Introduction

Flight Data Products (An Overview)

There are two data records of interest here which are merged post- flight. The experiment data record comes down over the medium rate telemetry channel (1.4 Mbits/sec) and consists primarily of CCD images from the GLO spectrographs and imagers. Post flight, these images are put into FITS format. A real time data stream from the shuttle contains everything you ever wanted to know about the shuttle operations. This is called ancillary data; we refer to it as UCAT data, derived from the official name. We extract shuttle attitude, thruster firings, dead band limit cycles, etc. The data rate is 18 Kbaud and gives most data on a one-second time base. We also calculate several parameters of interest - like geometric tangent height, solar zenith angle (SZA) at the orbiter, SZA at the tangent point, etc.

File Structure

File names are generated with respect to the mission event time (MET). For instance, the first file in the data directory is 2120801B which means that the record was read out on day 2 at 12:08:01 MET. The B identifies the detector. The first five letters A-E are the five spectrographs, A being the red end. The imagers follow: F, G are the two six-channel imagers, while H is the IR support imagere

Each CCD readout file has a standard FITS header which is attached at the readout time. It contains all of the instrument characteristics, the file name, several times to define the exposure, pointing information, high voltage conditions, CCD read out parameters (how many pixels were summed), etc.

The readout takes a fraction of a second, but still the file name of successive related records, such as a set of simultaniously exposed images, will change by one or two seconds. We have tools to work with individual CCD's at the raw data level. Although raw data is not included here, this point is relevant to the processing which takes place.

Directories:

All data files from a single instrument configuration are identified as a data set and are grouped into one directory. For example, in a typical orbit of limb observations there would be one dayglow data set and one night airglow data set. The directory is named from the first file in the series, 2_1208. The underscore indicates the mission was STS-53 (GLO1). For subsequent missions we use an A for the STS-63/GLO2 mission (e.g. 2A1208), B for STS-69/GLO3 (e.g 2B1208), and C for STS-74/GLO4 (e.g. 2C1208).

Lists:

The software tools do their work by reference to lists of files rather than the files themselves. For instance, a DOS directory sort on spectrograph C (???????C.FIT) to a file C.LST will give the programs access to all the C spectrograph data when LIST C is used to initiate a process. Data processing proceeds in the order of files contained in C.LST.

Initial Data Processing

Once the FITS image data has been prepared in a DOS directory format, it is sorted by name to be certain the data is in ascending time order, which is expected by subsequent programs. The utility program GETMET produces a listing (GETMET.DAT) containing each FITS file name in the directory. This file is processed against the UCAT flight data record. The data in the FITS headers are used to interpolate the UCAT data and prepare the parameters to be returned and merged into the FITS headers in the GLO data file. The returned file, UCAT.DAT, is then merged with the data files. The effect is to expand the FITS headers and add the attitude and position data. The merge process is followed by a procedure called MAKESET, which changes the slightly different file names of related data so they can be identified as a set. Although the data is recorded at nearly the same time, there are enoughseconds involved that the file name of successive reads may be different by one or two seconds. The utility MAKESET senses the first record in the ascending series AB_____H and changes the names of succeeding records to the prefix of first record resulting in a SET of spectra or images. The real time is still preserved in the header. The MAKESET program produces a MAKESET.LST which is the complete list of the data "SETS" in the directory and includes selected UCAT data for each data set. The MAKESET.LST is contained in each experiment data set. The MAKESET.LST can be printed out across a page in condensed type (landscape format).

When all of the above steps have been performed on the data, the experiment sets are corrected by leakage subtraction, hot pixel removal, and brightness calibration. The data is then ready for analysis.

o Last Updated: 29 January, 1996

This page is maintained by:
Jesus A. Ramirez