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Old 9th May 2011, 20:37
  #1028 (permalink)  
auv-ee
 
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Noise

Originally Posted by RR_NDB
Signals 20 dB or more below the noise floor can detected using this method!
I think you are confusing wide-band vs. in-band noise. Note that I said in answer to DJ77 in post 1005, that a traditional detection starts with a narrow band filter. This accepts and compares only the signal and noise that is within 100-200Hz of the target signal; the bandwidth of a 10ms pulse being about 100Hz .

The pictures you have shown appear to be of signals that are at least 10 and maybe 20db above the in-band noise (I don't know what the color scale is for those images). Your sonograms are performing the narrow-band filter operation, by using an FFT to break the spectrum into narrow slices, then separately displaying each slice.

gums points out correctly that GPS works at very low SNR; but GPS uses a long pseudo-random code that an auto-correlation algorithm can detect. Unfortunately, these old pingers just send a 10msec tone burst. One can certainly do better using techniques possible with a DSP than with the traditional detector, but I'm certain that the performance of a system like GPS, which uses engineered signals, cannot be exceeded when the signal is a tone burst filling only 1% of time (10msec/1sec).
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