6 ms·

I think it's amazing how they thought it had landed and it was still moving. This is a very very tough thing they are trying to do. Couldn't Rosetta have told

I think it's amazing how they thought it had landed and it was still moving. This is a very very tough thing they are trying to do. Couldn't Rosetta have told them that Philae had bounced? I might have the wrong scale in my head of just what Rosetta could see. In any case this is all still wondrous.

jessriedel 12y ago
I can't for the life of me understand why Rosetta doesn't have equipment to determine Philae's relative position to cm resolution, except perhaps in the case where the comet is between them.
- leeoniya 12y ago
  i was thinking, couldn't they afford to put a simple signaling mirror or corner cube reflector on the lander so rosetta can just scan the surface and see any reflection? they've had them on the moon for decades. http://en.wikipedia.org/wiki/Corner_reflector http://en.wikipedia.org/wiki/Corner_reflector
- alimoeeny 12y ago
  I don't think there is such technology available. All the options that come to my mind need aiming and aiming is not an option as far as I can imagine.
  - sp332 12y ago
    Rosetta had to aim to launch Philae in the first place, right? Why can't it aim now?
    - codeulike 12y ago
      Rosetta is about 50km from the comet. To Rosetta's camera, Philae is about 1 pixel.
- joshvm 12y ago
  Because positioning doesn't work like that. It's possible that Rosetta could have carried a detector that could get the time of flight of Philae and so get a rough sphere where it could be. However, over the distances that we're talking about, you'd probably need a powerful laser with a very low divergence in order to get any signal there/back. LIDAR will break a power budget easily. If you're thinking why couldn't we use some kind of GPS type positioning, remember that you need four satellites minimum for that (plus ground stations to correct things). There was some discussion about using multiple flybys of Rosetta, but it's not clear whether there'll be enough time to do this before the batteries die. We can locate things on Mars because we have excellent cartography of the planet and the landers were the right way up. We're able to figure out where things are based on the images they send back. Once we know where a lander is the first time, it's easier to figure out where it is afterwards based on this imagery.
  - 13 12y ago
    Why are four satellites needed for positioning, wouldn't three do the trick?
    - joshvm 12y ago
      The first satellite gives you a sphere (a shell really) The second satellite gives you a circle of intersection between the spheres. The third satellite gives you two points where three spheres intersect. One of these is usually in space so it's discarded. The fourth one lets you correct for timing errors in the reciever. A microsecond uncertainty corresponds to around 300m (i.e. c * 1µs). You could get coarse information with three satellites, for instance if you already know your altitude, but four+ is preferable.
    - tonyarkles 12y ago
      In Earth-based GNSS systems, it's for clock synchronization. You've got 4 unknowns (x,y,z,delta t) and 4 signals; if you only had 3 signals, you couldn't solve the system of equations.
  - jessriedel 12y ago
    > Because positioning doesn't work like that. Yes it can. You don't need LIDAR when you the thing you are tracking can have a purpose-built transmitter attached to it. Obviously I'm not talking about GPS, I'm talking about some sort of EM radiation (e.g. the IR on your remote control) being emitted from Philae and being simply seen, as a bright dot in space, by Rosetta. Distance, obviously, you do with timing or with luminosity. Doubtlessly there is a list of many other things I haven't considered, at the top of which are nonintuitive weight, power, and reliability restriction. But your comment doesn't answer my question at all.
- acqq 12y ago
  1) It is a very cheap mission 2) It's a first one. It wasn't really known up to the last detail what to expect. If there's the next mission, some things will certainly be done differently. Especially if this one results in more willingness to fund the next. Also note that the precise location on Earth (GPS) needs a lot of satellites in orbit providing only that. There's only one device now orbiting around the comet's core. The position will probably be found based on all we have now (1), but having it very fast is significantly harder. 1) https://twitter.com/FrancescoTop/status/533417398662336512 https://twitter.com/FrancescoTop/status/533417398662336512

jessriedel 12y ago

I can't for the life of me understand why Rosetta doesn't have equipment to determine Philae's relative position to cm resolution, except perhaps in the case where the comet is between them.

leeoniya 12y ago
i was thinking, couldn't they afford to put a simple signaling mirror or corner cube reflector on the lander so rosetta can just scan the surface and see any reflection? they've had them on the moon for decades. http://en.wikipedia.org/wiki/Corner_reflector http://en.wikipedia.org/wiki/Corner_reflector
alimoeeny 12y ago
I don't think there is such technology available. All the options that come to my mind need aiming and aiming is not an option as far as I can imagine.
- sp332 12y ago
  Rosetta had to aim to launch Philae in the first place, right? Why can't it aim now?
  - codeulike 12y ago
    Rosetta is about 50km from the comet. To Rosetta's camera, Philae is about 1 pixel.
joshvm 12y ago
Because positioning doesn't work like that. It's possible that Rosetta could have carried a detector that could get the time of flight of Philae and so get a rough sphere where it could be. However, over the distances that we're talking about, you'd probably need a powerful laser with a very low divergence in order to get any signal there/back. LIDAR will break a power budget easily. If you're thinking why couldn't we use some kind of GPS type positioning, remember that you need four satellites minimum for that (plus ground stations to correct things). There was some discussion about using multiple flybys of Rosetta, but it's not clear whether there'll be enough time to do this before the batteries die. We can locate things on Mars because we have excellent cartography of the planet and the landers were the right way up. We're able to figure out where things are based on the images they send back. Once we know where a lander is the first time, it's easier to figure out where it is afterwards based on this imagery.
- 13 12y ago
  Why are four satellites needed for positioning, wouldn't three do the trick?
  - joshvm 12y ago
    The first satellite gives you a sphere (a shell really) The second satellite gives you a circle of intersection between the spheres. The third satellite gives you two points where three spheres intersect. One of these is usually in space so it's discarded. The fourth one lets you correct for timing errors in the reciever. A microsecond uncertainty corresponds to around 300m (i.e. c * 1µs). You could get coarse information with three satellites, for instance if you already know your altitude, but four+ is preferable.
  - tonyarkles 12y ago
    In Earth-based GNSS systems, it's for clock synchronization. You've got 4 unknowns (x,y,z,delta t) and 4 signals; if you only had 3 signals, you couldn't solve the system of equations.
- jessriedel 12y ago
  > Because positioning doesn't work like that. Yes it can. You don't need LIDAR when you the thing you are tracking can have a purpose-built transmitter attached to it. Obviously I'm not talking about GPS, I'm talking about some sort of EM radiation (e.g. the IR on your remote control) being emitted from Philae and being simply seen, as a bright dot in space, by Rosetta. Distance, obviously, you do with timing or with luminosity. Doubtlessly there is a list of many other things I haven't considered, at the top of which are nonintuitive weight, power, and reliability restriction. But your comment doesn't answer my question at all.
acqq 12y ago
1) It is a very cheap mission 2) It's a first one. It wasn't really known up to the last detail what to expect. If there's the next mission, some things will certainly be done differently. Especially if this one results in more willingness to fund the next. Also note that the precise location on Earth (GPS) needs a lot of satellites in orbit providing only that. There's only one device now orbiting around the comet's core. The position will probably be found based on all we have now (1), but having it very fast is significantly harder. 1) https://twitter.com/FrancescoTop/status/533417398662336512 https://twitter.com/FrancescoTop/status/533417398662336512