Spirit Sol 450

I have to go give an RSVP demo, but first I have to know how Spirit did on my first attempt to drive it for months. I borrow a Web browser window from Colette Wilklow and quickly bring up the automatically generated Web page that summarizes the results.

The first line says more than eight meters -- not too shabby, for this terrain. Then I notice the next line, the usually more accurate line -- the one that tells you not just how far the wheels spun but how far the rover actually moved. About 87cm. That's all.


Just then my cell phone rings.


It's Mark Maimone. "Well, I ruined my record," he says. "I fataled a spacecraft."

"Fataled" doesn't mean the rover is dead, but it is a bad thing. It means the rover rebooted. And it's our fault.

After I get back from my demo, I get the story. What happened here, it transpires, is that we screwed up something really stupid. It has to do with a feature in the new release of the flight software. The original flight software let you write sequences that made decisions based on a limited set of parameters -- so that the rover could decide to do one thing or another based on its distance to a defined target, whether the previous command had succeeded or failed, and about a dozen more. The new flight software added a bunch more of these parameters, one of them being the rover's current heading. (So now we can write a sequence that says, for instance, "if you're not already heading more or less east, turn to the east.")

Now, there was only one way to add these new parameters in a way that was backward-compatible with the old command dictionary (a requirement imposed from above), and that was sort of a kludge. The trick was to take one of the old parameters -- one we picked because we never actually used it in flight -- and use a command to redefine that one so that it's an alias for one of the new ones, and then tell the rover to test that. The "if" command itself resets the alias to an undefined state afterward. And when you test this parameter when it's in an undefined state, the flight software fatals.

Now, we did everything right. In the main sequence, that is. What we forgot is that we also had a background sequence running, one that checks periodically to see if the rover is too close to an obstacle, so that it can abort driving. And this sequence is basically a long string of "if you're too close to obstacle #1, stop driving; if you're too close to obstacle #2, stop driving ...." One of these commands just happened to undefine the special parameter set in the main sequence, triggering the fatal.

Ashitey and I didn't realize that any "if" reset the special parameter -- we thought only the ones that used that parameter reset it. Mark Maimone knew better, but he was awfully tired and stressed out yesterday. I fear he's overloaded.

John Callas declares by fiat that the team is too tired to work this weekend. We're just going to clean up after our boneheaded error and do a minimal weekend plan. No driving, no IDD. It's a hint that we should go home and rest up. But do I? Nope: too much other work to do.[1]

I'm not surprised the team is tired. This whole hill-climbing campaign with Spirit looks like it could become frustrating. For the rover, it's a long climb. We've been at it three months, and we're only halfway up. The hell of it is, two decently strong men could pick the damn rover up and carry it to the summit in half an hour.

When Andy's new schedule comes out, it shows I'm not downstairs just temporarily. Looks like I'm back on Spirit at least through the end of May, which probably means for good. I'm surprised that I'm disappointed by this. I'm almost ashamed to admit it, but there was a lot to be said for working on Opportunity. The team's more laid back. It's easier. You drive really far. It's a cheap kind of glory but I'll take it.

But in the end, I'm not too disappointed. This rover and I have been through a lot together already, and this just means there's more adventure ahead.

I can't wait to get started.

[Next post: sol 453, April 12.]


[1] Easy for me to say now, but that was a bad decision: incompatible with rover safety. Shoulda gone home and slept. Shame on me.

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