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Is a highly advanced radio signal indistinguishable from noise?
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Tom Mazanec Send message Joined: 19 Aug 15 Posts: 79 Credit: 6,938,247 RAC: 4 |
I have heard that we may never detect "leakage" from an alien civilization because the most efficient possible transmission within an advanced civilization is indistinguishable from noise. Also that spread spectrum signals are the ultimate for security and are indistinguishable from noise. Is this true? |
William Rothamel Send message Joined: 25 Oct 06 Posts: 3756 Credit: 1,999,735 RAC: 4 |
Not all transmissions would be spread spectrum. Television, radar etc would not be. Also any intentional signal would not be encrypted whatsoever. I think that this is not an issue at all. |
KLiK Send message Joined: 31 Mar 14 Posts: 1304 Credit: 22,994,597 RAC: 60 |
I have heard that we may never detect "leakage" from an alien civilization because the most efficient possible transmission within an advanced civilization is indistinguishable from noise. Also that spread spectrum signals are the ultimate for security and are indistinguishable from noise. Yes, over some distance they go down in background noise...totally indistinguishable! :( non-profit org. Play4Life in Zagreb, Croatia, EU |
rob smith Send message Joined: 7 Mar 03 Posts: 22160 Credit: 416,307,556 RAC: 380 |
A spread spectrum transmission is actually quite distinguishable from noise, it is just very hard to demodulate and thus decode. Bob Smith Member of Seti PIPPS (Pluto is a Planet Protest Society) Somewhere in the (un)known Universe? |
Mr. Kevvy Send message Joined: 15 May 99 Posts: 3776 Credit: 1,114,826,392 RAC: 3,319 |
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River Song Send message Joined: 30 Jul 15 Posts: 268 Credit: 1,735,966 RAC: 0 |
Detection of signals from Deep Space... 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" |
William Rothamel Send message Joined: 25 Oct 06 Posts: 3756 Credit: 1,999,735 RAC: 4 |
The integration that you are referring to is cross correlation and autocorrelation and relies on the expectation of a specific pulse form and the fact that random (thermal noise) does not correlate. if you slide a square wave into another square wave then you will get the summation of the area as the one square wave slides past another. The resultant wave will be a triangular wave that is higher in amplitude than either of the square waves and hopefully above the thermal noise. Since we tend to know the spectrum of thermal noise this is made more easy. I think that you are referring to sonar in your analogy of submarines. |
River Song Send message Joined: 30 Jul 15 Posts: 268 Credit: 1,735,966 RAC: 0 |
"I think that you are referring to sonar in your analogy of submarines." No, radar, not sonar... A good operator, back then, could pick out the periscope of a slow moving 'diesel/electric boat,' moving at maybe 2 to 6 knots, out of 'sea return.' However, in the case of a nuclear sub, which is much faster submerged, the case is different. The Nautilus, SSN-571, the first nuclear boat, when first entering San Diego, CA harbor around 1957, did about 30 knots at flank speed submerged, and easily eluded a screen of several destroyer escorts during a 'war game.' The DE's were given a 24 hour period during which to expect the new boat and it left them in the dust. :) This is perhaps a little known historical fact. River Song (aka Linda Latte on planet Earth) "Happy I-Phone girl on the GO GO GO" |
William Rothamel Send message Joined: 25 Oct 06 Posts: 3756 Credit: 1,999,735 RAC: 4 |
Ok, That's interesting. My father (now deceased) was in charge of the memorial service for the Thresher at the New London Sub Base. |
River Song Send message Joined: 30 Jul 15 Posts: 268 Credit: 1,735,966 RAC: 0 |
"Ok, That's interesting. My father (now deceased) was in charge of the memorial service for the Thresher at the New London Sub Base." Wow, really? The Thresher accident was a tragedy......... A very sad happening... The reason I know about what I posted is that my husband was the man on that "radar stack." :) Stay here on Earth. It's the only planet with DARK CHOCOLATE !! River Song (aka Linda Latte on planet Earth) "Happy I-Phone girl on the GO GO GO" |
Lynn Send message Joined: 20 Nov 00 Posts: 14162 Credit: 79,603,650 RAC: 123 |
50 years later, mystery of bizarre radio echoes solved More than 50 years after weird radio echoes were detected coming from Earth's upper atmosphere, two scientists say they've pinpointed the culprit. And it's complicated. In 1962, after the Jicamarca Radio Observatory was built near Lima, Peru, some unexplainable phenomenon was reflecting the radio waves broadcast by the observatory back to the ground to be picked up by its detectors. The mysterious cause of these echoes was sitting at an altitude of between 80 and 100 miles (130 and 160 kilometers) above sea level. http://www.cbsnews.com/news/bizarre-radio-echoes-mystery-solved-50-years-later/ |
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