Reception of Signals from Deep Space...

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Profile River Song
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Message 1779941 - Posted: 17 Apr 2016, 19:33:52 UTC
Last modified: 17 Apr 2016, 19:36:01 UTC

Hi All,

I've read many posts to this message board, and there appears to be a misconception by a few SETI Users about the reception of signals from deep space?

The misconception takes the form of likening earthly RF/Microwave signal reception to deep-space signal reception. The two are usually NOT the same.

To put it simply, ALL earthly signals, to be received, like those used in radar or two-way, MUST be strong enough to be 'seen' above the thermal noise floor. This floor is, roughly, -174 dBM/Hz; better yet, it should be at least -171 dBm/Hz or higher.

It is NOT the same for the scheme used to track a probe, like Voyager, and others. I've designed receivers for NASA while working as an RF/Microwave engineering consultant. Early on in my employ, I was literally "blown away" when I discovered an important difference between earthly and interstellar data transmission.

The further out a probe gets from Earth, the weaker its received signal is. We all know that, and its expected. What is NOT known by some is the answer to the following question: Is it necessary that the received signal ALWAYS be above the thermal noise floor? The answer is NO.

A deep-space receiver in a probe, like its counterpart on Earth, can receive signals as far as 20 to 30 dB BELOW the thermal noise floor! This is WHY 2-way communication is possible. Telemetry is sent repetitively, over and over, until received correctly at each end; using what some 'digital types' call a 'CRC' code, i.e., a 'cyclical redundancy code.'

HOW is this possible you may well ask? The reason is simple and lies in the definition of noise. Noise is RANDOM; it is not PERIODIC. A deep-space receiver integrates over time, and, in doing so, 'sees' a periodic signal thus making below the noise floor transmissions receivable.

How does such a receiver 'integrate?' Here is a simple explanation; not exact, but enough to give you an idea. Imagine a large capacitor connected to a source that sends periodic pulses. Over time the capacitor will 'fill up,' right? The threshold, where the charge is high enough to be measured, can be likened to the thermal noise floor. It's as simple as that. :)

How did the idea of 'low-level signal integration' come about?' I've heard theories, and here is ONE of them.

In WW2, in the Atlantic, Nazi U-boats were a major threat. To attack them you needed to detect them. A good ship-borne radar of the time, using a PPI (plan position indicator), type display, had the ship at the center of a circular display; the screen was maybe 10 inches in diameter. The radar 'beam' could be seen revolving around the center. The area around the ship, and extending out perhaps hundreds of yards, was filled with noise; this noise was called "sea return." A skilled radar operator, sitting on what was called by some, 'the stack,' in CIC, the (Combat Information Center), could, by carefully studying the sea return pick out the presence of a submarine periscope and warn the ships captain.

While it is easily possible to 'chit chat' in deep space, like in Star Trek, or Star Wars, using voice, if the craft are far enough apart, some form of repetitive transmission must be used. This is our current 'state of the art.'

I would like to think that SETI uses signal integration of received data, but have no clue if it does? Anyone know?
River Song (aka Linda Latte on planet Earth)
"Happy I-Phone girl on the GO GO GO"
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Message boards : SETI@home Science : Reception of Signals from Deep Space...


 
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