Spectrometer for Sky-Scanning, Sun-Tracking Atmospheric Research (4STAR)

1
Spectrometer for Sky-Scanning, Sun-Tracking
Atmospheric Research (4STAR)
Background. Since 1985 the NASA Ames Airborne
Tracking Sunphotometers (AATS-6 and -14) have
made measurements on a wide variety of aircraft
in missions over most of the worlds continents
and oceans. AATS measurements have been analyzed
to yield aerosol optical depth and extinction
spectra, aerosol size distributions, water vapor
columns and profiles, and ozone columns. The
diverse scientific uses, described in gt80 journal
publications, include validations of measurements
by 12 satellite instruments, by two airborne
simulators of satellite instruments, and by
several airborne and ground-based lidars, plus,
via collaborations, studies of aerosol radiative
forcing of climate, aerosol light absorption
spectra, and consistency (closure) between in
situ and radiometric measurements. The AATS
instruments measure the direct solar beam
transmission through the atmosphere at 6 or 14
discrete wavelengths (near ultraviolet through
visible to near infrared) using individual
photodiode detectors and filters.
Entrance aperture
Optical fiber
Sun-tracking head
A concept for a sun-sky spectrometer smaller than
AATS-14 and usable on manned and unmanned
aircraft, both small and large.
Current developments. To provide additional
measurement capabilities and also to broaden the
types of aircraft usable, we have been exploring
advanced instrument concepts and investigating
their feasibility. The advanced concepts, called
Spectrometers for Sky-Scanning, Sun-Tracking
Atmospheric Research (4STAR) are smaller than
AATS-14 while extending AATS capabilities in two
ways Sky scanning By adding the capability
to measure the angular distribution of sky
brightness, the new instrument concepts would
enable retrievals of aerosol type (via complex
refractive index and shape) and aerosol size
distribution extending to larger sizes. These
capabilities, currently provided on the ground by
NASAs AERONET network, would be extremely
valuable in an airborne instrument. Wavelength
resolution By using a spectrometer in place of
the discrete photodiodes and filters of AATS, the
new concepts would improve accuracy of water
vapor and ozone measurements, enable measurements
of other gases (e.g., NO2, SO2) and improve
accuracy of aerosol measurements via better
aerosol-gas separation.
Earth Science Division - NASA Ames Research
Center 2006
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Spectrometer for Sky-Scanning, Sun-Tracking
Atmospheric Research (4STAR)

• Ground prototype. 4STAR feasibility depends on
  overcoming three technological hurdles
• 1. Maintaining calibration to 1 stability over
  a period of months.
• Demonstrating stray light rejection to permit
  measuring skylight within a few degrees of the
  sun.
• 3. Devising a fiber optic coupling that
  maintains 1 calibration stability with as many
  as possible of the following desirable
  characteristics detachable during assembly
  before calibration detachable between
  calibration and scientific measurements
  rotatable during measurements.
• To investigate ways to overcome these hurdles we
  have developed a ground-based prototype,
  4STAR-Ground. Examples of data from 4STAR-Ground
  are shown.
• 4STAR research is being conducted as a
  collaboration between NASA Ames and Battelle,
  Pacific Northwest Division.

Photo of the ground prototype Spectrometer for
Sky-Scanning, Sun Tracking Atmospheric Research
(4STAR-Ground).
(a)
Direct (sun)
(b)
Diffuse (sky)
Points of Contact Dr. Phil Russell 650-604-5404,
Philip.B.Russell_at_nasa.gov http//geo.arc.nasa.gov/
sgg/AATS-website Dr. Beat Schmid Battelle,
Pacific Northwest Division 509-375-2996,
beat.schmid_at_pnl.gov
Data from 4STAR-Ground. (a) Output counts for
four wavelengths during an elevation scan that
combines data from diffuse (sky) and direct (sun)
channels (b) Direct-sun spectrum.