31–40 of 205 posts
You know, I never really thought of lower wavelengths than light as being carried by photons, but I suppose it's all EM. Antennas are technically just really red light bulbs.
Earlier quoted context omitted.
There's another major factor. I suppose it falls under the encoding. Frequency stability. In a certain sense, having an extremely precise oscillator at both the receiver and the transmitter, is the same thing as just having a better more frequency-stable antenna, or less noise in the channel (because you know what the signal you're listening for should look like). I'm no physicist here so take this with a major grain…
Coding counters the uncertainty principle by allowing multiple measurements, which can then be averaged. Thant counters the contribution of random noise. There are practical signals we use every day that are "below the noise floor" before we decode them. So while there is an ultimate limit of the maximum coding rate for a given signal-to-noise ratio, this is expressed in terms of a data rate (i.e. bits per second). I…
I love these kinds of questions. So what does that conclusion mean about when the probe will be so far away that we are below the Shannon limit? And can we beat the Shannon limit somehow, eg collect for longer, put the dish outside the atmosphere, and so on?
With each doubling of distance the number decreases by the inverse square law, so with the current setup we'd have a maximum distance of log2(60) = 5.9 times the current distance (about 163 astronomical units (AU)) which is 961 AU.
In comparison, the closest star to our sun, Proxima Centauri, is 268774 AU away!
Which means we would need something 268774/961 ~= 280 times SQUARED = 78222 times more sensitive than the current setup at the Shannon limit to communicate with it if it managed to get that far.
I am confused. I thought photons were just visible light but I guess these little buggers are everywhere. Also very surprised voyager is using 2.3ghz, that's crazy saturated on earth due to wifi. How these engineers make this all work, is magic to me.
Didn’t realize the math would be that straightforward. Is there something the author isn’t taking into account or is that a decent plausible range?
The TMU encodes the high rate data stream with a convolutional code having constraint length of 7 and a symbol rate equal to twice the bit rate (k=7, r=1/2).
So the effective symbol rate is 320 baud[2], and thus a factor of two should be included in the calculations from what I can gather.
Note that the error correction was changed after Jupiter to use Reed-Solomon[3] (255,223) to lower the effective bit error rate, so effectively I guess the data rate is more like 140 bps.
[1]: https://web.archive.org/web/20130215195832/http://descanso.j...
[2]: https://destevez.net/2021/09/decoding-voyager-1/
[3]: https://destevez.net/2021/12/voyager-1-and-reed-solomon/
I am confused. I thought photons were just visible light but I guess these little buggers are everywhere. Also very surprised voyager is using 2.3ghz, that's crazy saturated on earth due to wifi. How these engineers make this all work, is magic to me.
Nope. It's one of those things that can take a bit to get used to, but everything on the electromagnetic spectrum is just light in the general sense. The only difference between radio-waves, x-rays, infra-red and (human) visible light is the frequency/wavelength. If the frequency is high enough then the waves of light can be detected by things as small as cells in the back of your eye, or the pixels in a camera senso…
This makes me think of the dual slit experiment. Does the universe treat everything as a wave to save CPU cycles or something?
If we think of light as little balls (at our size i think that would make sense). If we were much bigger, we would think of these longer waves as balls too?
It's crazy to me how many theoretical limits Shannon predicted way before the hardware was there.
I love these kinds of questions. So what does that conclusion mean about when the probe will be so far away that we are below the Shannon limit? And can we beat the Shannon limit somehow, eg collect for longer, put the dish outside the atmosphere, and so on?
I think the practical limit right now is that the Voyagers are losing power.
"The radioisotope thermoelectric generator on each spacecraft puts out 4 watts less each year. [...] The two Voyager spacecraft could remain in the range of the Deep Space Network through about 2036, depending on how much power the spacecraft still have to transmit a signal back to Earth."
I love these kinds of questions. So what does that conclusion mean about when the probe will be so far away that we are below the Shannon limit? And can we beat the Shannon limit somehow, eg collect for longer, put the dish outside the atmosphere, and so on?
Not my field, but assuming transmitting hardware (including beam forming) is constant and that atmosphere can mostly be ignored (see comments about it usually being a non-impact in the transmission frequencies), two approaches would suggest: 1. Increase the effective receiving dish size, to capture more of the signal. Essentially, this would be effective in direct proportion to beam spread (the more beam spread, the…
Of course that would mean sending (a) giant receiver dish(es) in the general direction a probe is sent. On the flip side, if using a single relay it could travel at roughly 1/2 the speed of the probe.
Note that signal strength weakens with distance^2. So if eg. you'd have 2 relays (1/3 and 2/3 between Earth & the probe), each relay would receive 9x stronger signal.
No doubt the 'logistics' (trajectory, gravity assist options, mission cost etc) make this impractical. But it is an option.
I am confused. I thought photons were just visible light but I guess these little buggers are everywhere. Also very surprised voyager is using 2.3ghz, that's crazy saturated on earth due to wifi. How these engineers make this all work, is magic to me.