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ATMOS Level 1 Data

Directory Architecture:

	README				-	this file
	at3.ss90.Tp-gpa.nmc.121394.mdl	-	model
	header.info			-	header
	r941111015626			-	spectra
	      .      			-	spectra
	      .     	 		-	spectra
	      .      			-	spectra
	r941111015941			-	spectra

You can find the spectra by anonymous ftp to thunder.jpl.nasa.gov/atmos/level1

References:

C. B. Farmer and R. H. Norton, "Atlas of the Infrared Spectrum of the Sun and the Earth Atmosphere from Space. Volume I, The Sun", NASA Reference Publication 1224, 1989.

C. B. Farmer and R. H. Norton, "Atlas of the Infrared Spectrum of the Sun and the Earth Atmosphere from Space. Volume II, Stratosphere and Mesosphere, 650 to 3350 cm-1 ", NASA Reference Publication 1224, 1989.

M. Geller, "Atlas of the Infrared Spectrum of the Sun and the Earth Atmosphere from Space. Volume III, Key to Identification of Solar Features", NASA Reference Publication 1224, 1989.

C. B. Farmer, "The ATMOS Solar Atlas", Infrared Solar Physics, 511-521, 1994.

Introduction:

High resolution infrared solar spectra have been obtained by ATMOS during four space shuttle flights. A set of spectra from one occultation, a sunset in this case, are provided as an example. These spectra were obtained during the ATLAS-3 mission (November 3 - November 12, 1994). They span the 600- 2500 cm-1 frequency range, and have already been ratioed against a high sun reference spectrum to form atmospheric transmission spectra.

Data Format:

Each spectrum file is a 1-D binary array of 2-byte signed integers (big endian: hi-byte, lo-byte). The number of points (Num_Pts below) is thus 0.5 * the file size in bytes.

On a MAC or Sun, these can be read directly to an integer array, using for example, a C fread() routine. On a DEC or PC, a byte swap would have to be performed following the read operation. (In FORTRAN the process is more complicated because the data is pure binary and has no record terminators.)

Important ancillary information necessary to recreate the frequency scale are provided in the file "header.info". For each spectrum the following information is given in a single line: 

Field                    Contents
=================        =================================================
Run_ID                   Run label, (yymmddhhmmss), giving GMT of measurement
                         optionally preceeded by an "r" for a ratioed
                         spectrum.  Spectrum file name is this Run ID
                         preceeded with a "p".
Filt                     ATMOS optical filter number (1-13)
Locat                    Observation location or mission name
F/R                      Forward/reverse instrument scan
Alias                    Alias number (1 or 2)
Loc_Mx                   Index of maximum spectral point
Num_Intf                 Number of interferogram points
N_Fring                  Number of laser fringes
FFT_sz                   Fourier transform size
I_First                  Index of first point saved
Num_Pts                  Number of points in file
Step                     Step size in fringes, 
                         between successive interferogram samples
SP_Max                   Maximum value of spectrum
ZPD_GMT                  GMT of ZPD point (seconds since midnight), 
                         transformed to give run label
Obs_Alt                  Observation altitude (km)
Obs_Lat                  Observation latitude (decimal degrees)
Obs_Lon                  Observation longitude (decimal degrees)
Tan_Ht                   Tangent height (km) - derived from data
TP_Lat                   Tangent point latitude (decimal degrees) - calculated
TP_Lon                   Tangent point longitude (decimal degrees) - calculated
Tan_Pres                 Retrieved tangent pressure (bars) - derived from data
Pres_Err                 Retrieved tangent pressure error (bars)
FOV_Mag                  Magnification for internal field of view 
                         (constant for ATMOS at 2.7)
Ext_FOV                  External field of view (mrad), 
                         variable according to optical filter used
SC_Dopp                  Estimated spacecraft doppler shift along line of sight
Las_Sem_Frq              Laser semi-frequency (cm-1)
Frq_Cor_Fac              Instrument line frequency correction
Sc_Factor                Rescaling factor to convert integer spectral values to real
Start_Frq                Approx. starting frequencies for spectrum (cm-1)
End_Frq                  Approx. ending frequencies for spectrum (cm-1)
Comments                 Comments, usually the occultation name

The file can be read using the following read and format statements:
      read (20,810) runlabl,afilter,obsloc,forrev,
     &              aalias,mxpptloc,numint,afring,afftsiz,afirst,
     &              numprim,step,spmaxval,zpdgmt,obsalt,latitude,
     &              longtude,tanht,tplat,tplon,tanpres,preserr,
     &              fovmag,extfov,scdopp,lasemfrq,wifact,
     &              spec_x_fac,sig0,sig1,acomment

810   format(a16,2x,a2,2x,a8,1x,a2,i5,i9,i9,i8,i8,i8,i8,f6.1,f10.5,
     &       f11.2,f9.3,f8.2,f9.2,f9.3,f8.2,f9.2,2x,e10.4,2x,e11.4,
     &       f9.2,f7.3,2x,e11.4,f11.4,f14.9,e11.4,f11.4,f11.4,2x,a72)

Data Scaling:

The spectra are stored as an integer array to conserve space. The spectral points can be rescaled to their original values by multiplying by Sc_Factor.

Frequency Scaling:

The frequency interval between successive points is determined from, 

        Freq_Int = Corr * Las_Sem_Frq / (Step *FFT_sz)
where the correction factor, Corr, is very nearly 1.0. 
e.g.    Freq_Int = 7899.0015/(2.0*524288) = 0.0075331 cm-1
The frequency corresponding to each spectral point is found using, 
        freq[i] = Freq_Int * [ (i - 1) + (I_First - 1) + FFT_sz*(Alias - 1) ]
where array indices are assumed to start at 1 (not 0).

For greater accuracy, the frequency interval can be corrected by, 

                             Frq_Cor_Fac * (1 + SC_Dopp)
        Corr = --------------------------------------------------------
              [1 - (Ext_FOV/1000)*(Ext_FOV/1000)*(FOV_Mag)*(FOV_Mag)/16]


Acknowledgements - This work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract to the National Aeronautics and Space Administration.

You can find the spectra by anonymous ftp to thunder.jpl.nasa.gov/atmos/level1

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File modified by: A. Chang 3-16-98 / Links updated by Bill Irion 3/19/01