Raw satellite pass data is stored online in disk partitions (or standard UNIX files) defined by the bad sector table. See BAST. Pass data on disk can be archived to tape or restored from tape using the archive function.
Raw pass data on disk or tape is read by TeraScan ingest functions, such as avin, dcsin, tipin, rtdin, gmsin, and vasin. While these ingest functions share a common programming interface to pass data on disk or tape, this interface is currently not available to the general user.
Although several passes can be stored in a single disk partition, there is typically one pass per disk partition, starting at the first block. Pass partitions contain telemetry data only. Header information for the passes on disk is stored in the file $PASSDIR/onlinecat.
The contents of the onlinecat file can be listed using the lspass and the llpass functions. Though the format of this file is documented in onlinecat, it is imperative that general users refrain from writing their own programs that may modify this file.
Pass data on archive tape is stored as an alternating sequence of header files and data files; each data file is preceded by a corresponding header. Each header file is a single 1024-byte record. Only the first 72 bytes are defined:
bytes 1- 8 : 1-rel pass number on tape, right justified bytes 9-16 : satellite name, upper case, left justified bytes 17-24 : telemetry name, upper case, left justified bytes 25-32 : pass date, left justified, yy/mm/dd GMT bytes 33-40 : start time, left justified, hh:mm:ss GMT bytes 41-48 : orbit number, always 0, right justified bytes 49-56 : number of scan lines, right justified bytes 57-72 : tape label, case dependent, left justified
Admittedly, the format of the header is strange, but it will likely never be changed for historical reasons. Most ingest functions skip over header files on tape anyway, because the raw telemetry data usually contains the same information. Also, ingest functions are often required to read non-TeraScan archive tapes, and cannot afford to count on the format (or even the presence) of header data.
Pass data can be archived to tapes with fixed 1024-byte blocks or variable length blocks. 4mm (DAT) and 8mm (Exabyte) tapes support both these options. Tapes with fixed 1024-byte blocks are similar to the old 1/4-inch cartridge tapes; tapes with variable length blocks are like the old standard 1600/6250 bpi CCT.
For a given telemetry type, data records on archive tape may differ from data records in pass partitions. Translation between pass partitions and archive tape are handled by the archive function. These differences are described below.
$PASSDIR//BAST, $PASSDIR/devtable
archive, lspass, llpass, avin, dcsin, tipin, rtdin, gmsin, vasin, onlinecat
Some precautions must be taken when accessing pass data on disk by non-TeraScan functions, such as the UNIX dd utility. At the very least, users should avoid accessing pass partitions when pass data is being written to them.
Last Update: $Date: 2000/11/20 19:10:34 $