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Extricating Spirit, Digging out from Troy
SteveM
post Apr 13 2010, 05:55 PM
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I don't know the techniques being used on the ranging experiment but two points seem (at least theoretically) relevant.

1) The signal's wavelength need not be an absolute lower limit for position. If they're measuring phase shift of the received signal, they can probably get down to a fraction of a wavelength. (Precise GPS surveys do that all the time although in the Earth-Mars case the signal strength is much lower and it may be much more difficult to do.)

2) The stability of the local oscillator on the rover is not relevant to a doppler signal, where the frequency is set by the stability of the signal transmitted from the Earth ground station, received by the rover and retransmitted (perhaps with some alteration) back to Earth.

Steve M (talking outside his area of expertise).
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alan
post Apr 13 2010, 06:34 PM
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If Spirit manages to move over a meter wouldn't they still be able to do the measurements if they knew the distance moved to an accuracy of less than a cm?
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tanjent
post Apr 14 2010, 12:26 AM
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Yes, that is another very reasonable question to ask - why can't they correct for known movements? But in that case they certainly could have performed the experiment during the 1+ Earth years spent at Winter Haven 3. Those little movements inching down the slope were very precisely calculated.
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ElkGroveDan
post Apr 14 2010, 04:43 AM
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QUOTE (alan @ Apr 13 2010, 11:34 AM) *
If Spirit manages to move over a meter wouldn't they still be able to do the measurements if they knew the distance moved to an accuracy of less than a cm?

I'm going to say "no" and go way out on a limb with why I think not. With all the variables involved in the absolute distance between Earth and Mars in three dimensions, my guess would be that the uncertainty for the location is not linear since we are looking at variations along all three axis in 3D space. The equation accounting for the uncertainty of the distance from Spirit and a defined point on Earth more than likely describes the volume of a sphere. By changing Spirit's position I would assume that the radius of that uncertainty sphere is being increased accordingly, and therefore its volume is increasing exponentially.

Scott mentioned to Emily they could move Spirit several centimeters. If that means the base uncertainty is something like 6 centimeters, the uncertainty sphere would be 904 cm(3) in volume. If you move the rover a distance of one meter (100 cm) then the radius would be 106 cm with a volume of 4,986,387 cm(3). Comparing the volumes of the two spheres [4,986,387 cm(3)]/[904 cm(3)] it would seem that by moving the rover one meter it creates an uncertainty over 5500 times as large as the 6 cm uncertainty where she now sits. It certainly shows why extreme minimal movement is necessary for this experiment.


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Deimos
post Apr 15 2010, 07:18 PM
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QUOTE (tanjent @ Apr 13 2010, 04:39 AM) *
IIRC Spirit was able to call home during the previous winter, and yet the stationary observations that could have been done then were apparently not accurate enough.

That presumes that any old observations will do--not true--or that "could have been done" = "were done"--not true (at least in the quantities needed). The experiment needs passes other than what would be chosen normally, extra DSN resources, and extra energy on Spirit. Precise speed measurements with doppler shift require a to/from velocity component, so Earth has to be low in the sky. Winter direct-to-Earth comm tends to be associated with Earth being high in the sky.
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Deimos
post Apr 15 2010, 07:28 PM
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I think there are a couple issues with motion. Personally, I think motion during a specific precession-pole-measurement would be bad. Such a measurement might be done in a sol, but might stretch over a week given whatever the energy situation is. Second, movement over the course of the campaign has never been ruled out, to my knowledge. It's just that the uncertainty added by that needs to be at the 1-3 cm level (balance between being sufficiently better than current knowledge and being lost in other noise sources--no I don't know details). So I'd guess that moving 3 m might be OK; moving 30 would very much not be. So that could give Scott et al. plenty of room to play before they get to the end of the leash.
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ElkGroveDan
post Sep 21 2010, 02:49 PM
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I dropped by the office of a few friends last week who happen to be rover drivers.

In case anyone was wondering, Spirit's sister in the testbed at JPL is also still stuck and waiting out the winter.

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ElkGroveDan
post Sep 21 2010, 02:57 PM
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And can we have a round of applause for Paolo and Kim, the two people who spent much of last year up to their eyes and ears in dust and dirt and many hours out in the back parking lot of JPL in the blazing sun concocting mixtures of various dry powders to simulate martian soil? While the rest of us were sitting around looking at images typing, "turn this wheel", "turn that wheel", "go faster", "use the arm" they were out there performing the difficult and unglamorous tasks of creating a pile of dirt in Pasadena like the one on Mars.
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Zeke4ther
post Sep 21 2010, 11:19 PM
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Yes! We definitely have to say 'Thank-you!" for all of the hard work.

My Heart felt thank-you. May we hear from Spirit soon!

wheel.gif wheel.gif wheel.gif


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nprev
post Sep 21 2010, 11:45 PM
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Applause indeed!!! smile.gif How is Paolo? Haven't head much from him of late.


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A few will take this knowledge and use this power of a dream realized as a force for change, an impetus for further discovery to make less ancient dreams real.
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Mixer
post Sep 22 2010, 12:32 AM
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So shall we say seconded, thirded and motion passed! Thanks Paulo and Kim!
pancam.gif

Lets hope our little buddy pokes her head up soon.

I know I get a little jump in my heart rate whenever I get a reply notification to the forum.
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PaulM
post Oct 18 2010, 03:50 PM
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QUOTE (alan @ Apr 13 2010, 07:34 PM) *
If Spirit manages to move over a meter wouldn't they still be able to do the measurements if they knew the distance moved to an accuracy of less than a cm?

Scott Maxwell has indicated on his Twitter account that Spirit may be moved a few meters before it starts its radio science experiment:
Spirit hibernated (in trap) for Martian winter. We've started trying to reestablish contact. Will resume extrication.

This makes sense to me. If Spirit is to become a stationary lander for two or three years then it seems sensible to me that it be moved 5m or so to a new location where it could better survive the next Winter. I believe that a slight slope to the North would provide sufficient power for both Summer and Winter operations.

I do believe that Spirit will eventually manage to traverse the 100m to Goddard crater. However, this epic journey may well have to wait until the radio science experiment is complete. In the past I have imagined a race between Spirit and Opportunity and wondered whether Spirit would reach Goddard before Oppy reached Endeavour crater. My ideal location for Spirit to park for a time as a stationary lander would be on the North facing slope leading up to Goddard crater.
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Guest_cassioli_*
post Oct 23 2010, 02:07 PM
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Guests






How can i know which day of the year is currently on Spirit position? I mean, is it today july-10th-like day rather than february-3rd? I know it's spring right now... but is it march-21-equivalent or june-21 equivalent?
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Phil Stooke
post Oct 23 2010, 03:06 PM
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Go here:

http://www-mars.lmd.jussieu.fr/mars/time/martian_time.html

Enter today's date, convert to Mars Date and Solar Longitude, and check out the explanation of Solar Longitude in the link at the top of the page.

Phil


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djellison
post Oct 23 2010, 06:00 PM
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Mars24 also shows LS and the season.
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