Joined: 30 Jul 15
Reception of signals from Deep Space...
I posted the below over a year ago, and it received hundreds of hits back then. I thought a re-post may be of value as many new people have joined SETI since then? Below is a slightly modified and up-dated version of the original post.
Previously posted & modified: 17 Apr 2016, 19:36:01 UTC
I've read many posts to this, and other, message boards, 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 radio signal reception to deep-space signal reception. The two are, almost always, NEVER the same.
To put it simply, ALL earthly signals, those that can be received, like those in cellular, two-way radio, radar, etc. MUST be STRONG enough to be 'seen' above the thermal noise floor. This floor is, roughly, -174 dBM/Hz; better yet, the signals should be at least -171 dBm/Hz (3 dB higher), the difference represents a doubling of signal power.
It is NOT the same for the scheme used to track a probe, like Voyager, and others, far away from earth. I've designed receivers for NASA while working as an RF/Microwave engineering consultant, years back. 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 it's 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, un-like its counterpart on Earth, can receive signals as far as 20 to 30 dB BELOW the thermal noise floor !! This is WHY data communications, earth to probe, is possible. The telemetry is sent repetitively, over and over and over, until it's received correctly at each end; using what some 'digital types' call a 'CRC' code, i.e., a 'cyclical redundancy code.' The code TELLS each receiver how MANY bits to expect. These bits are then counted, at each end, and checked against what the number should be. If the number of bits is correct, the data is accepted; if not, it must be re-sent, and listened for again, until all is well at both ends. You cannot say "Hello" just ONCE, as in earthly voice communications. You may have to say "Hello hello hello hello, and hello," MANY times for it to be heard.
HOW and WHY is this type of data communications possible you may ask? How does it WORK? The reason is simple and lies in the definition of noise. Noise is RANDOM; it is not PERIODIC. A deep-space receiver "integrates" noise over time, and, in doing so, eventually "sees" a very weak periodic signal thus making below the noise floor reception possible, if, albeit, such communications are likely slow.
How does a receiver 'integrate?' Here is a simple explanation; it's not exact, but close enough to give you the general idea. Imagine a large capacitor connected to a source that sends it periodic pulses. Over time the capacitor will "fill up," right? The threshold, where the charge is just high enough to be measured, can be likened to a signal just above 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 I learned from a U.S. Navy relative.
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 navy ship, likely a destroyer, 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 of the scope. The area around the ship, and extending out perhaps hundreds of yards, as seen on the radar scope, was filled with noise; this noise was called "sea return." A skilled radar operator, (like my relative), sitting in front of what was called, by some, "the stack," (the radar PPI), in CIC, the (Combat Information Center), could, by carefully studying the sea return pick out the presence of a submarine periscope and sound the alert.
While it is presumed easily possible to 'chit chat' in deep space, as seen in Star Trek, or Star Wars, and other sy-fy, using voice, in REALITY, if the crafts are FAR enough apart, some form of repetitive transmission would have to be used. This is our CURRENT 'state of the art.' Perhaps, in the far future, once a "faster than light" drive is invented, we will be able to "chit chat" from one solar system, or one galaxy to another? Who knows, but NOT now.
I would like to think that SETI may use signal integration of received data, but have no clue if it does? Anyone know?
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