Opportunity Sol 521 (Spirit Sol 542)

I come in early to get a jump on things. This was a mistake -- the downlink's been delayed (Spirit's data was queued ahead of ours on ODY), so there's nothing for me to get a jump on. At least it's being delayed for a good reason. Spirit has an earlier start and a tighter deadline, and they're also downlinking part of a huge panorama they took over the Independence Day weekend. I see part of this as it's coming down, and it's clear the full version will be spectacular.

Two hours later, our data starts flowing. The news isn't great, but it's okay. We'd planned about a 16m traverse, and we actually made about 10m progress. We'd have made more, but visual odometry -- which we rely upon for our slip checks -- produced a couple of erroneous results, making the rover think it was slipping a lot more than it actually was. As it happens, the rover thought it was slipping 32.5%, just barely over the 30% limit then in effect. So it stopped short.

Thisol we're just carrying on. The rover hasn't dug in and isn't in any apparent danger, so there's no reason not to just shoot for another 15m or so. The project issued new Rules of the Road that allow us to accept up to 40% slip, but this makes me a little nervous. The good slip estimates -- the ones produced during the larger portion of the drive, where visodom was working normally -- were in the neighborhood of 5% or less. So wouldn't it be better to actually lower the allowed slip percentage? We could set it to, say, 20% -- well above what we actually expect a good update to produce -- so that if visodom suddenly produces a bogus update that exceeds that threshold, the rover will stop moving instead of thinking it's somewhere it's not and driving into a ripple instead of along the trough.

But Jeng talks me out of this. The rover can actually climb these ripples, he points out. As long as it keeps heading more or less south -- there's essentially no chance of anything else, bogus update or no bogus update -- it'll be doing the right thing. And if it should get bogged down, visodom will fail to converge (because it doesn't expect the terrain not to change, an interesting result I'd never thought of). If that happens, the visodom failure-count limit will be exceeded, and the rover will stop shortly after.

The other thing that worries me is that we don't really know why the bogus updates occurred. We didn't get much data down; in particular, the data that would let us debug this problem is still aboard Opportunity. So if it was related to the unusual stuff we're doing with visodom -- looking over our right shoulder at the tracks we're leaving behind ourselves -- we'll just get more bogus updates at the start of this drive and fail again, probably without making much real progress. But with no debugging data available until tomorrow and no other real options anyway, there's little else we can do but cross our fingers and continue along the Blue Line toward Erebus.

No comments: