Contents of Overpass Data Files

Contents of Overpass Data Files

The overpass data files contain the data derived from the best-matched TOMS field-of-view (FOV) to a site for every day the TOMS instrument was operational.

Each overpass file contains four (4) header records, followed by all the data records.

Header

The file header consists of four (4) records:

Site name, ID, and location.
Overpass program run information
Column headings
Marker record.

An example of the first header record is:

   Edmonton/Stony Plain, Canada  ID:  21   Lat:  53.55  Lon: -114.10  Alt:  766

In this example, the site ID number is 21, the latitude is 53.55^oN and longitude is 114.10^oE, and the verified elevation is 766 m.a.s.l.

The following FORTRAN code will read the first header:

      CHARACTER*28 site_name
      INTEGER*4    site_id, site_alt
      REAL*4       site_lat, site_lon

      . . .

      READ(lun,2) site_name, site_id, site_lat, site_lon, site_alt

    2 FORMAT(A30,4X,I3,7X,F7.2,7X,F7.2,7X,I4)

      . . .

An example of the second header record is:

   Nimbus-7 TOMS V.7 Archive Overpass.  Generated: 14-Apr-1998

This indicates the spacecraft instrument, the data version used to generate the overpass data, and the date on which the file was generated (created or updated).

The third header record contains the column headings for the data records. The header looks like this:

    MJD   Year Day sec-UT SCN  LAT     LON  DIS  PT  SZA  OZONE   REF   A.I.   SOI

The contents of each column is described below in the section on Data Records.

The fourth header record simply contains a pound-sign (#) in the first column, and signals that the data begin with the next record in the file.

Data Records

Each data record contains fourteen (14) values. They are as follows.

MJD Modified Julian Day. Astronomical Julian Day number*, less 2,400,000.5 The number is given to the nearest 1/10 day.

Year The four-digit Gregorian year number of the TOMS measurement.

Day The day number (day 1 through 366) of the TOMS measurement.

sec-UT The number of seconds from midnight, Universal Time. on the day specified by Year and Day.

SCN TOMS instrument scan position (1--35 for N7 and EP; 1--37 for Adeos)

LAT Latitude of the center of the IFOV.

LON Longitude of the same.

DIS Distance from site and IFOV center position, in km.

PT Terrain pressure at IFOV center, in (atm x 100)

SZA Solar zenith angle, in degrees, at time and location of IFOV

OZONE TOMS Version-7 best total ozone, in Dobson Units (DU)

REF TOMS Version-7 reflectivity at 380 nm (N7, M3) or 360 nm (EP, Adeos).

A.I. TOMS Version-7 aerosol index.

SOI TOMS Version-7 Sulfur dioxide index.

(*) The astronomical Julian day number (JD) is the number of Greenwich mean noons that have occurred since Greenwich noon on 1 January 4713 B.C.E., on the Julian proleptic calendar. This provides an unambigous time index. The Modified Julian Date (MJD) is defined as MJD = JD - 2,400,000.5 . Using the MJD saves having to store the leading two digits, which, for the purpose of this data set are always `24', and makes integral values of MJD begin at Greenwich mean midnight. Additional information about these may be found in the Explanatory Supplement to the Astronomical Almanac.

A FORTRAN (or IDL) format specification that would be appropriate for reading a data record is:

  (F7.1, 1X, I4, 1X, I3, 1X, I5, 2X, I2, 1X, F6.2, 1X, F7.2, 1X,
   I3, 1X, I3, 1X, F5.2, 1X, F5.1, 1X, F5.1, 1X, F6.2, 1X, I4)