Airborne Sunphotometer 
                        OTTER Data Description

Date:
March 6,1991

Instrument Description:
The instrument consists of a solar-tracking system, detector module, 
temperature-control system, nitrogen-purge system, mechanical drive 
chain, and data-collection system.

The two-axis solar-tracking system is designed to 1) be able to acquire the 
sun starting from a position several degrees away and 2) to track the sun 
with an accuracy of plus or minus one-tenth of a degree in the presence of 
aircraft movement.  A large field of view (FOV) is required because the 
initial pointing is manually controlled until solar acquisition occurs.  The 
large FOV simplifies the initial pointing and, in addition, enables the 
system to reacquire the sun if tracking is lost because of abrupt 
movements of the aircraft.

The sensors used are Claires photoresistors that have been matched to 
track each other over the operational range of sun intensities.  The sensing 
technique uses a shadow mask that bisects each detector when the system 
is in balance.  This design allows for very accurate tracking, yet at the 
same time provides a FOV of plus or minus twenty-five degrees.  The 
dome rotation is referred to as azimuth motion.  The central section of the 
dome is free to rotate within the dome, perpendicular to the azimuth, and 
is referred to as elevation motion.

The detector module is a cylindrical unit that plugs into the main unit 
through a connector.  It contains six separate silicon photodetectors, each 
with its own optical filter; a sun sensor for sun-tracking purposes; and a 
temperature sensor and heater to control the temperature inside the 
module.  The filters range for 380 to 1020 nanometers with a nominal 
bandwidth of 10 nm.  The detectors used are silicon Detector Corp. devices 
that combine a detector and preamplifier inside a TO-5 style can.  The FOV 
of each detector is set by the entrance aperture to two degrees, the inside 
surfaces of the aperture assembly are anodized a dull black to reduce 
internal reflections, and a baffle is included to further reduce reflections.  
The 2-degree FOV allows for plus or minus one degree of tracking error 
without affecting the solar-radiation signal.  The entrance aperture is 
protected from the airstream with a fused quartz window; no lenses are 
used in the system.

The six detectors located inside the detector module require absolute 
temperature control and are temperature controlled to 45 degrees C plus 
or minus one degree by an analog temperature control system located 
inside the aircraft.  To reduce heat loss the dome shell and the detector 
module are constructed of fiberglass.

The wavelength bands of the six detectors are centered at 380, 
450, 526, 600, 940, and 1020 nm.  Their full-width, half-maximum bands 
are 12.1, 5.5, 12.1, 10.3, 14.4, and 12.1 nm, respectively.  Aside from the 
Rayleigh scattering, the 380- and 450-nm bands are affected by aerosol 
extinction and nitrogen dioxide absorption, the 526- and 600-nm bands 
are affected by aerosols and ozone, the 940-nm band is situated at a water 
vapor absorption feature but is also affected by aerosols, and only the 
1020-nm band is affected by aerosols alone.

The six detector signals, detector temperature, altitude, latitude, longitude, 
tracking error, suntracker azimuth and elevation position, and Greenwich 
mean and local times are recorded on floppy disks and (optionally) printed 
on hardcopy for backup.  A microcomputer is used to process the data and 
to graphically display channel voltages and optical depth plots in real time.

To prevent condensation from forming on the window, a dry-nitrogen 
purge system is included.  The nitrogen is is on during decent. 


OTTER Use of Sun Photometer:
Provide quantitative atmospheric correction to remotely sensed data of 
forest reflectance and radiance.


OTTER Data Acquisition (GMT):
            6-19-90
            6-20-90
            6-21-90
            8-13-90
            8-14-90

Note (August 12, 1991):  
	Data for only the August 1990 flights have been and will be
		processed.
	The August data have been processed and offered in two files,
		one for each day, August 13 and 14.  The data were not
		divided into separate files corresponding to
		individual flight lines or runs.

Data Decommutation:
OTTER contact:    Mike Spanner
                  TGS Technology, Inc.
                  Mail Stop 242-4
                  NASA/ Ames Research Center
                  Moffett Field, California  94035
                  415/604-3620


References:
Matsumoto, T., Russell, P., Mina, C., and van Ark, W.  1987.  Airborne 
tracking sunphotometer.  J. of Atmospheric and Ocean Technology vol.4, no. 
2, pages 336-339.

Pueschel, R. F., Livingston, J. M., Russell, P. B., Colburn, D. A., Ackerman, 
T. P., Allen, D. A., Zak, B. D., and Einfeld, W.  1988.  Smoke optical depths:  
Magnitude, Variability, and Wavelength Dependence.  J. of Geophysical 
Research vol. 93, no. D7, pages 9399-8402.

Pueschel, R. F. and Livingston, J. M.  1990.  Aerosol spectral optical depths:  
Jet fuel and forest fire smokes.  J. of Geophysical Research vol. 95, no. D13, 
pages 22,417-22,422.

Spanner, M. A., R. C. Wrigley, R. C., Pueschel, R. F., Livingston, J. M., and 
Colburn, D. S.  1990.  Determination of atmospheric optical properties 
during the First International Satellite Land Surface Climatology Project 
Field Experiment.  J. of Spacecraft and Rockets vol. 27, no. 4, pages 373-
379.


author and date of extract:  Jay Skiles  March 6, 1991
text on OTTER data processing added:  Gary Angelici  August 12, 1991