screen high water vapor pixels (cloud, 

rain, etc) – Estimate minimum TB from histogram

• Use OE or Merra/ERA‐I data to correct for EIA of each sensor since TB minimum is EIA dependent Ruf, 2000; IEEE Trans. Geosci. Rem. Sens., 38(1), 44‐52

Vicarious Cold Calibration technique developed at U. Michigan by Prof 

Chris Ruf

Page 35: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

5. Warm Calibration• Other techniques give estimates at cold end of TBs 

– really need an estimate at warm end– This would allow estimation of gain (ie: slope)

• Warm Calibration technique developed by UMichgroup uses Amazon rainforest – This is a warm, (near) black‐body (= unpolarized) 

target• First, estimate surface (temperature and 

emissivity) and atmospheric (water vapor, cloud liquid water) parameters– Forward RT model used with non‐linear least 

squares fit• Forward model then used to remove EIA 

differences– Diurnal cycle correction also necessary

• Technique applied to SSM/I by Darren McKague(U. Michigan)

Brown and Ruf, 2005; J. Atmos. Ocean. Technol., 22(9), 1340–1352

Sample Amazon Targets

Page 36: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

Combining aproaches• Have estimates from five techniques, most with several realizations of sensor differences

Technique SimulationofEIAdifferences

Sensorscovered

Notes

PolarMatchups

ERA‐I(on1 equalareagrid) F10,F11,F13,F14,F15

Direct overlapsnearpoles;restrictedtoopenoceanonly

ReanalysisTransfer

Merra,ERA‐I(bothon1grid)

F08,F10,F11,F13,F14,F15

SimulateTbfromreanalysismatchedtoSSM/I;modelistransferstandard

TMIMatchups

Merra,ERA‐I,OptimalEstimation(allon1 grid)

F11,F13,F14,F15

Crossoversbetween TMIandSSM/I;usedTMIasatransferstandard

VicariousCold

Merra,ERA‐I(1),OptimalEstimation(atnativeres.)

F08,F10,F11,F13,F14,F15

Estimate stableminimumTboverclear‐skyopen‐ocean

AmazonWarm

PhysicalmodeldevelopedbyBrownandRuf (2005)(atnativeresolution)

F13,F14,F15 Used homogeneousAmazonwarmtarget;usedTMIasatransferstandardtoremovediurnalcycle

Page 37: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

Combined analysis• Plot shows intercal estimates 

for F13‐F15 as a function of  F13 Tb– Vicarious Cold cal and Amazon 

warm cal are valid only for single Tb values

• Agreement is extremely good– Spread amongst colder 

techniques ~0.5K for most channels

– Amazon Warm cal in less good agreement, but has 0.57K error

– Larger (~1K) for F08 and F10 (not shown)

F13‐F15

Page 38: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

Beta (B6) Calibration (April 2012)• Combination of techniques to get Beta Intercal requires some care

– TMI only available for F11, F13, F14 and F15– Amazon Warm Cal technique not available for all satellites (requires TMI to remove 

diurnal effects)– F08 is a special challenge due to lack of overlaps: use VCC and Model

• Since variations of each technique are not independent, average these first, then take mean and SD of four techniques to make Beta Cal– Amazon warm results not used in order to be consistent amongst sensors

SSM/I Beta Cal 19V 19H 22V 37V 37H 85V 85HF08 0.74 ‐0.02 1.53 0.84 1.56 1.98 ‐0.7F10 0.14 ‐0.03 1.38 ‐0.19 0.4 0.44 ‐0.07F11 0.19 ‐0.16 0.15 0.67 0.36 ‐0.55 ‐1.22F13 0 0 0 0 0 0 0F14 0.34 ‐0.2 0.06 ‐0.2 0 0.45 0.35F15 0.29 ‐0.26 ‐0.13 0.24 0 0.26 0.09

Sapiano et al., Towards an Intercalibrated Fundamental Climate Data Record of the SSM/I Sensors, Trans. Geosci. And Rem. Sens., in revision.

Page 39: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

SSMIS Beta Version (April 2012)• SSMIS has several major, known issues

– Worse for F16 than for F17, F18– Preoperational data not used currently

• Applied corrections for– Solar/lunar Intrusions – Emissive antenna

• Estimated attitude for EIA calculation– SSMIS has 6 feedhorns: use common satellite attitude with a fixed 

sensor alignment offset for each feedhorn• Spillover and cross‐pol corrections from operational code 

used, but not scene‐dependent correction.• SSMIS beta intercalibration

– Same approach as used for SSM/I. Only applied to the 7 SSM/I equivalent channels

Intercal 19V 19H 22V 37V 37H 85V/91V 85H/91HF13 ‐ F16 0.67 ‐0.75 0.36 ‐2.04 ‐2.13 1.59 0.38F13 ‐ F17 0.25 ‐0.95 ‐0.4 ‐2.39 ‐1.54 3.59 2.51F13 ‐ F18 0.73 0.28 0.12 ‐1.38 ‐0.76 3.01 2.28

Page 40: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

Intercalibration Issues• Non‐physical trends can exist in 

satellite estimates – Can be difficult to quantify 

these given sampling requirements of some techniques

• No techniques calibrate in rainy part of TB spectrum– Difficult to get idea of how 

calibration should change with TB

– Vicarious warm calibration helps here, but the middle of the spectrum is not sampled

Page 41: Matt Sapiano1 Wes Berg - CNRipwg/meetings/saojose-2012/training/Sa... · 2016. 6. 7. · • XCAL is a working group of the NASA Precipitation Measurement Missions (PMM) project whose

Summary of key issues• A number of specific calibration‐related issues need to be addressed before 

sensors can be effectively intercalibrated (cross‐track biases, geolocation and pointing errors etc.)

• Although conical‐scanning radiometers designed to have same view angle across the scan, differences of as little as 0.1 degrees can impact calibration as well as geophysical retrievals, particularly over oceans.

• Calibration needs to be ongoing as sensors change/degrade, orbits drift, and new sensors are launched

• As demonstrated by biases in TMI due to heating of the antenna (Jones et al.), calibration errors often change over time (as with spacecraft heating) and can mimic real physical signals (i.e. diurnal cycle).

• Using multiple approaches to intercalibration helps to confirm results, identify issues, and determine residual errors.

• As a result, calibration should be considered an ongoing, continually evolving endeavor that involves collaboration by multiple groups with expertise in both the engineering and scientific aspects of the sensors.