If the rovers 'fall asleep' due to underpowered batteries and some time later there are enough amps to
reawaken them, how do they go about finding Earth and re-establishing communication once the clock has
restarted?
Think cosmic,
TomT

To answer your question... absolutely no idea.

But welcome aboard TomT!

There is not really need. The low gain antenna is omnidirectional.

Of course without the clock, the rover wouldn't know when Earth is above the horizon. It would need to keep trying periodically, until communications is established again.

Ah, of course, elegant answer!
Thanks

05-2866.pdf (google) is a document by Tracy Neilson that talks about all the fault modes.

A totally flat battery - or as the fault would be classified, Battery failure - would result in a loss of mission clock and the vehicle trying to figure out local noon for itself and opening X-Band windows at that time.

Doug

To pursue TomT's question (oddly enough, I was wondering about the same thing), am I correct that the rovers locate earth by spotting the sun and computing earth's location? How low must the tau level be to permit the rovers to spot the sun?

TTT

Thanks Doug, enteresting answer : http://rms1.agsearch.agropedia.affrc.go.jp...ety/05-2866.pdf

Not a real answer, but, you can see here on sol 1261 that Spirit could still see the sun : http://marsrovers.jpl.nasa.gov/gallery/all/spirit_p1261.html

Yeah - err - the one I was reading was on the JPL technical reports server

Doug

Or, alternatively, how high does tau need to be to effectively shut down any direct component of the sunlight. I imagine even a very large value for tau would still leave enough unscattered sunlight to be detectable even using the darkening pancam solar filter with a sufficient exposure.

Right, and when tau gets so high that the required solar filter exposures are too long, then just switch to L2 or L7 or whatever. Though of course if tau got that high, I can't imagine there being enough power for any imaging in the first place!

Don't know, but I'd imagine they'd figure the local noon from the power input from the solar arrays - when it trends down, it's likely that noon has passed.

Assuming that the Pancam's can support a dynamic range of around 12 bits per pixels then the light levels from the suns disk would have to drop by a factor of >4096 in order to be below the detectable level. That would correspond to a Tau of something between 8 and 9. However increasing the exposure time could be used to compensate provided the general noise levels are low. I seem to recall that the Pancam sun shots are fairly quick (10th of a second?) so there could be some leeway there.

At the same time the diffuse lighting will increase the ambient "noise floor" however some back of the envelope calculations show that it wont ever be sufficient to swamp the direct flux at any tau value. The solar disk represents approximately 1/200,000th of the solid angle area of the sky. For the solar disk to "disappear" its brightness would have to be substantially less intense (say an order of magnitude or two, maybe even as much as 10 - 12) than the diffuse background which would require a situation where total diffuse insolation was 2-20 million times higher than the beam insolation. None of the diffuse insolation formulas I've seen ever allow for that. However we regularly see this happen on earth so there's either something wrong with the diffuse lighting approximations (highly likely) or our earth's water laden atmosphere behaves quite differently to Mars (also highly likely).

This is interesting. Do you mean that the ratio of the surface brightness (intensity per steradian on the sky) of the sun to the surface brightness of the sky in these models does not approach zero as tau approaches infinity? What does the ratio approach? Presumably this would mean that these models predict that the sky surface brightness goes to zero as fast as the sun surface brightness?

To add to what you said, it seems likely that these models were only designed to apply in the low-tau regime, so that the high-tau behaviour they predict isn't reliable. On the other hand, scattering due to dust strikes me as something that would be pretty straightforward to model, even in the high-tau limit. I'd guess that above a certain total mass of suspended dust (this no doubt would be dependent on the composition, size, etc of dust) any direct light ray from the sun would hit dust and be absorbed or scattered. Basically, enough suspended dust completely covers up the sun. (Some might recognize this as an inverse Olber's paradox.) Some of the scattered light should still make its way down and contribute to the sky illumination. So my intuition, for what it's worth, predicts there would be a point at which the sun would be invisible.

I thought I had read in one of the recent reports that the sun wasn't visible at the height of the storm. I can't find the reference now, and it could be that what I recalled was the comment in Emily's report that "no direct sunlight reaches the rover," which I guess is not the same thing.

You may be thinking of the quote in here. The highest tau we have images for so far is from Oppy sol 1235, where the sun is still clearly visible in the solar filter pancams. Tau peaked a bit above 1235 levels on sol 1237, but I can't see that a slight increase in tau would cause the sun to wink out completely, so I think that Zurek must have been in error.

That probably came from a Rich Zurek interview (the video is on the JPL web site). I think he was exaggerating a little. It's just hard to see. Assuming I knew where to look, I suspect I'd still be able to make out the Sun, even at tau = 5. Tau measurements were taken without any difficulty as high as 4.7 before they stopped doing them to save power. That requires seeing the disk of the Sun. I have no doubt that tau measurements much higher than that could be made, with the appropriate exposure time. The problem is you need energy to take a measurement, which you probably don't have to spare at higher tau, if you happen to be solar powered.


"No" is a bit extreme. Also in Emily's article is the formula for this, which is that e^-tau of the direct light comes through. So at tau = 5, you're still getting about 0.7% of the direct light you'd get through a vacuum. At typical Mars taus of 0.5 to 1, you're getting 40% to 60% of the direct light.

http://trs-new.jpl.nasa.gov/dspace/handle/2014/38695 - that the link to Tracy's PDF about fault protection.


Doug

Hi Mark

This is right as written, but especially for high tau, the phase function of the dust
can matter a lot (as it does e.g. for Titan's haze*). If some of the light is scattered forward
rather than being absorbed, you can still push a lot of light through several optical depths
(IIRC Chris McKay explained to me once it goes as 1/(1+tau) or something.) Of course,
the phase function and optical depth are different for different wavelengths, so you may end
up knocking out all the blue light, but red still leaks through.

As descriptors for atmospheric optics go, tau is a good number, but it is only one number
and so may not tell the whole story.

* I think Titan's haze was measured by DISR to have an optical depth of 3 or more, which
shouldnt allow Cassini ISS to see the surface at all. But because the haze has such a strong
forward-scattering peak, the light preserves some information on where it came from.

Ralph

" so you may end up knocking out all the blue light, but red still leaks through"

Note that solar cells are able to use a LOT of the 0.7 to 1.0 micrometer near infrared. Blue light's massively absorbed in multiple scattering, but the long wave half of the spectrum's much less strongly absorbed.

"Note that solar cells are able to use a LOT of the 0.7 to 1.0 micrometer near infrared. Blue light's massively absorbed in multiple scattering, but the long wave half of the spectrum's much less strongly absorbed."

The Rovers use triple junction vertically stacked solar cells. Each cell layer converts a portion of the 300 nm to 1,800 nm solar spectrum. They are electrically connected in series. As they are current sources, the weakest in the chain of the 3 cells controls total cell current output. Lose the spectrum that any one cell converts (such as the UV / Blue) and it matters not what happens to the other portions of the spectrum, the cell output drops.

Without the clock, the rover can't compute the earth's location. That is time dependant...

Anyone who has spent time under the smoke of a massive brush fire will concur. In 1975 the Big Tujunga fire in the mountains above Pasadena (of all places) blanketed the entire LA Basin with thick, dark smoke for the better part of a week. That's what these high tau images remind me of. The orange-red hue illuminating everything for several days in a row was indeed other-wordly.

And the rain of ashes covering our cars afterwards....

I hope the rovers escape a rain of dust at the end of this.

At the risk of being alarmist, I hope your comparison does not apply in this case to the MERs!

Several years ago there was a serious bushfire in and around the Royal National Park south of Sydney Australia which broke out on Christmas Day. At its height the highway between Sydney and the city of Wollongong fifty miles to the south was severed for several days. (There were other fires south of the city which broke out much the same time, severing road links in that direction as well, and so for all practical purposes effectively isolating Wollongong from the outside world.)

The day after the northern road was reopened I drove along it up to Sydney. On the way you could see blackened bushland, sometimes accompanied with scorched (and even partially melted) road signs, interspersed with untouched areas the fires had miraculously avoided. However, even in the untouched areas there was not a trace of green. The entire landscape had turned an eerie gray. Every brush and every tree that I could see--and I mean EVERY SINGLE ONE--had been blanketed in a layer of ash.

Let's hope Spirit and Opportunity avoid an equivalent fate when the present storm on Mars finally peters out and the martian dust settles back to the ground again.

======
Stephen

"...As they are current sources, the weakest in the chain of the 3 cells controls total cell current output. Lose the spectrum that any one cell converts ..."

To quote the watergate tape transcripts: "EXPLETIVE DELETED"

I'd forgotten the rovers were high efficiency triple junction cells.... and didn't know about the spectral response problem AT ALL.

I suppose Opportunity might be able to make it back to those dark streaks again for some more of that cleaning wind. (And to think that I was worried that those streaks would wind up depositing dust on Opportunity. I guess the wind eddies have a beneficial effect on slightly elevated surfaces such as the rover deck.)

There's another good reason to exit in that direction if and when Oppy leaves Victoria. I love that I'm again thinking months ahead!

The only direction they're going to exit is back out of Duck Bay.

Doug

I think some of the strongest winds we had were sitting here on Cabo Verde. The winds we've been feeling recently have been from the northish, so I don't think it matters where you are on the north side of Victoria. Hopefully the winds continue as the dust settles!

A belated thank you to Fredk and Mark Adler for providing the reference to Rich Zurek's interview, and an extra thank you to Mark Adler for sharing his expertise as a rover engineer on this forum.

TTT