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Dust Storm
marsophile
post Jun 17 2018, 06:57 PM
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QUOTE (mcaplinger @ Jun 17 2018, 07:41 AM) *
...solution that requires state to be recorded and used from sol to sol is usually not used in deep fault responses...

Not to mention the fact that flash memory is unavailable, so only measurements from a single Sol could be considered, and the CPU would have to stay on throughout the Sol.

In principle, the rover might be able to autonomously reset its clock if there was enough energy to take images of the sun position but (as the previous post points out) such complex algorithms would be unlikely in fault protection. Waking up every 4 hours is a simple and seemingly adequate solution.

The biggest danger I see from the dust storm is a poor dust factor going into the winter season leading to desperate measures. If I recall, the issue with Spirit was precipitated by over-winter issues that upset the original plan of driving along Home Plate. Hopefully we will get cleaning after the current dust storm but we may also get a second dust storm in the current summer season.
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scalbers
post Jun 17 2018, 07:55 PM
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How severe is this dust storm compared with others weathered by Opportunity? Estimating the total solar radiation in heavy dust conditions is an interesting exercise and the scattered light will be the main contributor when the direct radiation diminishes. I'm trying to model all this with my simulated weather imagery package. A key factor would be how much light gets absorbed for each scattering event (a single scattering albedo of about .90 in green light and .97 in the IR see figure 8). A tau of around 10 would only cut the light down about 50% if absorption wasn't a factor. I can note a Titan analog with a tau of around 8 and estimates of 10% of the visible band light getting through when absorption is factored in. Deimos' formulation in post #28 is reasonable. As mentioned more light in the IR will help for the solar panels. Serpens' YouTube video is pretty impressive.


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mcaplinger
post Jun 17 2018, 08:11 PM
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QUOTE (scalbers @ Jun 17 2018, 11:55 AM) *
How severe is this dust storm compared with others weathered by Opportunity?

The worst previous was back in 2007: "Due to extensive dust storms in Mars' southern hemisphere causing record atmospheric opacity levels, Opportunity is currently experiencing its lowest power levels to date. The tau measurement as of sol 1225 is 4.12, resulting in a mere 280 watt-hours of array energy. A tau measurement of 5.0 would result in approximately 150 watt-hours."

https://mars.nasa.gov/mer/mission/status_op...07.html#sol1382

Keep in mind that we have two competing measurements, the tau determined by analysis from Pancam images, and the actual solar production. From an engineering perspective, only the second one is of direct interest. I'm sure they have a model to go from the first to the second, but once you have an actual number for the second you should use it. And of course for any model, you not only have to know the irradiance but also how much dust is on the panels.


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Disclaimer: This post is based on public information only. Any opinions are my own.
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djellison
post Jun 17 2018, 11:42 PM
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QUOTE (fredk @ Jun 17 2018, 06:34 AM) *
I guess the question is: how does the rover decide if it's midday?


It doesn't. Assuming a loss of clock fault - it simply boots up when 2 amps are on the array. The IDD heater is not a significant concern when you're at 2 amps.
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mcaplinger
post Jun 18 2018, 12:11 AM
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QUOTE (marsophile @ Jun 17 2018, 10:57 AM) *
Not to mention the fact that flash memory is unavailable, so only measurements from a single Sol could be considered...

While the flash is not available, the rover also has 11MB of EEPROM thst could be used to store state information. I think, for example, that this is where Earth position as a function of SCLK is kept. Without time reference, no HGA comm will work until this can be updated.


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serpens
post Jun 18 2018, 12:19 AM
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QUOTE (scalbers @ Jun 17 2018, 08:55 PM) *
How severe is this dust storm compared with others weathered by Opportunity? ..... A key factor would be how much light gets absorbed for each scattering event (a single scattering albedo of about .90 in green light and .97 in the IR A tau of around 10 would only cut the light down about 50% if absorption wasn't a factor......

This may help (or not). https://www.swsc-journal.org/articles/swsc/...swsc150027.html
The maximum tau assessed by Opportunity in 2007 was 5.5. The maximum assessed this time around before she went dark was 10.8 and this may have increased. This doubling of the tau, ignoring the airmass variable means that direct insolation would be 0.005 that enjoyed by opportunity at the height of the 2007 storm.

EDIT. An update by A.J.S. Rayl. http://www.planetary.org/explore/space-top...torm-sleep.html
An interesting extract: “The dust here is thicker than anything I have ever encountered, going back to Viking missions,” said MER Deputy Principal Investigator Ray Arvidson ........It’s dark, like the end of twilight dark.”
Paolo. described it as basically "the difference between a full sunshine day and a full moon night kind of state". Even a minor increase in tau over the 10.8 value and all ambient light would go.
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