by Calilasseia wrote:What's the transmission rate from that spacecraft? Only I seem to recall it was 1 kilobit per second. If that's the case, then 50 gigabits is going to take nearly 18 months to transmit back to Earth.
Yes, that's about right although when Pluto is high in the sky, data rates get up to 4 2 megakilobits (:doh:) per second for an hour or two... but still, downloading data is going to be the main job for the next year plus months. There's not much else to do whilst waiting for the next Kuiper belt object fly-by in 2018.
They have a neat trick that can nearly double New Horizons' data transmission rate, but it comes at a cost of doing simultaneous science. New Horizons' radio system includes two Traveling Wave Tube Amplifiers or TWTAs (pronounced "twittas," like a Bostonian would say "twitters"). The TWTAs amplify the radio signals before they get broadcast from New Horizons' 2.1-meter dish. There are two TWTAs for redundancy: if one fails, the mission will still be able to return data to Earth. But the two TWTAs are not quite identical. One of them transmits radio signals with left-hand circular polarization, and one of them transmits with right-hand circular polarization.
Because they transmit with different polarization, both TWTAs can simultaneously transmit the same data through the dish antenna. On Earth, special hardware at the Deep Space Network can separately receive the two differently-polarized signals, and then combine them to make the signal stronger. Stronger signal means New Horizons can transmit at a higher data rate, about 1.9 times the rate with a single TWTA.
This two-TWTA mode wasn't developed until after launch; they deployed it early in the mission, and it worked well. But radio transmitters are power-hungry. New Horizons' nuclear power source has decayed since it launched nearly a decade ago, and there is no longer enough power to run both TWTAs at the same time as all the other spacecraft subsystems. If they want to nearly double their data rate and reduce their backlog, they need to shut something else down.
Emily Lakwallada blog source
I'm not doing a very good job at researching this but the confusion is entirely mine (I keep remembering snippets of stuff from the past few weeks without proper references to check). But, today, I found another link that talks about 4000 bits per second, some of the time...
But again, once the critical data collection phase is over, the New Horizons team is hopeful that we’ll be able to downlink somewhat faster. The craft is currently configured in what NASA calls ‘three-axis pointing mode’ (aka, Pluto observing mode), but it’ll transition over to ‘spin-stabilized mode’ after the encounter is over. In spin mode, New Horizons will be pointing itself arrow-straight at the Earth, spinning along its axis for increased stability. As a result, NASA reckons we’ll be able to boost downlink speeds to something in the neighborhood of 4,000 bits per second over the next few days. That’ll help New Horizons send us back a sampling of the key scientific data its collecting right now.
4,000 bits per second may be double our current downlink speed, but downloading planetary science data over 3 billion miles is still quite a bit slower than loading your email on a 56K connection. Hence the reason it’s going to take us an estimated 16 months to send home all the data we collect in the next several days.
Gizmodo source
