The macro language is used by the application program xtract to extract data from the GMRT visibility database. Most common use of xtract is to extract a data in the form of an ASCII table for display and/or further processing (e.g., to compute the antenna pointing errors). The output of xtract can be supplied to another program in two ways.
By default, xtract writes the output on the standard output. Hence if xtract is started as
xtract | MyProg
the output of xtract will be piped to the standard input of the
program named MyProg. The other, probably more convenient,
method of piping data is to set the out keyword to
'|MyProg'.
The output will be written in ASCII format, preceded by a simple header. Apart from other fields, the header contains information about the number of rows and columns and the labels for each of the columns. This header always ends with a string ``#End'', after which the data is written. A line beginning with '#' is also written per LTA-scan. It is hoped that users will utilize these facilities to generate more filters to process and display data externally.
If the output file name begins with a '*', the file name is constructed after stripping the initial '*' character and the data is written in binary format (floating point numbers of size determined by the operator sizeof(float) of C or C++). The data itself is preceded by the ASCII header mentioned above. Hence, out=*tst.bin will produce a file tst.bin, which will contain the output in binary format and out=*|MyProg will pipe the binary data to MyProg.
For convenience of usage, a filter has been incorporated on the output stream of xtract which will supply the data directly to the QDP line plotting package. This filter can be invoked by setting out=>QDP. The output, in this case will be displayed as a stack of line plots using QDP.
A more general and usable graphical interface to the multiplot features of the freely available line plotting program Gnuplot has been developed by (Kudale & BhatnagarNCRA Tech. Rep. - in prepration). The data to this software can be supplied using the piping mechanism described above. A graphical user interface then allows the user to select the available baselines/antennas and plot them interactively in a flexible manner.