Thursday, February 2, 2012

CASARA, Electronic Searches - Science vs Pareidolia - Part 2

In Part 1 I presented some still photographs of the audio spectra of signals recorded during an Off-Tuned/Aural Null/Cardinal Pass electronic search. Here are some videos of the spectra of the passes. I intentionally left the audio track out of the video so that you can concentrate on finding the peak through direct measurement of the signal volume. The crew claimed to be able to determine the position to within 0.125nm, assuming a search speed of approximately 100kts 0.125nm would be covered in 4.5 seconds. So to do as well you will need to identify the 4.5 second segment of maximum signal in each clip. With the advantage of being able to re-play and pause the video you should be able to pick out the one or two sweeps that are stronger than all the others. Remember signal strength varies with the inverse of the square of distance so the closer the search airplane is to the ELT the faster the rate of signal strength change. All of this assumes the receiver is actually passing that on to the crew, and the Automatic Gain Control (AGC) isn't throwing that information away. If so you should be able to see a very clear peak in these videos*.

In the FFT trace on the top the ELT signal will look like a bunch of sports fans doing the "wave" from right to left as the frequency sweeps from high to low. The FFT Waterfall display at the bottom may be easier to read. Colours change to indicate volume level. Leave a comment with the time index for each clip if you think you can locate the ELT. Include an email address if you want me to return your results (I will remove the email from the comment before it gets posted).

In a stroke of irony, while I was producing these videos I came accross a portion where the airplane flies overtop of the ELT. The navigator says "And we're right over top of the target." To which the pilot replies "Don't know that because of the signal, we know that because that's where we put it." The navigator agrees "That's right."



Here is the other pass


* Of course you won't see a peak, because the AGC does control the signal volume and there is no peak.

No comments:

Post a Comment