Why is bandwidth so much lower at Pluto? I can't see how the increased lag has anything to do with this.
Why is bandwidth so much lower at Pluto? I can't see how the increased lag has anything to do with this.It seems to me that if New Horizons can transmit at 38 kbit/s from Jupiter, it can transmit at 38 kbit/s from Pluto. In both cases Earth will receive a 38 kbit/s data stream, the only difference is that in the second case it will receive the stream a few hours later.Can someone explain why the increased distance affects the bandwidth?
Quote from: Sesquipedalian on 01/20/2015 01:34 pmWhy is bandwidth so much lower at Pluto? I can't see how the increased lag has anything to do with this.It seems to me that if New Horizons can transmit at 38 kbit/s from Jupiter, it can transmit at 38 kbit/s from Pluto. In both cases Earth will receive a 38 kbit/s data stream, the only difference is that in the second case it will receive the stream a few hours later.Can someone explain why the increased distance affects the bandwidth?Signal strength drops off with the square of the distance and bandwidth scales with received power. Pluto is at around 40 A.U., Jupiter at around 5.2 A.U.Yes, one way light time has nothing directly to do with bandwidth. Of course that increasing distance for a *given* transmitter power will decrease bandwidth, but nothing in principle prevents you from putting a kilowatt transmitter at Pluto and getting back a more reasonable bitrate than the trickle New Horizons will get.
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.
I'm just worried about how complex and filled with debris the Plutonian system is. With those recently-discovered mini-moons, the area close to the planet could easily have several debris fields (although I would think the area between Pluto and Charon should have been cleared out by their gravity and motion through that space). I know that missions control was looking at ways to minimise collision risk but there has to be only so much they can do, given the probe doesn't have an MPS like Cassini (or, earlier, Galileo).
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.
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.