Radiometry and Uncertainties from SORTIE

(Spectral Ocean Radiance Transfer Investigation and Experiment)

Kenneth Voss and Howard Gordon, Univ. of MiamiMarlon Lewis, Scott McLean and Mike Twardowski, WetSat

Carol Johnson, NISTMark Yarbrough, Stephanie Flora and Mike Feinholz, Moss Landing Marine Lab

Chuck Trees, NURC, NATO

(SORTIE also includes Ron Zaneveld, Andrew Barnard, Susanne Craig)

2008 NASA Carbon Cycle and Ecosystems Joint Science Workshop, May 1

Sortie Goals

• To test radiometric techniques to augment the current vicarious calibration methods.

• The basic idea is to use measurements of inherent optical properties (IOP’s) to extend the radiometric measurements and determine subpixel variability

• To determine the suitability/ability to collect a usable vicarious calibration data set in Case II waters, in the best possible case.

SORTIE method• Start with the very well characterized radiometric

instrumentation• Collect a complete suite of radiometric and

apparent optical property (AOP) measurements during overpass time (Lu, Ed, L(,), KLu, KEd, +)

• Collect IOP’s synchronous with these measurements, and then use a towed vehicle to investigate the IOP field around the measurement site before and/or after the overpass

First experiment in Hawaii, second in San Diego

• Start with clear water experiment, cross-over with MOBY and the MOBY team (experiment done last March)– If you can’t make clear water work, Case II is crazy

• Move to coastal region, San Diego (experiment done in January)– More difficult optical field (variation in IOP’s within

pixel)– Important for vicarious cal…different “colored”

sources…out of band issues accentuated

Hawaii experiment

• Cross over with the MOBY buoy, the “gold standard” for vicarious calibration

• Vicarious calibration in clear water, clear atmosphere

• Laboratory component, before field work, to set a baseline….if it doesn’t work in the lab why expect it to work at sea.

Responsivity uncertainties from laboratory calibration:Temperature uncertainty is after correction, as is stray light.

HyperOCR Uncertainty Budget (Lu) – Preliminary

Uncertainty Component (%) 412.4 nm

442.7 nm

498.8 nm

530.3 nm

547.1 nm

665.1 nm

Comments

Radiometric Calibration

NIST Spectral Irradiance 1.05 1.04 1.01 0.98 0.97 0.92 NIST Report

Reflectance Target (0/45) 1.52 1.52 1.52 1.52 1.52 1.52 Labsphere Report

Transfer to HyperOCR

Radiometric Transfer 1.73 1.73 1.73 1.73 1.73 1.73 SIRREX-7 TM - typical uncertainty estimate

Interpolation 0 0 0 0 0 0 4 Point Lagrange – no change in original data

Reproducibility 0.46 0.76 0.81 0.89 0.91 1.04 Pre-post SORTIE-1 cals

Wavelength Accuracy 0.28 0.27 0.18 0.15 0.13 0.07 0.06nm accuracy

Stray Light 0.10 0.09 0.08 0.02 0.04 0.09 3 SIRCUS realizations in SLC matrices

Temperature 0.01 0.00 0.01 0.02 0.03 0.07 estimate of uncertainty in thermal correction at 25.5C

Combined Standard Uncertainty (Lab)

2.59 2.65 2.65 2.65 2.66 2.69 %

Laboratory comparison experiment

•Compare MOBY and Satlantic E and L instruments•Look at radiometric calibration sources•Look at filtered sources, which can point out issues with out-of-band or stray light.•If it doesn’t work at sea, data set gives another place to look.

Some examples, L from MOBY and Satlantic HPL

1.21.00.80.60.40.20.0

Radi

ance

( W

cm

-2nm

-1sr

-1)

1000800600400Wavelength (nm)

Radiance: OL420 NIST MOBY Middle Arm MOBY Top Arm Satlantic HPL180

-4

-2

0

2

4

% D

iffer

ence

1000800600400Wavelength (nm)

Radiance: OL420 MOBY Middle Arm MOBY Top Arm Satlantic HPL180

-4

-2

0

2

4

% D

iffer

ence

1000800600400Wavelength (nm)

(MOBY-HPL180)/NIST

Colored Radiance source0.10

0.08

0.06

0.04

0.02

0.00Rad

ianc

e (

W c

m-2

nm-1

sr-1

)

650600550500450400Wavelength (nm)

OL420 with BG28 Filter NIST OL420 BG28 Satlantic 180 MOS

-10

-5

0

5

10

% D

iffer

ence

600550500450400Wavelength (nm)

MOS Satlantic 180

-4

-2

0

2

4

% D

iffer

ence

600550500450400Wavelength (nm)

(MOS-HPL180)/NIST

Irradiance, standard calibration source

20

15

10

5

0

Irra

dian

ce (

W c

m-2

nm-1

)

1000800600400Wavelength (nm)

NIST F431Satlantic HSE192 MOBY 237 -3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

% D

iffer

ence

1000800600400Wavelength (nm)

MOBY 237 Satlantic HSE192

-3

-2

-1

0

1

2

3

% D

iffer

ence

1000800600400Wavelength (nm)

(MOBY-HSE192)/NIST

At Sea comparison• Obviously harder….as close to

contemporaneous as possible (within 900m)• Newest Satlantic algorithm (see Scott

McLean’s poster)• Day shown was very clear (cloud free) and

calm

MOBY and Satlantic Hyperpro1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Upwe

lling

radi

ance

(uW

/cm

2 nm

1 sr

1)

900800700600500400Wavelength (nm)

Satlantic Cast 00:25:10 Satlantic Cast 00:31:29 MOBY 00:25:00 Lw calculated with propagating top arm MOBY 00:19 Lw calculated by propogating mid arm

Different view20

15

10

5

0

-5

% D

iffer

ence

700650600550500450400Wavelength (nm)

2*(HPL180-MOBY)/(HPL180+MOBY)*100 New Satlantic Multicast processing

Still working…..

• But the SORTIE data set, very high quality IOP and AOP, gives us all sorts of other places to explore:– Nurads Radiance distribution with MASCOT VSF

(and other Wetlabs IOP’s)– Inversion of Satlantic profile to get IOP’s to

compare with measurements

Predicting Radiance distribution with VSF

Inversion techniques to go from AOP’s to IOP’s

• Based on algorithms detailed in: – H.R. Gordon and G.C. Boynton, “A radiance -

irradiance inversion algorithm for estimating the absorption and backscattering coefficients of natural waters: Stratified water bodies”, Applied Optics 37, 3886-3896 (1998).

– G.C. Boynton and H.R. Gordon, “An irradiance inversion algorithm for absorption and backscattering coefficients: Improvement for very clearwaters” Applied Optics, 41, 2224—2227 (2002).

• Really sensitive to reflectance and gradients (K) so is not dependent on absolute radiometry

Example inversion of single wavelength

Fairly constant offset on order of 0.005 m-1, within published instrument uncertainties

Hyperspectral bb and a inversion

Hyper spectral c from inversion(?)

No real reason it should be this good. b estimate depends on phase function assumed..

San Diego Experiment

• Have most of the data processed, not all• Some very good days• a good contrast to the Hawaii data set in

terms of optical properties in the water.

Conclusions• These are preliminary results. Still determining, in a collaborative

manner, if there are areas for improvement.• The laboratory intercomparisons between MOBY, Satlantic, NIST

radiance and irradiance sensors are quite good and within 2%, well within the instrumental uncertainty budget.

• There are some biases between MOBY and HyperPro derived water-leaving radiances in the field data, which remain unexplained at this point, but are likely due to differences in deployment and environmental variations.

• Measured IOP's plus RTE (both Gordon and Morel approaches) provide excellent estimates of the upwelled radiance distribution.

• The inversion of the HyperPro AOP profiles provides a remarkably good agreement with direct measurements of IOP's.

• While the agreement for c is fortuitous, the agreement with a and bb emphasizes the excellent calibration (and baseline correction) of the AC-9 for use in these very clear waters.

• Next step is to apply same approach to San Diego Data set.• We will be doing another field experiment in the fall in the Ligurian

Sea, near the Bousolle site.