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Calculation of environmental correlations using the routines presented in this chapter proceeds through the establishment of a configuration file, called here 40m.config with the following structure:

#  Correlations between 40m Channels  over a period
#  of approximately 266 seconds. The IFO sample rate is 
#  9868.4\dots Hz (hence 9868.4 x 266 = 2621440 samples).
#  The sample rate for the `slow' channels is
#  (1/10)th that of the `fast' so (1/10)th the
#  number of samples are requested 
IFO_DMRO        S        2621440
IFO_Mike        S        2621440
IFO_Seis_1      S        262144
IFO_SPZT        S        262144

The file may begin with any number of comment lines beginning with an initial #. The next line is a character string describing the detector, this is just used for naming intermediate files. The following line gives the total number of channels (signal plus environmental). There follow this number of lines each containing three columns, the first of these lines pertains to the signal the remainder to environmental channels. The three columns are:

  1. the name of the channel,
  2. the data type of the channel (here short) - see animateT.c or corr_init.c for a description of the possible types, and
  3. the number of data points from that channel to be analysed - these should correspond to the same period of time.

Finally, there is a line containing a single number. This should be set to 1 if the user wants to obtain `cleaned' output and 0 if the user just wants to see correlation data. (Note: This line is not used by corr_init described below, but only by env_corr. Thus it is possible to change one's mind about whether to find the cleaned signal without having to rerun corr_init.)

The configuration file is used by the two basic programs:

  1. corr_init which calculates the Fourier transforms and writes binary data files in a data directory named `configuration name'_fft, so 40m_fft in the above example. Only those frequencies appropriate to the slowest channel are saved.
  2. env_corr which calculates the correlations between each environmental channel and the signal channel and pops up a graph of these correlations. The data for this graph is stored in the same data directory as the FFT data. env_corr also produces a file corr_view... which enables this graph to be reproduced later without running env_corr again. If the configuration file asks for the signal to be cleaned env_corr will also produce an ASCII data file giving the (FFT of the) `cleaned' signal and also the total fractional reduction in noise obtained by the method. Again this file is stored in the same data directory, its first line gives the frequency spacing and the following lines the real and imaginary parts of the FFT of the cleaned signal. (To avoid plotting difficulties with xmgr the DC component is arbitrarily set equal to that of the first bin.)

Thus, having created the appropriate configuration file one would type corr_init 40m.config and then env_corr 40m.config. (Of course, the environment variable GRASP_FRAMEPATH must first be set to the directory containing the appropriate frames.)

Note: These programs perform linear algebra by calls to clapack routines. These may be obtained from These routines use f2c and, in particular, use complex numbers defined in f2c.h through the structure:

typedef struct {
float r; /* real part */
float i; /* imaginary part */
} complex;

next up previous contents
Next: Function: calc_rho() Up: GRASP Routines: Signal-to-noise enhancement Previous: Signal-to-noise enhancement by environmental   Contents
Bruce Allen 2000-11-19