How to Make Use of the Calibration

Joachim Köppen DF3GJ Kiel/Strasbourg/Illkirch Summer 2004

(A) With the data from the calibration steps, we can convert the audio voltages measured by the sound card and recorded with the SkyPipe software into antenna voltages or more convienently into temperatures of thermal noise giving the same signal. This is done after the observations with some suitable software... (see here for more details)

(B) Straight comparison of the data readings with the readings of the calibration steps gives a rough estimate in which range the noise temperature of the signal lies. Here is a small table of the calibration steps for our calibrator whose maximum level is +35 dB over thermal noise (linked to measurements by Keith Gooley VK5OQ). For convenience, the temperatures are given in kK (thousands of K - note that k for kilo is written in small letter!):

attentuation [dB]noise level [dB(kT_0)]noise temperature [kK]
0 +35 917
-5 +30 290
-10 +25 92
-15 +20 29
-20 +15 9.2
-25 +10 2.9

(note that for convenience only I list the attenuations as negative dB numbers ... of course, more properly a gain of -10 dB is an attenuation of 10 dB)

(C) But from taking just the readings during the calibration sequence, we can already well estimate these noise temperatures:

This procedure is accurate for readings well above the internal noise level of the receiver.

So what is the level of night-time background noise you measure at your site?

At ISU we have had something just below 100 kK on nights without interference from the blinking streetlamp. During the day, it can be about 100 kK, but also as large as 300 kK - a combination of radio noise reflected by the ionosphere and local noise from computer electronics in the building. In winter 2004/05 we had quiet nights with less than 50 kK, but could not yet observe the Galactic centre. However, at other times a steady noise of about 2 million K can be seen to be suddenly switched on, lasting for some hours or even a few days, before being switched off again. Sometimes, one can identify two levels of noise. We haven't yet been able to locate the source in our neighbourhood...


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