TITLE: 

Aircraft Twin Otter Radiation Downwelling and Upwelling Solar Spectral 
Irradiance; 380 -1700 nm (Pilewskie/Rabbette)

AUTHORS(S): 

        PI: Peter Pilewskie
	NASA Ames Research Center
	MS 245-4
	Moffett Field, CA 94035
	Phone: (650)604-0746
	Fax:     (650)604-3625
	email: ppilewskie@mail.arc.nasa.gov

	Co-PI: Maura Rabbette
	(Same address as above)
	Phone: (650)604-0128
	email: mrabbette@mail.arc.nasa.gov
	
	Co-PI: John Pommier
	(Same address as above)
	Phone: (650)604-3374
	email: jpommier@mail.arc.nasa.gov

1.0   DATA SET OVERVIEW:

The Ames Solar Spectral Flux Radiometer (SSFR) was flown on the Twin otter 
during the ACE-Asia experiment. The instrument acquired both downwelling and 
upwelling solar irradiance spectra (Wm-2nm-1) covering a spectral range 380 -
1700 nm. 

SSFR spectra were collected on the following dates during ACE-Asia 2001:
March 31, April 2, 4, 6, 8, 9,12, 13, 14, 16, 17, 19, 20, 23, 25, 26, 27, 28, 
May 1

2.0   INSTRUMENT DESCRIPTION

The SSFR is a moderate resolution flux (irradiance) spectrometer with a 
hemispherical field-of-view. The SSFR is comprised of an identical pair of 
spectrometer modules for simultaneous zenith and nadir viewing. Each module 
contains a Si linear diode array detector that covers the spectral range 380 nm 
to 973 nm and an InGaAs linear diode array covering 973 nm to 1700 nm. The 
complete wavelength range is 380 nm to 1700 nm. Thermal control is applied to 
the Si detector to maintain its temperature at 27 deg C and the InGaAs detector 
is thermoelectrically cooled to 0 deg C. The instrument precision is ~0.2% and 
its accuracy is ~3%. Spectral sampling rate is approximately 1 Hz.

3.0   DATA COLLECTION AND PROCESSING

Prior to the deployment, laboratory calibrations were carried out for angular 
response and absolute radiometric response. During the experiment the SSFR was 
calibrated using a Li-Cor field calibration device (standard lamp). Prior to 
each flight hundreds of dark spectra were acquired. The following are the data 
processing steps:

i)   The average dark signal is subtracted from the raw data.
ii)  Apply absolute spectral calibrations which converts the raw signal to 
     spectral flux measurements (Wm-2nm-1) .
iii) A filter was applied to the spectra for quality control so as to eliminate 
     periods of unusable data (e.g. during aircraft steep climb/descent or 
     spiral maneuvers). Hence, the archived SSFR spectra were extracted from 
     level flight legs only.

4.0   DATA FORMAT

SSFR data is archived in NetCDF format. There is one NetCDF file per date.
Each file contains the following variables:

varid1 = ncdf_vardef(id, 'Wavelength', [xdim1], /float)
varid2 = ncdf_vardef(id, 'seconds', [xdim3], /float)
varid3 = ncdf_vardef(id, 'zen_spec', [xdim2, xdim3], /float)  ;Downwelling flux
varid4 = ncdf_vardef(id, 'nad_spec', [xdim2, xdim3], /float)  ;Upwelling flux

Local Attributes:

ncdf_attput, id, varid1, 'units', 'nm'
ncdf_attput, id, varid1, 'long_name', 'Wavelength'
ncdf_attput, id, varid2, 'units', 'UTC'
ncdf_attput, id, varid2, 'long_name', 'UTC Time_Data'
ncdf_attput, id, varid3, 'units', 'Wm-2nm-1'
ncdf_attput, id, varid3, 'long_name', 'Zenith Irradiance'
ncdf_attput, id, varid4, 'units', 'Wm-2nm-1'
ncdf_attput, id, varid4, 'long_name', 'Nadir Irradiance'

5.0 DATA REMARKS

i)   Navigational filter: Whenever the cosine of the solar zenith angle with 
     respect to the aircraft (?') deviated from the ideal cosine (?o) by more 
     than 5%, the corresponding flux spectra were eliminated.
ii)  It should be noted that this filter does not eliminate any cloudy spectra, 
     they are included in the archived data.
iii) In the NetCDF file there is a time array called varid2. The entries in this
     array correspond to the time stamp on the retained spectra.
iv)  There is spectral noise between 950 nm to 1100 nm due to poor detector 
     sensitivity at these wavelengths.

6.0   REFERENCES:    

http://geo.arc.nasa.gov/sgp/radiation_aug01/rad8.html