[X] My Assistant

[fredk]

What's rising?
http://mars.jpl.nasa.gov/msl-raw-images/pr...NCAM00545M_.JPG

[DeanM]

Quoting Emily: "Point a camera with a 5-degree FOV at a moving speck crossing a moving spot from a rover that was only briefly paused.."

This can have been no easy feat!

Indeed, how is such precise 'pointing' achieved: gravity provides one vector but Mars (now) has no magnetic field to provide a second.

Dean

[Deimos]

Indeed, how is such precise 'pointing' achieved: gravity provides one vector but Mars (now) has no magnetic field to provide a second.

Sun images show the Sun's position and (optionally) direction of motion in the rover's frame. These allow a full attitude solution or a yaw-only solution. Initial measurement units (gyro-based) update attitude between sun images. MER used a subset of Pancam Sun images. MSL uses Navcam Sun+sky images. The IMU propagates attitude forward, but error builds with drive-time. So the attitude, especially for Sun aims, is precise after a Sun update and before the next drive, and degrades until the next Sun update. The midnight planets page for sol C/369 shows such an update after the drive (and mid-drive imaging). Sol B/3387 shows an update for Opportunity. (In both cases, look for the Sun image/images after "driving...".)

[fredk]

MSL uses Navcam Sun+sky images.

I didn't realize that - I had assumed MSL used mastcam. Why navcam? Surely it's harder to pinpoint the sun's position in a navcam frame, or in other words the uncertainty on the sun's position would be much larger in a navcam frame than a mastcam frame. That's both because of the lower resolution of navcam, but also because of the overexposure due to lack of solar filter. (I guess you could pinpoint the position from navcam pretty well in one direction, due to the CCD bleeding.)

FredK: "Subtracted the average..." Brilliant idea... just freaking brilliant.

Thanks a lot Ed, but I have to say I was inspired by what we did several years ago with Spirit's dust devil images. I have no idea who started differencing the images to show DD's more easily.

[Gerald]

Why navcam? Surely it's harder to pinpoint the sun's position in a navcam frame, or in other words the uncertainty on the sun's position would be much larger in a navcam frame than a mastcam frame. That's both because of the lower resolution of navcam, but also because of the overexposure due to lack of solar filter. (I guess you could pinpoint the position from navcam pretty well in one direction, due to the CCD bleeding.)

Here a graphical sketch, how a good reference point can be determined from this NRB image:

[fredk]

good

The question is how good. Being able to image the sun's un-overexposed disc means you will be able to pinpoint it's position considerably better with mastcam (or pancam).

But mcaplinger has answered this - the accuracy from navcam is sufficient. One thing I suppose they can do is calibrate - image the sun simultaneously with mastcam and navcam, and use that to create a model for the sun's true position in the navcam overexposed blob. There will still be uncertainties due to differing tau, dust on optics, etc. All of these effects could distort the shape of the overexposed "blob".

[mcaplinger]

The question is how good.

In the accuracy stackup, the uncertainty in the actual RSM pointing from gear backlash, encoder/resolver precision, etc, is probably many times larger than any centroiding error from sun overexposure.