Plan for calibration and maintenance of AHAB
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Plan for calibration and maintenance of AHAB Uncertainty Budget: Environmental Components Ken Voss University of Miami. Environmental Uncertainties. Variability in downwelling light field during measurement time Wave focusing BRDF effects in Satellite matchups Instrument self-shading

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Environmental Uncertainties

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Environmental uncertainties

Plan for calibration and maintenance of AHAB Uncertainty Budget: Environmental ComponentsKen VossUniversity of Miami


Environmental uncertainties

Environmental Uncertainties

  • Variability in downwelling light field during measurement time

  • Wave focusing

  • BRDF effects in Satellite matchups

  • Instrument self-shading

  • Polarization Effects

  • Bio-fouling

  • SITE LOCATION!


Variability in downwelling light field

Variability in downwelling light field

(std MOBY style processing - std simultaneous)

std simultaneous

  • Current MOBY design uses sequential measurements from arms at different depths. Entire measurement sequence takes 20 minutes, both solar zenith angle and other environmental effects can change the downwelling light field.

  • New design has simultaneous measurements at different depths.

  • Improvements in std deviation can be estimated by graph on right.

  • The graphs shows that AHAB should have a 20% reduction in std and the red should have 60% in std.

Wavelength (nm)


Wave focusing effects

Wave focusing effects

  • While most people worry about wave focusing in the downwelling light field, the upwelling light field also has relatively large (compared with the uncertainties we desire) fluctuations due to wave focusing. The upwelling radiance distribution, at 412 nm, in clear water is shown below. This image was taken with an exposure on the order of 0.5 sec.


Averaging over wave focusing events

Averaging over wave focusing events

  • How many Simultaneous data sets need to be averaged to get a specific std? (I.e. average across wave focusing.)

  • Each measurement takes 1.5 seconds for all depths. 50 sets would be less than 2.5 minutes (including some overhead) and would achieve most of the reduction in std.

  • This is one example experiment, 4 were done with identical results.

% std

Number of data sets averaged


Brdf effects in satellite matchups

BRDF effects in Satellite matchups

Upwelling radiance distribution not uniform and depends on illumination and viewing geometry.

The effect is strongest in the red wavelengths in clear water due to the high water absorption.

Vicarious calibration should be done in region which has stable/predictable water properties that the BRDF can be determined experimentally or modeled.

Example of MOBY and MODIS/Terra


Environmental uncertainties

MODIS, Terra, view geometry of MOBY

ONLY CERTAIN VIEW DIRECTIONS


Variation of lview lnadir with day of year and scan time

Variation of Lview/Lnadir with day of year and scan time.

Note: Blue is Moby on west side of scan


Spectral variation for early moby on west side orbit

Spectral variation for early(moby on west side) orbit


Spectral variation for situation with lview lnadir approx 1

Spectral variation for situation with Lview/Lnadir approx. 1


Brdf conclusion

BRDF conclusion

NEED BRDF MODEL for ViCal site! Nothing you can do with the design to get around this.


Self shading

Self-shading

Ratio of disk to bare fiber (670 nm)

Instrument or platform self shadowing is a problem for all ocean optics instruments.

The effect is strongest in the red wavelengths in clear water due to the high water absorption.

This shows the result of a shadowing experiment done in June 2006. A fiber optic input is used to measure the upwelling radiance in the center of 3 different disk sizes simultaneously (size in diameter).

This shows the increase in error as the disk diameter increases or solar zenith angle decreases (sun more directly overhead) as predicted by Gordon and Ding.

Zenith Angle (Deg)


Self shadowing in the blue in clear water

Self shadowing in the blue (in clear water)

  • Same idea but at 412nm. In clear water not as large an effect, but still 5-10%.

  • At both wavelengths this can look like a seasonal effect, as the solar zenith angle at measurement time varies.

  • Note that in turbid water, shadowing will be much more important in the blue.

Ratio of disk to bare fiber

Solar Zenith angle (Deg)


Self shadowing in the blue in clear water1

Self shadowing in the blue (in clear water)

  • Same idea but at 412nm. In clear water not as large an effect, but still 5-10%.

  • At both wavelengths this can look like a seasonal effect, as the solar zenith angle at measurement time varies.

  • Note that in turbid water, shadowing will be much more important in the blue.

Ratio of disk to bare fiber

Solar Zenith angle (Deg)


Ahab solution

AHAB solution

  • Multiple Arms, select arm with minimum shading affect from platform.

  • Arm itself has small profile.


Polarization in upwelling field

Polarization in upwelling field

  • Upwelling light field polarized. Example on right shows 412 nm and 550 nm.

  • Both AHAB and MOBY designs are made polarization insensitive by using fiber optic inputs.


Bio fouling

Bio-fouling

  • Example of the effect of using a UV light as an anti-bio-fouling tool.

  • This was in the Chesapeake Bay and the small UV lamp (at other end of light tube) was turned on every 3 hours for 10 mins.


Solutions for r o buoy anti bio fouling

Solutions for R&O BuoyAnti-bio-fouling

  • Diagram of the way the UV light would be reflected onto the collector head.

  • Tests need to be conducted to determine the length of time is needs to be on and the how often it needs to be on.

  • Also look at optimum wavelength in UV, LED‚Äôs available from 265 nm and up.


Site location for vical

Site Location for ViCal

  • Want the least stressful, most accurate, frequent satellite retrieval of nLw

    • Deep site (no bottom effects to stress atm. Corr)[Hi good]

    • Spatially and temporally homogeneous[Hi good]

      • Enable good matchups easily, particularly with necessary scale mismatch

    • Clear, non-absorbing atmosphere[Hi good most of the time]

      • Easy Atm. Corr with errors minimized in final nLw

    • Large nLw signal relative to atmosphere

      • Points to blue rich site: clear water

    • Good frequency of clear weather[Hi relatively good]

      • Want as many possible matchups as possible

    • Satellite overpass fairly near Nadir[Hi as good as any for polar orbiter]

      Hi not as good as some spots for MODIS because of glitter


R o buoy technical modifications to reduce effects of environmental uncertainties

R&O BuoyTechnical Modifications to reduce effects of Environmental uncertainties

  • Simultaneous multi-arm observations, also shortens measurement time and allows more measurements to be averaged.

  • Two arms at each depth to help with platform self shading.

  • Fiber optic inputs act as polarization scramblers.

  • UV LEDs for anti-bio-fouling


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