NASA-Ames files - Header

The header of a NASA-Ames format file must contain the following descriptors in the order listed below:

1. Overall Format description
2. Data Source Description and Storage Volume Information
3. Description(s) of the Independent variable(s)
4. Description(s) of the Primary variable(s)
5. Descriptions of the Auxiliary variable(s) - note this could be a single zero indicating no auxiliary variables
   
6. Special and normal Comments - note that either one or both of these could be a single zero indicating no comments
   

1. Overall Format Description

The first record of a NASA-Ames file always contains two integers. The first integer represents the number of header records in the file (NHEADER). This should include the first record itself. The second integer is a four-digit number called the file format index (FFI). The first digit of FFI indicates the number of independent variables.

2. The Data "Dource" Description and Storage Volume Information

The data source description requires six records:

• Records 2,3 and 4 are generally used to describe the name, address and affiliation of the originator(s) of the file.
• Record 5 is generally reserved for the name of the associated campaign.
• Record 6 details the current volume and the number of volumes in the dataset. This information dates from the days when data were distributed on diskettes - for most files today the numbers are 1 and 1.
• Finally record 7 provides two important dates, The first is the date on which the data starts while the second is the date when the file was created. All dates are given in in "YYYY MM DD" format.

3. Description of the Independent variable (or variables)

The description of the independent variable changes significantly depending on the number of variables and the variable type. The possible options for variable type are numeric or character and bounded or unbounded. With bounded variables the range and interval must be specified either within the NASA-Ames file header or as in the case of some formats as an auxiliary variable. With an unbounded variable this range is unknown until after the file has been completely read. It is meaningless to have more than one unbounded variable in a NASA-Ames format file.

Since a large proportion of ACSOE data comprise straightforward, in many cases unbounded, time series, it is worthwhile examining this example more closely. In this case it is only necessary to specify the interval between independent variable marks and the name of the independent variable. Note that if the spacing between independent variable marks can change then this interval should be set to zero.

4. Description of the Primary variable (or variables)

Three quantities are needed to describe each primary variable: the scaling factor, the value which has been substituted for missing data and the name of the variable; the latter must include the units.

The NASA-Ames format stipulates that all scale factors for the different primary variables should be held in the same record, all "missing-data" values should be held in the next record and each primary-variable description should be written in separate subsequent records.

5. Description of the Auxiliary variable (or variables)

Three quantities are needed to describe each auxiliary variable although these vary depending on whether the variable is numeric or a character string. Numeric auxiliary variables require a scaling factor, a default value to be substituted when data are missing and the name of the variable. Character auxiliary variables require the length of the string, a default string for missing data and the name of the variable(s). Character auxiliary variables are used to record quantities such as the name of the filling gas used to inflate a balloon.

6. The normal and special comments fields

The format makes provision for two comment fields structured in identical ways - first record number of comments, subsequent records actual comments.

For ACSOE data files only, four comment fields are mandatory. These are THIS-FILE-NAME, THIS-FILE-VERSION, THIS-FILE-ENDS and E-MAIL-CONTACT. These fields may appear in either the special or normal comments, but the keywords must be given exactly. The contents should be formatted as follows:
THIS-FILE-NAME=mh960708.an
THIS-FILE-VERSION=1
THIS-FILE-ENDS=1996 08 09
E-MAIL-CONTACT=j.jones@somewhere.ac.uk
Version numbers start at 1 and increment by 1. Dates should be formatted as YYYY MM DD.

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