Development of a L-band On-orbit Calibration Reference Model for the Marie-

Byrd Antarctic Region: Application to Aquarius, SMOS and SMAP

Shannon Brown and Sidharth MisraJet Propulsion Laboratory, California Institute of Technology

L-band Radiometer Calibration

• Calibration at L-band has become an important issue: SMOS, Aquarius, SMAP

– These radiometers use an internal calibration approach; internal switches and noise diodes

– Requires external end-to-end calibration reference – pre-launch and on-orbit

• Calibration techniques developed for radiometers on-board satellite altimeter missions applicable

– Altimeter radiometers also employ internal calibration

– For Jason series radiometers, calibration referenced to stable on-Earth references

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TB References 18-37 GHz

• Tune TB to hot and cold absolute brightness temperature references

– Vicarious Cold Reference (Ruf, 2000, TGARS)

• Stable, statistical lower bound on ocean surface brightness temperature

– Amazon pseudo-blackbody regions (18-40 GHz) (Brown and Ruf, 2005, JTECH)

• THOT(frequency, incidence angle, Local Time, Time of year)

SSM/I 37.0 GHz V-pol – H-pol TB

Hot Reference Targets

• Techniques recently used to generate 13-year climate data record from Topex radiometer data (Brown et al. 2009, JTECH)

• Used on-Earth references to remove long-term drift, instrument temperature dependence and antenna pattern correction errors

Developing On-Earth TB Calibration References at L-band

• Natural targets for L-band radiometer calibration over on-Earth dynamic range

– Calm, flat ocean scenes – Cold reference

– Ice sheets: Antarctica (e.g. Dome-C), Greenland – Mid-range reference

– Land areas: flat, dry deserts; homogeneous heavily vegetated regions – Hot reference

• Use to assess absolute calibration, monitor stability and assess residual instrument calibration errors

37 V-H

23 V-H

18 V-H

10 V-H

6 V-H

AMSR-E De-polarization

• Use match-ups between Aquarius and ocean altimeters to identify observations over calm seas

• Compare Aquarius to modeled TB

– Calm ocean surface reduces model uncertainty – nearly specular emission

– Model inputs (e.g. SST, SSS) from ancillary data sources

– Sort comparisons to identify residual errors in corrections (e.g. solar, galactic, ionosphere)

• Significant number of match-ups with minimal temporal and spatial difference (1 hr/100km)

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Cold TB Reference

Number of match-ups per 1o bin – all horns

Cold Scene Stability Monitoring

• Used simulated data to assess resolution of method

• Compare TBs to model to look for jumps/drifts

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Daily StdDev Horn 1 Horn 2 Horn 3

H-pol 0.26 K 0.20 K 0.24 K

V-pol 0.23 K 0.26 K 0.21 K

H-pol/V-pol Double Difference Daily StdDev

Horn 1 0.08 K

Horn 2 0.07 K

Horn 3 0.13 K

Over range of 0 < WS < 5 m/sAssumes 0.5C SST knowledge and 0.5psu SSS knowledge

Inter-channel double difference :

Horn 1

TBV – Model : Horn 1

Antarctic Calibration Reference

• Recent work has shown Dome-C as suitable candidate for an on-Earth L-band reference (Floury et al., 2002; Macelloni et al. 2006 ; Macelloni et al. 2007)

– Region is heavily instrumented and studied, but small in size

• Particularly for Aquarius, larger site desired due to fixed independent radiometer beams

• Used AMSR-E to search for other suitable Antarctic calibration sites

– Identified other regions with low spatial and temporal variability of surface and deep ice temperature

Aquarius 3-beams

Temporal stability at 6 and 37 GHz

6.9 GHz 37 GHz

• Regions below 0.5K std.dev chosen for 6GHz • Regions below 4K std.dev chosen for 37GHz

• Spatial stability of region evaluated by searching for contiguous thermally stable sets within a 150km radius

Marie Byrd Region:

• Marie-Byrd region identified as suitable site

• Approximate area of stable region ~160,000km2

• Two automated

weather stations (AWS) in region

Characteristics of Marie-Byrd Region

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• Accumulation in Marie-Byrd region ~30cm/yr, higher than in East Antarctica• Gentle upward slope from north to south across the region• Surface density ~350kg/m-3 with firn-ice transition around 64 m (Gow 1968)

Accumulation Rate

Cuffey and Patterson 2010

Rubin and Giovinetto 1962

Characteristics of Marie-Byrd Region

• Warmer surface temperatures in Marie-Byrd region than East Antarctica

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Cosimo 2000

Mean Surface Temperature

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6.9 GHz H-pol

6.9 GHz V-pol

37 GHz H-pol

37 GHz V-pol

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37 GHz H-pol

37 GHz V-pol

6.9 GHz H-pol

6.9 GHz V-pol

Marie-Byrd vs. Dome C

Marie-Byrd

Dome-C

Marie-Byrd Site

Dome-C Site1

Tb Variability 6GHz <0.5K <0.5K

Tb Variability 37GHz <4K ~8K

Area ~160,000km2 ~10,000km2

Aquarius Overpass (rad – 1) 155 samples/day 43 samples/day

In-Situ equipment Less More

Tb37_pp = 15K1Macelloni (2007)

Tb37_pp = 30K

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AMSR-E V-pol Dome C

AMSR-E H-pol Dome C AMSR-E H-pol MB

AMSR-E V-pol MB

`

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AMSR-E 6 GHz V-pol AMSR-E 6 GHz H-pol

AMSR-E 37 GHz H-polAMSR-E 37 GHz V-pol

Long Term Temperature Stability at Marie-Byrd

• Annual averaged surface temperature from Byrd AWS stable to ~1C from 1980 – 2000

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• AMSR-E 6 GHz TB stable to ~0.2K from 2003 to 2011

• 6 GHz TB stable to <0.1K over last 5 years

Development of Coupled Forward Model

• Use model to transfer calibration from higher frequencies radiometers to L-band

• Model couples an ice heat-transport equation and radiative-transfer equation

– Constrain model using AMSR-E and in situ AWS data

• constrain density profile, temperature profile and grain size

– Predicts brightness temperature at L-band

• Tracking calibration stability over time

– Sensitive to heat-transport model, but temporal variability small

– Lower uncertainty on monthly or longer time scales

• Inter-satellite calibration

– Use region to assess calibration between sensors – daily observations

– Model used to account for differences in incidence angle

• Assessing absolute calibration

– Uncertainty dependent on radiative transfer model

– Evaluate several models to estimate uncertainty

Use as a calibration reference

summerwinter

autumnspring

2

2

z

T

c

k

t

T t

Surface temperature values obtained from AWS stations used as top boundary condition, with its mean as the bottom boundary condition

Only considered annual harmonic

Used simple radiative transfer model assuming layered ice to estimate L-band and C-band V-pol TB annual signal

• df

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6.9 GHz H-pol

6.9 GHz V-pol

SMOS 55o H-pol

SMOS 55o V-pol

Time Series Comparison

• Monthly averaged SMOS TB at 55o incidence angle compared to AMSR-E 6.9 GHz channel for June 2010 to June 2011

• Observed annual signal at L-band higher than expected

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AMSR-E 6.9 GHz and SMOS V-pol AMSR-E 6.9 GHz and SMOS H-pol

Conclusions and Future Work

• Marie-Byrd region identified as a candidate L-band calibration site in West Antarctica

– Large in extent, thermally stable

– Complementary to Dome-C site

• Analysis of AMSR-E indicates good long term stability of region

• Developing model to transfer calibration from higher frequency radiometers (e.g. AMSR-E, WindSat) to L-band over region

Next steps

• Evaluate several radiative transfer models, constrain using AMSR-E

• Inter-compare Aquarius and SMOS over region

• backup

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TB References• Tune TB to hot and cold absolute brightness temperature references

– Vicarious Cold Reference (Ruf, 2000, TGARS)

• Stable, statistical lower bound on ocean surface brightness temperature

– Amazon pseudo-blackbody regions (18-40 GHz) (Brown and Ruf, 2005, JTECH)

• THOT(frequency, incidence angle, Local Time, Time of year)

Sample references over various instrument temperature

states– Multiple independent

estimates– Additionally, constrain

solution to minimize slope of RMS error vs. instrument temperature

SSM/I 37.0 GHz V-pol – H-pol TB

Hot Reference Targets

Salinity Retrieval Validation

• Form database of Aquarius co-locations with in situ data

– Argo float array, the Shipboard Sensor Database (SSD) and the Global Temperature-Salinity Profile Program (GTSPP).

• Analyze global mean differences between Aquarius retrieved salinity and in-situ measurements

– Assess over time and instrument temperature

• Analyze regional differences– Assess antenna pattern correction, faraday

rotation correction and atmospheric and roughness corrections

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Faraday Rotation Correction

• Dual-frequency altimeter match-ups also useful for assessing Faraday rotation correction

• Match-ups cover large range of TEC values

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Aquarius orbit overlap

Calibration Approach for Aquarius

-8

-4

0

4

8

12

16

20

0 20 40 60 80 100 120 140 160

JMR(VA)-GPSJMR(VA)-SSMIJMR(VA)-TMIJMR(VA)-Model

Jason Repeat Cycle

Cycle 31

Cycle 68

Monitor instrument level parameters and diagnostics

Compare brightness temperatures to natural on-Earth reference targets

Compare retrievals to in

situ ground truth or models

Objective is to identify the cause of any observed calibration or retrieval error or instability in order to apply a suitable correction at the appropriate level of processing

AMSR-E vs AWS: Yearly Temperatures (2008)

• Data curve fit to the following equation, with a period of 365 days

tctctccT fit 3210 )cos()sin(

Annual Temperature Variations

• Annual amplitude decreases and phase lag increases with decreasing frequency

• Penetration depth near 10 m at 6 GHz

• Nearly 50 m at 1.4 GHz

Radiative Transfer Model

• Used simple radiative transfer model to estimate L-band TB from estimated temperature vs depth

– Snow is assumed to be dry and pure

– Density was varied from 0.3g/cm3 to 0.916 g/cm3 to fit with AMSR-E values

– Simple empirical scattering correction

0.17 K peak to peak

1.4 GHz

6.8 GHz

10.7 GHz

18.7 GHz

36.5 GHz