# Presentation: 196445

## NASA’s operational approach for the vicarious calibration of 

- on-orbit ocean color satellites (using Gordon and Wang 1994)
- Bryan Franz, Sean Bailey, the OBPG, and Jeremy Werdell
- MODIS Science Team Meeting, 1 Nov 2006

## The vicarious calibration ...

- ... makes use of a single set of fractional gains, where unity indicates no correction
- ... (*L**t** = counts * K**rad** * K**time** * K**x,y,z** *... ** g*
- ...  (minimize difference between satellite *L**w* and ground-truth *L**w*)

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## The vicarious calibration ...

- ... makes use of a single set of fractional gains, where unity indicates no correction
- ... (*L**t** = counts * K**rad** * K**time** * K**x,y,z** *... ** g*
- ...  (minimize difference between satellite *L**w* and ground-truth *L**w*)
- ... modifies the integrated instrument-atmospheric correction system
- ...  (effectively accounts for undetermined post-launch instrument changes
- ... (and atmospheric correction biases)

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## The vicarious calibration ...

- ... makes use of a single set of fractional gains, where unity indicates no correction
- ... (*L**t** = counts * K**rad** * K**time** * K**x,y,z** *... ** g*
- ...  (minimize difference between satellite *L**w* and ground-truth *L**w*)
- ... modifies the integrated instrument-atmospheric correction system
- ...  (effectively accounts for undetermined post-launch instrument changes
- ... (and atmospheric correction biases)
- ... assumes that temporal trends are independently removed

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## The vicarious calibration ...

- ... makes use of a single set of fractional gains, where unity indicates no correction
- ... (*L**t** = counts * K**rad** * K**time** * K**x,y,z** *... ** g*
- ...  (minimize difference between satellite *L**w* and ground-truth *L**w*)
- ... modifies the integrated instrument-atmospheric correction system
- ...  (effectively accounts for undetermined post-launch instrument changes
- ... (and atmospheric correction biases)
- ... assumes that temporal trends are independently removed
- ... was updated for SeaWiFS Reprocessing 4 (July 2002)

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## Lt = Lr,g,wc,... + La + td Lw

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## Lt = Lr,g,wc,... + La + td Lw

- _pre-Reprocessing 4 (launch – July 2002)_
- gain calculated using *L**w*
*g =* *L**w**target** / L**w*
- assign *g* and iterate until mean radiance ratio reaches unity

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## Lt = Lr,g,wc,... + La + td Lw

- _pre-Reprocessing 4 (launch – July 2002)_
- gain calculated using *L**w*
*g =* *L**w**target** / L**w*
- assign *g* and iterate until mean radiance ratio reaches unity
- _post-Reprocessing 4 (July 2002 – present)_
- gain calculated at *L**t*
*g = L**t**target** / L**t*
- satellite atmospheric parameters used to propagate *L**w**target* to TOA
- * no iteration (less computationally expensive)
- * permits exploration of gain offset
- * requires disabling correction for non-negligible *L**w*(NIR)
- methods agree to within 0.04%

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## Lt = Lr,g,wc,... + La + td Lw

*L**t**target** = L**r,g,wc,...** + L**a** + t**d** L**w**target*
- _post-Reprocessing 4 (July 2002 – present)_
- gain calculated at *L**t*
*g = L**t**target** / L**t*
- satellite atmospheric parameters used to propagate *L**w**target* to TOA
- * no iteration (less computationally expensive)
- * permits exploration of gain offset
- * requires disabling correction for non-negligible *L**w*(NIR)
- methods agree to within 0.04%

- all terms are computed for the full relative spectral response of each sensor band

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## Lwn = Lw / ( cos(sun) td Fdist )

**TARGET**

*L**wn* = *L**w* / ( cos(*sun*) *t**d* *F**dist *)

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## Lwn( sun≠0, sens =0, rel =0 )

**TARGET**

*f*/*Q* from Morel et al. 2002

*L**wn*( *sun*≠0, *sens *=0, *rel *=0 )
*L**wn*

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## Lwn( sun=0, sens =0, rel =0 )

**TARGET**

**SUN**

*f*/*Q* from Morel et al. 2002

*L**wn*( *sun*=0, *sens *=0, *rel *=0 )
*L**wn*
*L**wn**ex*

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## Lwn( sun≠0, sens ≠0, rel ≠0 )

**TARGET**

**SUN**

**SUN**

**SATELLITE**

*L**wn*( *sun*≠0, *sens *≠0, *rel *≠0 )
*L**wn*
*L**wn**ex*
*L**wn*

*f*/*Q* from Morel et al. 2002

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## + Lr , td , ...

**TARGET**

**SATELLITE**

**TOP OF ATMOSPHERE**

- from the satellite
*+ L**r** , t**d** , ...*

*L**t**target*

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## our operational targets ...

- _our operational targets_ ...

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## Lt(NIR) = Lr,g,wc,...(NIR) + La(NIR) + td Lw(NIR)

- _assumptions_:

**TARGET**

**SATELLITE**

**TOA**

- from the satellite
*+ L**r** , t**d** , ...*

*L**t**target*

*L**w**target*

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## Lt(NIR) = Lr,g,wc,...(NIR) + La(NIR) + td Lw(NIR)

- _assumptions_:
- (1) target sites exist where aerosol type is known
- (1) and *L**w*(NIR) is negligible

**TARGET**

**SATELLITE**

**TOA**

- from the satellite
*+ L**r** , t**d** , ...*

*L**t**target*

- 0

- M90

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## Lt(NIR) = Lr,g,wc,...(NIR) + La(NIR) + td Lw(NIR)

- _assumptions_:
- (1) target sites exist where aerosol type is known
- (1) and *L**w*(NIR) is negligible
- (2) 865-nm perfectly calibrated,
- (2) such that *g*(865) = 1.0

**TARGET**

**SATELLITE**

**TOA**

- from the satellite
*+ L**r** , t**d** , ...*

*L**t**target*

- 0

- M90

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## Lt(NIR) = Lr,g,wc,...(NIR) + La(NIR) + td Lw(NIR)

- _assumptions_:
- (1) target sites exist where aerosol type is known
- (1) and *L**w*(NIR) is negligible
- (2) 865-nm perfectly calibrated,
- (2) such that *g*(865) = 1.0
- _implementation_:
- knowledge of the aerosol type
- and *L**a*(865) permits the
- estimation of *L**a*(765)
- once *L**a*(765)*target* known,
- calculate *L**t*(765)*target*

- 0

- M90

**TARGET**

**SATELLITE**

**TOA**

- from the satellite
*+ L**r** , t**d** , ...*

*L**t**target*

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## the NIR calibration is completed prior to the VIS calibration ... the g(NIR) are now fixed

- the OBPG use data from the Marine Optical Buoy (MOBY) for the VIS calibration
- MOBY provides hyperspectral *L**wn*(VIS) measured during the satellite overpass

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## Lt(VIS) = Lr,g,wc,...(VIS) + La(VIS) + td Lw(VIS)

- _implementation_:
- calibrated NIR bands used to determine
- local aerosol type and concentration,
- which provides *L**a*(VIS)*target*

**TARGET**

**TOA**

- from the satellite
*+ L**r** , t**d** , ...*

*L**t**target*

*L**w**target*

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## Lt(VIS) = Lr,g,wc,...(VIS) + La(VIS) + td Lw(VIS)

- _implementation_:
- calibrated NIR bands used to determine
- local aerosol type and concentration,
- which provides *L**a*(VIS)*target*
- calculate *L**t*(VIS)*target* using
*L**a*(VIS)*target* , *L**w*(VIS)*target *,
- and satellite *t**d* , ...* *

**TARGET**

**TOA**

- from the satellite
*+ L**r** , t**d** , ...*

*L**t**target*

*L**w**target*

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## various complications:

- (1) *f*/*Q* requires an estimation of *C**a*
- (2) non-hyperspectral targets (conceptually) require adjustment to the
- (2) satellite spectral bandpass
- (3) non-clear water targets require an estimation of *L**w*(NIR)
- (4) polarization can be problematic as the correction depends on the
- (4) observed radiances

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## target data

- extract 5x5 box

- locate L1A files
- extract 101x101 pixel box
- process to L2

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## calculate gpixel for each pixel in semi-interquartile range; then:

- extract 5x5 box

- identify flagged pixels:
- LAND, CLDICE, HILT,
- HIGLINT, ATMFAIL,
- STRAYLIGHT,  LOWLW
- require 25 valid pixels

- calculate *g**pixel* for each pixel in semi-interquartile range; then:
*g**scene* =  *g**pixel* / *n**pixel*

- limit to scenes with average values:
- < 0.20 *C**a*
- < 0.15 (865)
- < 60 sensor zenith
- < 75 solar zenith

- locate L1A files
- extract 101x101 pixel box
- process to L2

- calculate gains for each matchup

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## calculate gpixel for each pixel in semi-interquartile range; then:

- extract 5x5 box

- identify flagged pixels:
- LAND, CLDICE, HILT,
- HIGLINT, ATMFAIL,
- STRAYLIGHT,  LOWLW
- require 25 valid pixels

- calculate *g**pixel* for each pixel in semi-interquartile range; then:
*g**scene* =  *g**pixel* / *n**pixel*

- limit to scenes with average values:
- < 0.20 *C**a*
- < 0.15 (865)
- < 60 sensor zenith
- < 75 solar zenith

- limit to *g**scene* within
- semi-interquartile range

- visually inspect all scenes

*g = * *g**scene* / *n**scene*

- locate L1A files
- extract 101x101 pixel box
- process to L2

- calculate gains for each matchup
- calculate final, average gain

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## Slide 27

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## Slide 28

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## Slide 29

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## g remains stable as a function of time, both long-term and seasonally

*g* remains_ stable as a function of time_, both long-term and seasonally
- * verifies temporal calibration
- * suggests consistency in MOBY deployments
- * scatter (5%) underscores need for independent temporal calibration
- * (SeaWiFS 443-nm has degraded 2% since launch)
*g* _consistent with both solar and sensor zenith angles_
- variations with geometry might suggest problems with
- * the atmospheric correction
- * the *f/Q* correction
- * the *in situ* determination of *L**w* under certain sky conditions

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## satellite-to-in situ biases and mean ratios approach zero and unity, respectively

- _input the calibration scenes into validation system_ ...

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## satellite-to-in situ biases and mean ratios approach zero and unity, respectively

- satellite-to-*in situ* biases and mean ratios approach zero and unity, respectively
- RMSs and absolute MPDs not entirely negligible
- compare with MPD of 13% for deep water validation set at 443-nm

- _input the calibration scenes into validation system_ ...

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## changes in g with increasing sample size ...

- _changes in __*g*__ with increasing sample size_ ...

- standard error of *g* decreases to 0.1%
- overall variability (min vs. max *g*) approaches 0.5%
- provides insight into temporal calibration

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## future ruminations ...

- ... statistical and visual exclusion criteria influence *g *only slightly, yet
- ... they reduce the standard deviations ... can uncertainties be quantified
- ... for the assigned thresholds?
- ... how do the uncertainties of the embedded models (e.g., *f / Q*, the NIR-
- ... correction, etc.) propagate into the calibration?
- ... what are the uncertainties associated with *L**w**target*?

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## Slide 35

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## 1993 SBRC / 1997 NIST

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## SeaWiFS validation for the deep water subset:

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## Slide 38

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## MODIS-Aqua validation for the deep water subset:

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## Backup slides ...

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## Slide 41

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## Slide 42

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## Slide 43

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## SeaWiFS validation for the global data set:

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## MODIS-Aqua validation for the global data set:

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## SeaWiFS validation statistics:

- _SeaWiFS validation statistics_:

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## MODIS-Aqua validation statistics:

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## SeaWiFS validation statistics:

- Vicarious Calibration, NASA OBPG, 1 Nov 2006

## MODIS-Aqua validation statistics:

- Vicarious Calibration, NASA OBPG, 1 Nov 2006