Quote from: JasonAW3 on 01/20/2015 03:02 pmQuote from: ugordan on 01/20/2015 02:53 pmQuote from: pagheca on 01/20/2015 02:39 pmSo, as the communication distance increases, the pointing error becomes more and more relevant on each side, and you cannot beam as the receiver could miss the information. That doesn't make sense. Any given directional antenna will focus its signal into a specific cone. As distance grows, the cone footprint at Earth increases so the pointing requirements become *less* stringent, not more.I don't know much about RF comms, but my understanding was that the radio wavelength chosen is what limits how well the signal can be collimated (to reduce "waste" of the footprint) due to diffraction. That's one of the biggest motives for going with optical comms, not that RF is inherently bad. Optical beams can be collimated so tightly that it really does get to a point where you need to have very precise pointing. For radio, not so much.Basic physics; As the signal is sent down the cone, the strength of the signal falls off as that same power is spread over a larger width, plus, the signal itself looses energy due to gas, dust, EM fields and general RF interference. Thus, like all forms of radiation, the signal strength falls of with the square of the distance.Uh, the inverse square law is exactly what I was talking about in my first post on bandwidth above. I don't see what's that got to do with pointing requirements pagheca talked about. A wider footprint caused by greater distance means that you have more of an angular deadband wiggle room before you need to repoint your antenna at Earth. It is true, though, that you need to keep the Earth withing the angular size of the cone at all times and that doesn't change, but you get progressively more and more of the Earth's orbit inside that cone as distance increases. For Voyager, the "half-power half-width of the antenna beam was 0.32 degrees at X-band and 1.1 degrees at S-band."
Quote from: ugordan on 01/20/2015 02:53 pmQuote from: pagheca on 01/20/2015 02:39 pmSo, as the communication distance increases, the pointing error becomes more and more relevant on each side, and you cannot beam as the receiver could miss the information. That doesn't make sense. Any given directional antenna will focus its signal into a specific cone. As distance grows, the cone footprint at Earth increases so the pointing requirements become *less* stringent, not more.I don't know much about RF comms, but my understanding was that the radio wavelength chosen is what limits how well the signal can be collimated (to reduce "waste" of the footprint) due to diffraction. That's one of the biggest motives for going with optical comms, not that RF is inherently bad. Optical beams can be collimated so tightly that it really does get to a point where you need to have very precise pointing. For radio, not so much.Basic physics; As the signal is sent down the cone, the strength of the signal falls off as that same power is spread over a larger width, plus, the signal itself looses energy due to gas, dust, EM fields and general RF interference. Thus, like all forms of radiation, the signal strength falls of with the square of the distance.
Quote from: pagheca on 01/20/2015 02:39 pmSo, as the communication distance increases, the pointing error becomes more and more relevant on each side, and you cannot beam as the receiver could miss the information. That doesn't make sense. Any given directional antenna will focus its signal into a specific cone. As distance grows, the cone footprint at Earth increases so the pointing requirements become *less* stringent, not more.I don't know much about RF comms, but my understanding was that the radio wavelength chosen is what limits how well the signal can be collimated (to reduce "waste" of the footprint) due to diffraction. That's one of the biggest motives for going with optical comms, not that RF is inherently bad. Optical beams can be collimated so tightly that it really does get to a point where you need to have very precise pointing. For radio, not so much.
So, as the communication distance increases, the pointing error becomes more and more relevant on each side, and you cannot beam as the receiver could miss the information.
Quote from: pagheca on 01/20/2015 02:39 pmSo, as the communication distance increases, the pointing error becomes more and more relevant on each side, and you cannot beam as the receiver could miss the information. That doesn't make sense. Any given directional antenna will focus its signal into a specific cone. As distance grows, the cone footprint at Earth increases so the pointing requirements become *less* stringent, not more.
[...] Centering the dish once the signal is acquired can probably be done in 60 seconds or so. (My guess)
Quote from: Nomadd on 01/20/2015 05:01 pm[...] Centering the dish once the signal is acquired can probably be done in 60 seconds or so. (My guess)You have a 9hs round trip. You can't know your dB drop with anything remotely close to real time. Unless you mean pointing the receptor towards a carrying wave sent from Earth.
Quote from: baldusi on 01/20/2015 05:03 pmQuote from: Nomadd on 01/20/2015 05:01 pm[...] Centering the dish once the signal is acquired can probably be done in 60 seconds or so. (My guess)You have a 9hs round trip. You can't know your dB drop with anything remotely close to real time. Unless you mean pointing the receptor towards a carrying wave sent from Earth. I didn't think of the carrier not being up all the time. It would have to be on a schedule. Just pointing using guide stars seems like a hard way to get 1/4 degree accuracy, but maybe that's how they do it.
Quote from: Nomadd on 01/20/2015 08:05 pmQuote from: baldusi on 01/20/2015 05:03 pmQuote from: Nomadd on 01/20/2015 05:01 pm[...] Centering the dish once the signal is acquired can probably be done in 60 seconds or so. (My guess)You have a 9hs round trip. You can't know your dB drop with anything remotely close to real time. Unless you mean pointing the receptor towards a carrying wave sent from Earth. I didn't think of the carrier not being up all the time. It would have to be on a schedule. Just pointing using guide stars seems like a hard way to get 1/4 degree accuracy, but maybe that's how they do it.Does the ground station send out a reference beacon 4 1/2 hours before a scheduled transmission to help set up the tracking ? It would help with the pointing, but the station would be on the wrong side of the earth 9 hours before it starts receiving.
[/font]Quote from: pagheca on 01/20/2015 02:39 pm[/font]So, as the communication distance increases, the pointing error becomes more and more relevant on each side, and you cannot beam as the receiver could miss the information. [/font]That doesn't make sense. Any given directional antenna will focus its signal into a specific cone. As distance grows, the cone footprint at Earth increases so the pointing requirements become *less* stringent, not more.
[/font]So, as the communication distance increases, the pointing error becomes more and more relevant on each side, and you cannot beam as the receiver could miss the information.
Quote from: ugordan on 01/20/2015 02:53 pmQuote from: pagheca on 01/20/2015 02:39 pmSo, as the communication distance increases, the pointing error becomes more and more relevant on each side, and you cannot beam as the receiver could miss the information. That doesn't make sense. Any given directional antenna will focus its signal into a specific cone. As distance grows, the cone footprint at Earth increases so the pointing requirements become *less* stringent, not more.The longer is the distance, the wider is the footprint, the lower is the received power per surface unit.You can't go below a specific power level, else you won't be able to distinguish signal from noise; so you need a thinner cone; "thinner" and "taller" (=more distant) means a lot more difficult to point. Try turning off light in your room using a broomstick. The longer it is, the more difficult it is.(snip)
Ceres and Pluto unfolding... will I get any work done this spring?
Can I feel old, remembering the first dead tree pre HST publication of a picture of Pluto and Charon resolved as two separate objects in an image?
Quote from: kevin-rf on 02/04/2015 10:39 pmCan I feel old, remembering the first dead tree pre HST publication of a picture of Pluto and Charon resolved as two separate objects in an image?Why would that make you feel old? Don't all these new discoveries make you feel young and lucky to be alive at a time when we learn such new and fascinating things every day?
Barring an insisted-upon expedition to Eris
Sadly this isn't a real-life mission, but it is an extremely cool concept. I had no idea until watching that how truly far away Eris is.