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Analysis of a spectral overview using IDL Written by Christian Monstein, ETHZ, Switzerland

The spectral overview is a very important function for optimizing a radio spectrometer like Callisto. We use it regularly to evaluate different locations in view of radio frequency interference. The spectral overview is ideal plot to compare observatory places around the world, see figure 1 below for Mauritius. We also use it several times every year to optimize existing frequency programs as well as to create new ones. And finally it can also be used for radio frequency monitoring. If one compares spectral overviews at different dates one may find out new transmitter signals. If this should be the case and if the signal is within bands which are reserved for radio astronomy or other passive services it may become a case for OFCOM (office for communication).

To get a spectral overview we need two separate measurements to create a spectrum plot. One measurement is with an antenna attached to the receiver while pointing to the sun or just to the sky and is denoted to ‘Sky’. A second measurement is needed with a 50 Ω resistor at ambient temperature as reference with about 300 Kelvin. It will be denoted to ‘Ref’. The measurement itself is a manual function of the Callisto application-software by just pressing the button ‘Save spectral overview’. This takes about 30 second for all 13’200 channels. The data is stored in a simple ASCII file containing two columns, the first with frequency expressed in MHz and the second with the ADC-value expressed in mV with 10 bit resolution. The plot can be produced by any spread sheet tool like EXCEL, Mat-CAD, Mathematica, Maple or IDL. The example shown in figure 1 was produced by a simple IDL program, see attachment below.

Figure 1: Spectral overview (raw data) at MRT site in Mauritius. Out of these two plots we can create a new plot expressed in dB with the knowledge of the detector gain, see figure 2. In case of Callisto this gain is 25 mV/dB. Zero dB in this context is referred to system noise level of the receiver with 50 Ω resistor attached to the input. The system noise level or the noise figure of the receiver (without a preamplifier) is in the order of 9 dB which is about 2’083 K noise temperature.

Figure 2: Spectral overview in dB at MRT site in Mauritius. In figure 2 we can recognize radio signals which are 45 dB above internal noise level of 2’083 K. This means that the radio environment produces antenna temperatures of up to 66 million Kelvin or even more. These temperature levels are much higher than everything what could be expected from the sun during a flare with a reasonable antenna. In the example presented we can see several DVB-T signals above 500 MHz and we can easily see the transponder signals form America military satellites near 250 MHz (UFO-2, UFO-7, Skynet, Fltsatcom, Sicral etc.). We often use these satellite signals as a kind of indirect calibration or at least as a mean for a simple plausibility test. If they appear with about 10 dB above noise floor then everything is o.k. in the given configuration.

Attachment IDL program:

PRO PlottOverviewMRT1

sunfile ='H:\MyDocs\Callisto\IHY\Korea\Essay\PlotOV\OVS_MRT_20090427_083402.prn'
sundata = read_ascii(sunfile, DATA_START=1, DELIMITER=";")
freq = reform(sundata.field1[0,*])
ysun = reform(sundata.field1[1,*])

reffile = 'H:\MyDocs\Callisto\IHY\Korea\Essay\PlotOV\OVS_MRT_20090427_083957.prn';T0
refdata = read_ascii(reffile, DATA_START=1, DELIMITER=";")
freq = reform(refdata.field1[0,*])
yref = reform(refdata.field1[1,*])

title = 'Callisto spectral overview Mauritius Radio Telescope (vertical log-per)'
xtitle = 'Frequency [MHz]'
Ytitle = 'Intensity [mV]'

Filename = 'H:\MyDocs\Callisto\IHY\Korea\Essay\PlotOV\MRTovplot100pwm.eps'
freqrange = [0,870];frequency range to plot
freqstep = 50; (50) stepsize frequency-axis
dbrange = [-5,45];y-axis range

;set_plot,'PS'
;DEVICE, /ENCAPSULATED, BITS_PER_PIXEL=8, /COLOR, FILENAME=filename,xsize=20, ysize=10

plot,freq,ysun,$
xrange=freqrange,xtickinterval=freqstep,$
yrange=[1200,2500],ytickinterval=100,title=title,xtitle=xtitle,ytitle=ytitle
loadct,5
oplot,freq,ysun,color=133
oplot,freq,yref,color=33
xyouts,100,2300,'Sky (red)' ,color=133,charthick=3, charsize=1
xyouts,400,2300,'50 Ohms (blue)',color=33 ,charthick=3, charsize=1

;device,/close
;set_plot,'Win'

image3d = TVRD(TRUE=1)
WRITE_JPEG, 'H:\MyDocs\Callisto\IHY\Korea\Essay\PlotOV\MRTovplot100pwm.jpg', image3d, TRUE=1, QUALITY=100

end

In the above program you need to edit pathnames and filenames suitable for local situation. Similar examples can be downloaded form here:
http://www.astro.phys.ethz.ch/instrument/callisto/ecallisto/applidocs.htm

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