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Next: Radar Parameters Up: Setup and Radar Parameters Previous: Setup and Radar Parameters

Setup

On 2003 April 1, we used the 1.15-1.73 GHz receiver on the GBT, which was sampled using the ADLink PCI-9812 sampling board at 10 megasamples per second. The computer that acquired the data using the PCI-9812 card saved the data over the network to another computer. Since the data transfer speed between the two computers was limited, a compromise between the sampling speed and observing efficiency was necessary.

The PCI-9812 card supports up to 20 MHz sampling rate per channel and 12-bit analog input resolution. We sampled two channels, which were from orthogonal linear feed polarizations, each at 10 MHz. The 12-bit samples were trimmed to 8-bit values by using bits 2-9 (bit 0 being the least significant bit). We took 24 scans, each of 180 seconds duration. These 24 scans consisted of 12 ON and 12 OFF scans. The OFF scans were 76.25 arcminues toward the higher RA than the ON scan position. This corresponds to 305 seconds of time, which is roughly the time it took for the 3-minute scan plus about 2 minutes to write the data to computer disk. The ON and OFF scans were alternated throughout the observing time. Each ON and OFF scan was saved in three files of 60 seconds each. The input to the A/D converter was saturated when the strongest pulses from the radar were received at the GBT.

The program to acquire data was in two parts: a C language backend, which used the ADLink libraries to read the data from the PCI-9812 card, and a set of Glish scripts which controlled the GBT and the backend. In order to automate the data acquisition process as much as possible, Glish scripts used ssh to execute the backend program with correct parameters for each scan. A part of the Glish script is as follows:

%
# ...

# Track at the ON position
status := __Track(ra, dec, duration);

# Start the data acquisition process
ssh_cmd := spaste('ssh datacq "cd ',
  '/export/home/datacq/rfisher/DataAcquisition/ADLink/;',
  '9812 ', object, '_on', sn, '"');
print shell(ssh_cmd);

ssh_cmd := spaste('ssh datacq "cd ',
  '/export/home/datacq/rfisher/DataAcquisition/ADLink/;',
  ' ls ', object, '_on', sn, '*"');
file_list := shell(ssh_cmd);
slew_flag := T;

# Copy the data over the network while we are waiting
# for GBT to slew to the OFF position
for (fn in file_list) {
  ssh_cmd := spaste('ssh datacq "cd ',
    '/export/home/datacq/rfisher/DataAcquisition/ADLink/;',
    'mv ', fn, ' /mnt/rfimit/rfisher"');
  shell(ssh_cmd);
  if (is_ready() & slew_flag) {
    slew_flag := F;
    slewto(hours2hms_string(off_ra / 15.0),
      degrees2dms_string(dec), coord_mode);
    }
}

# ...
%
The J2000 coordinates of the galaxy are: RA 08h14m35s.95$ \pm$0s.09, dec +19$ \mbox{$^\circ$}$21$ \mbox{$^\prime$}$6$ \mbox{$.\!\!^{\prime\prime}$}$6$ \pm$1$ \mbox{$.\!\!^{\prime\prime}$}$3 ([Cotton, Condon, & Arbizzani(1999)]). The galaxy was known to have Heliocentric velocity  $ \approx$ 30220 km/s. Thus we set the GBT receiver center frequency corresponding to a velocity of v = 30100 km/s, which corresponds to a center frequency of  $ \nu$ = 1290.69 MHz. Note that the radar carrier frequency is 1292.01 MHz, and therefore the passband center frequency of 1290.69 MHz is about half way between the galaxy line-profile and the radar signal, thus enabling us to get both in the observed spectrum.


next up previous
Next: Radar Parameters Up: Setup and Radar Parameters Previous: Setup and Radar Parameters
Alok 2003-10-28