Document title: Updated format.sfd file for NDADS DE NACS_ASCII datatype
Project: DE
NDADS Datatype: NACS_ASCII         
Super-EID: DOCUMENT                      
There may be other documents also identified by this super-EID.
NDADS filename: NACS_FORMAT_DE.SFD
TRF entry B46556.txt in NSSDC's controlled digital document library. Feb. 1998.
Document text follows:
----------------------

FORMAT UPDATE ------------ April 1995 ------------- Dieter Bilitza

FORMAT ON PI-PROVIDED TAPES:

The file attached below is the SFDU format file that describes the
format of the PI-provided NACS data tapes. We found, however, that
this file requires a number of correction:

* The NACS data files on the PI-provided tapes were found to be of 
variable record length with a maximum record length of 26 bytes.

* The first word (time) in each record was found to be an INTEGER
(VI4) rather than a REAL variable.

FORMAT ON NDADS/ARMS:

* For better accessibility of the data records in an UNIX environment,
we have converted the records to fixed-length records of 28 bytes
record length.

FORMAT UPDATE ------------ April 1995 ------------- Dieter Bilitza




CCSDYDNM000200NSSD0007SMRK0001
TYPE_OF_FILE_NAME: MAF AMBIENT DENSITIES
FILE_ATTRIBUTES: Variable length, sequential file, unformatted, with
  fixed length logical records
LOGICAL_RECORD_LENGTH: 24 bytes
TYPE_OF_FILE_DESCRIPTION:
  Each file corresponds to an original Sigma 9 MAF file.  Each logical
  record of the file has ambient densities averaged over one second.
  The data date is contained in the file name.  File names are of the 
  form Nydddsssc, where: the letter N is at the beginning of all files, 
  y is the last digit of the year 1981 to 1983;  ddd is the daycount 
  (Jan 1 = 1); sss is the high order hundreds of seconds of the first
  data point (time=sss*100 sec); and c is 0 or other arbitrarily assigned
  integer to ensure unique file names.
FILE_STRUCTURE:
  MAF files are written as VAX unformatted sequential with logical
  records of equal length. 

FORMAT_OF_THE_LOGICAL_RECORD:

    WORD      FIELD NAME   TYPE   DESCRIPTION
   NUMBER                  I/R 

     1        TIME         VR4    Universal time of day in milliseconds
     2        DENSITY1     VR4    Atomic Oxygen density and percent
                                  error
     3        DENSITY2     VR4    Molecular Nitrogen density and
                                  percent error
     4        DENSITY3     VR4    Helium density and percent error
     5        DENSITY4     VR4    Atomic Nitrogen density and percent
                                  error - not reliable data
     6        DENSITY5     VR4    Argon density and percent error


   The field names DENSITY1 through DENSITY5 refer to pairs of
   parameters whose values have been packed into DEC VAX 4-byte reals
   designated type VR4.  Each pair of parameters consists of a density
   for the species indicated and an associated percentage statistical
   error (not including calibration error) of the measurement.  The
   representations for the pair are as follows:

   Density              Use the VR4 value directly to get the ambient
                        density in units of #/cm**3.  This is sufficiently
                        accurate to ignore the presence of the Percent Error.

   Percent Error        Extract the lower order 8 bits of the VR4 field
                        and treat as an unsigned integer.  Obtain this
                        value, convert to a real, and divide by two to
                        get a value that is the percentage statistical
                        error.
   
FIELD_RELATIONSHIPS:
  TIME_DENSITY: The time value from the TIME field corresponds to the
  mid-point of the one second time interval used to compute each density
  and percentage error given in a DENSITYx field.

CCSDYDNM000200NSSD0007EMRK0001