Sunday, January 29, 2012

100 Sols on Cape York — An Absurdly Brief (Read: Long) Summary (Part 8/8)

Part 8: Where we stand — and where we're going tomorrow

This, the belated Part 8 of my "100 Sols on Cape York" series, is late for a few reasons:

1) I fly lots of spaceships and they all started getting jealous. I keep telling them, "No no no, see, MER and I just friends."
2) I was waiting for Opportunity to turn 8 years* old so it could look like I was being cute — "8 years for Part 8… Get it?! Get it?!" **
3) There hasn't been a whole lot going on.

There actually has been a lot going on, but it's all invisible to the folks outside of tactical. Opportunity won't be moving for a good long while; we've got nearly 15 degrees of tilt in the northerly direction, plenty to allow us to survive the depths of the Martian winter while still collecting good, quality science. Opportunity's current view is summed up nicely in what we're calling the "Greeley Pan", named after the current location (Greeley Haven) and the instrument collecting the images (Pancam).

(Click for full-res awesomeness.)

Our team estimates that once we get past winter solstice on March 31***, it'll take a few months to get back to where we are right now in terms of available power. 

So, Matt, until then, what can Opportunity do in the meantime? 

We can certainly do everything that Opportunity has up her sleeve: driving; IDD'ing; remote sensing; backflips. We just can't do a lot of it. 

What are your real science collection options, and what are their relative priorities?

Astute question! The activities are, in decreasing priority, as such: 

1) Radio Science DTE ("RS-DTEs"): By analyzing the signal coming from Opportunity's High Gain Antenna (HGA), somebody way smarter than me can figure out how "wobbly" Mars is. It's much more complicated than the word "wobbly," but I'll settle for it. The wobbliness of the planet will cause the Doppler shift of the HGA signal to vary in predictable ways; a totally solid planet would wobble in a very different way than a planet that isn't so solid on the inside. The math to describe these variances is well known so all we need to close the error bars on what we already know about Mars' interior is a stationary radio source on the planet, or at least a radio source whose position is very well known. Hence, Opportunity is a prime candidate for this kind of measurement. Brilliant!
2) APXS+Mössbauer investigations of Amboy; MI Mosaic: Right at our feet is a bit of outcrop. On it is a nice smooth surface that is itching to be poked at by the IDD. One of these spots on this surface has been named "Amboy." The content of Amboy will help the scientists know what's going on at Cape York. Combinations of APXS and Mössbauer measurements will tell them all sorts of cool things about it. And, hey, since we're there, why not use the Microscopic Imager (MI) to make a nice mosaic? Exactly! We're currently queuing up a 20x2x5 (20 images long, 2 images across, 5 images deep) mosaic. It'll probably be more like 17 images long, depending on reachability.
3) Greeley Panorama: See image above. This is being taken piecemeal because of the sheet amount of time and data it uses, with similar lighting conditions across the board. A funny "feature" the Pancam team will have to work around is that we have sequenced most of the Greeley Pan segments in the morning science block (or "AM block"), when the HGA may be articulating as it tracks the Earth for the uplink communications passes every morning. You can see in the image above that we did indeed capture, in two Pancam images, the HGA in two different places within several minutes of each other.
4) Everything else: Your usual collection of random remote sensing observations. We have weekly/monthly/whatever-ly liens that we still like to cover.

Yours truly is one degree removed from NDT as of last night. This is what I like to call "cool."

So we can move, but we're not. Really, why aren't we boogying?

At our feet is a potentially rich source of science targets and we would like to be able to skirt along it and IDD it all to death, but we can't. Errrr, shouldn't. Why not?The primary reason is, of course, available power. This is the over-arching constraint that drives what we do every day. However, there are two benefits we gain from not moving.

1) RS-DTEs
2) AXPS+MB investigations

As I mentioned above, RS-DTEs require that we be in the same place; if we have moved, we have to know exactly where and how much down to, roughly, the centimeter level. These things matter when you're measuring single-digit Hertz doppler shifts. Using that nice little piece of flight software called VisOdom, we can track rover movements to that accuracy. However, that doesn't mean we should move. We want to completely characterize Amboy with the IDD and any movements would mean Amboy might not be in reach. (This is certainly true for azimuthal/rotational movements, and could be true for translational movements along the surface. Think about Oppy's damaged IDD shoulder joint and you'll understand why.)

Therefore we have incentive to stay put. The only thing a small "bump" drive would gain for us is some extra northerly tilt. We can get another couple degrees if we want it because the nearby terrain has it for us. However, at 15 degrees, we're doing alright. Better is the enemy of good.

It sounds like things aren't so busy if you're scheduling only extra communication windows and Pancam sequences.

Couldn't be further from the truth! 

We're living sol to sol on the power analyses. We're not really cutting it very close, but we love to be conservative. Think about our constraints over time: Every bit of the campaign on Cape York has been constrained for one reason or another. Previously, it was downlink volume and knowledge of the state of the rover, or it was the successful collection of APXS data, or it was [insert thing]. Now, we're living sol to sol on power constraints. Every day, the TAP/SIE (read: me, sometimes) gets to take the activity plan for the next sol(s) in the plan and model it. Using lots of big fancy math and computer simulations and things that look nothing like Hollywood would portray it, we come back to the team and tell them what we found. 

In technical terms, this verbal report ranges from, "Yah, we're cool," to, "We're cool if we pull out the [observation]," to, "Nah, we're going to have to do some surgery to this plan," to simply, "Nah." Every day, the team lines up the priorities and we allow the science team to pack it on. From there, we trim — it's easier to take away than to add, as it turns out. 

There are many permutations of combined activities that we can bless with the canonical "this won't kill the rover" stamp. We also have to be mindful of strategic (long term) decisions while we do this tactical decision making. This permutation process takes several hours, so even though there really isn't that much science activity going on, the days are still mind-bending and long.

Lately, this permutation process has resulted in a few interesting patterns of activity on the rover. For instance, for a given sol we have to choose between doing a UHF downlink relay pass with an orbiter and doing an RS-DTE. This is apparent to the amateur MER follower when there are a few days between image data on the ground when they expect to be getting some data every day. Because the data we're collecting is low-priority for rover health and tactical decision making, we can afford to not downlink the large (I speak relatively) amounts of data from the UHF relay. RS-DTEs give us a tiny snapshot of rover health, and although they don't tell us much, they tell us something useful. We can usually move on planning the next sol(s) with just the DTE "snapshot."

Another pattern is the placement of the Greeley Pan observations. The more perceptive MER data followers will note that most Pancam Greeley Pan images come down the sol after they were collected. This often happens anyways when we're tight on data and the low-priority Pancam images get shoved to the bottom of the downlink queue and don't make it down for a few sols. However we're not in this data-tight situation. Why, then, are they "late"? A sol's sequence of events will tell us why:

[ Start sol's plan ] >>> [ Do stuff ] >>> [ UHF Relay ] >>> [ Nap overnight ] >>> [ AM Block stuff ] >>> [ Next sol's plan starts ]

We straddle the local midnight barrier for each sol's plan. To save on power on a given sol for the UHF relays, RS-DTEs, or APXS/MB observations, we've removed most of our remote sensing blocks from the early afternoon before the UHF relay. Instead, we make more use of the AM block. This gives another advantage in that the time of day of the AM block is roughly the same, so shadowing and light in the images will be roughly the same for the whole panorama. The AM block of a given sol's plan occurs on the next sol, and this is some time after the UHF relay has occurred. In fact, anything that is after a sol's UHF window won't get down to the ground until at least the next sol,. maybe later. Therefore, many of the Greeley Pan observations show up "late".


This sorta, maybe, possibly wraps up the 8-part series. Right. We now return to your regularly scheduled MER blog.


*Surface years. They're like dog years, with the exception that they're the exact same length as normal years. So, normal years… on the surface.

**Mike the Mission Manager's idea. Though nobody reads my footnotes so he'll never get credit!

***Solstice is actually a few weeks after minimum solar insolation (insolation = amount of photons hitting Mars) at Opportunity's latitude. Insolation at the surface is defined mostly by two parameters: distance from the sun and tilt relative to the orbital plan about the sun. Solstice is defined by the latter (tilt), which doesn't have to coincide with the minimum insolation, though it does very closely. If you're of the number-crunching type, crunch those numbers and tell me if you agree! I know you will; as Ben Goldacre says, "If, by the end, you reckon you still might disagree with me, then I offer you this: you'll still be wrong, but you'll be wrong with a lot more panache and flair than you could possibly mange right now."

1 comment:

pdp8e said...

I could barely read this blog over the sound of how awesome science is...