The Radio Science Campaign, Part 1/2 — The Basics

Matt, you've had another one of those drastically long periods of time without a MER Blog post.

I know, I know. But I have an excuse!

If you say "work," we're no longer friends.

Can I say, "Partly work, partly that I just bought a new guitar amp, partly that there are lots of words to write"?

I'll allow it. On one condition...

Yes?

Don't make it boring.

Done.

We Talk With Our Rover

Since the 1st of January this year, priority number one for the Opportunity rover tactical team has been to sequence communication sessions with antennas at the Deep Space Network (DSN). 

Yawn.

I mean, yes, it's a critical bit of the rover surface operational scenario. But, I mean, you know…

Yawn.

Why "yawn"? Because we have done this every roughly sol since the beginning of the mission. 

There are several ways that Opportunity has to communicate with us. Most of them are "canned": We determine them weeks or months ahead of time and load them up to the rover, and then the rover's flight software autonomously does all the hard work for us when the time comes, unless of course we choose to tweak a particular track* tactically.

But, hey, wait a minute, there are a few things that have been different about these communication windows…

More DTEs, Less DFEs ("she's shouting back at us")

First, I must be clear: I am speaking about the UPLINK path only; DOWNLINK is another beast altogether as it also involves relay satellites, occurs at a different time of (Martian) day, and can actually piggyback on uplink sessions. There are subtleties here that I will omit the sake of the reader. (You're welcome!)

Historically speaking, most of the communications passes with Earth are "Direct From Earth" (DFE): meaning, the rover only listens for data — 0's and 1's that map to instructions for the upcoming sol(s) — coming from Earth and sends nothing back except for the infamous "beep" signal to confirm that it has successfully received the data and has "handed over" to the next sol's set of sequence**. 

We have another type of communication session that we still consider to be on the uplink side of the data flow to the rover: Direct To Earth (DTEs). In DTE passes, the rover sends back a limited amount of information — like, almost nothing, but a little — that allows the tactical downlink team to somewhat confidently assess the health and status of the rover. Although there is "downlink" going on here, we colloquially treat DTEs as uplink windows, because DTEs are inclusive of the activities performed in DFEs and they often (but not always) occur during the time of the sol when we typically execute uplink windows with the rover. 

DTEs provide the opportunity for additional, non-critical information, which brings me to the next thing…

There's a Scientific Goal to Achieve — Radio Science ("the vindication of our woes")

The additional information that comes down with a DTE allows engineers to derive timing information to help correct what the rover thinks "now" is. A non-perfect, drifting clock — something that plagues all spacecraft — causes many problems. Some of them are serious, like that the rover could suddenly think's a different sol and try to execute the wrong set of commands. Some of them are mere inconveniences, like that 360-degree panoramas don't show the horizon at the same local-level elevation. This effect is particularly notable in Stu's infamous vertically exaggerated images, even after rotating it to get it so that horizontal is actually "flat" on the Martian surface:

(Notice the strange bias of the horizon, not all of which is explained by the fact that we are peering into the giant Endeavour crater in this image.)

Again speaking historically, DTEs have been used primarily to get this timing information. This past winter, however, we are doing DTEs for another reason — with the added benefit of timing information. In these DTEs, we also get very precise Doppler-shift data (more on how in a bit) that helps us achieve a scientific, rather than engineering, goal. That goal is our famous Radio Science. From these are borne the infamous acronym, "RS-DTEs" (Radio Science DTEs).

Winter DTEs Create Tactical Sequencing Challenges ("sometimes we win, sometimes Mars wins")

We were able to do radio science this Martian winter primarily because we knew we weren't going to move for at least a few months while we passed through the depths of Martian winter. Again, more on why non-movement is important in a bit. The problem is that this is difficult: RS-DTEs require lots of power from the rover when the rover may not have that power available due to low solar insolation and inability to charge up her batteries fully. Balancing the goals of Radio Science with keeping the rover alive and healthy was a day-to-day, sol-to-sol battle. 

Although the end result of the challenge was a simple RS-DTE window, the means of getting there were brain-busting. I have never had to think so hard about uplink windows and how to fit them into the day's plan until these last few months. I'll give some insight into this process later on.

Right, so: Now we know how this winter has been a little different than past winters. We have done this four-month campaign of RS-DTEs, getting a little over of 1 hour's worth of Doppler-shift data per week to meet the requirements of the science. So how does this all work?

Before We Get There: A Brief Overview of Cool Things

To get the basics of the concept behind our beloved RS-DTEs, you cats first need to learn a little. First, some pretty:

Hastily created. Deal with it.

This summarizes all of the communication paths that can occur between the rover and Earth:

1) Relay, Forward and Return Link: We use Return link (rover to orbiter to Earth) most often. Forward link (Earth to orbiter to rover) has not been used regularly in years.

2) DFE: Earth to rover only. 

3) DTE: Everything that a DFE is, plus some small downlink back to Earth.

Now: The DSN antennae and the spacecraft (rover) can be in one of three Doppler states, shown below: 

0-way (not shown): No signals are traveling between the spacecraft and DSN.

1-way: Downlink from the spacecraft to the DSN. Fun fact: Whenever you have a scheduled time slot with a given DSN antenna, that means you at least have 1-way (downlink only) capability. Most of the time you get the uplink, too, but not always.

2-way: Downlink from the spacecraft to the Earth with simultaneous uplink from the DSN to the spacecraft. 

3-Way: The spacecraft simply radiates its downlink signal at Earth and this signal's "footprint" is bigger than the Earth itself due to the spreading of the signal over the vast distances of the solar system. Therefore any DSN antenna that can see the spacecraft will see its downlink signal. The DSN stations are located so that "view periods" of any given point in the sky overlap in time — and therefore DSN antennae on separate ends of the Earth (not to mention those at the same local complex) can see the same signal. When the spacecraft is 2-way with a station and another station starts listening for the same downlink signal, it is said to be 3-way with the spacecraft. (Subtlety: Only a DSN antenna is said to be in a 3-way state.)

Now: More detail. 2-way and 3-way aren't simply 2-way and 3-way. They need another qualifier, something called "coherency state."

When a spacecraft has a track scheduled with a particular antenna, there are several complications regarding the dependence of uplink and downlink paths on one another. Let's say you're a newbie and you want to listen to the spacecraft's downlink signal. You get out some documentation and find out that the spacecraft downlinks data on a radio signal that oscillates with a frequency of, say, 1 bazillion hertz. So you fire up your antenna and start listening at 1 bazillion hertz — but you can't detect the signal. What's going on?

Well, several things. Most importantly, the signal is Doppler-shifted (not to be confused with the Doppler state) because the spacecraft is moving relative to you (or you to it, same thing). That is, not only is the spacecraft hurtling through empty space at well above Earth-escape velocity, but you are on a planet that is moving and rotating through space at a high speed as well. The signal from the spacecraft appears to come down at a different frequency from 1 bazillion hertz due to this relative velocity between observer (Earth) and source (spacecraft). It's just like that canonical example of an ambulance driving by you: the frequency of the sirens increases right as the vehicle moves by you because of the relative speed to your ears.

This Doppler shift data is the bread and butter of not only deep space navigation, but also the science behind our beloved RS-DTEs. Another nice bit about Doppler information is that is gives us ranging data that allows us to determine velocity toward or away from the observer. A little magic math later and you can place this velocity in a reference frame at the center of the solar system, which gives you better geometric context of the spacecraft's position. 

Now, for a dose of reality: the Doppler-shifted signal coming from the spacecraft is noisy and is almost unusable to do accurate ranging measurements. So why have I told you that we do them?

Coherent downlink, is why. The "noise" and "error" in a downlink signal generated by the spacecraft comes from the imperfect reference oscillator source, among other things. There's some hardware in the spacecraft that generates a reference signal about which the spacecraft sends "modulated" signals that represent actual data, actual 0's and 1's. Due to these engineering imperfections and limitations, the frequency of this signal isn't stable and varies in time. Although the changes are small and imperceptible to the layman, it is significant — significant enough to render Doppler ranging data almost useless for high-precision navigation. It is impossible to predict this noise and error, as it is random, thus it is very difficult to extract good data from the Doppler-shifted downlink signal.

The elegant solution devised by the DSN is something called "coherency." What you need in order to have an ultra-stable downlink reference is a whole lot of heavy equipment cooled with… very cold things. Like, a handful of degrees Kelvin kind of cold. You can't put this on a spacecraft because it would be bigger than the spacecraft itself, quickly rendering all deep-space spacecraft un-launchable. Instead, the antennas at the DSN all have this equipment themselves to generate ultra-stable uplink signals. 

The spacecraft turns off its internal oscillator reference signal and instead listens for the uplink signal coming from the station. The spacecraft can measure this uplink signal very accurately and therefore the ultra-stable nature of the uplink signal is not lost. it takes this signal's frequency, multiplies it by a very precise number, and shoots that signal out as its downlink. The downlink signal is then said to be coherent. The communications link between station and spacecraft is also designated as "2-way coherent" (not just "2-way"):

With this ingenious method of uplink-downlink dependence, the antenna on Earth can receive downlink signals that are as stable and accurate as their uplink signal. This means much more precise Doppler shift data — so precise, in fact, that we can use this data to see if a planet is wobbling by placing a stationary radio source on the surface at an arbitrary location. 

How to Measure Wobbliness

If you make the assumption that the planet Mars is spinning perfectly and immutably about its normal spin axis (coming out of its North Pole, like Earth), you can easily*** subtract all so-called "systematic" effects of the Doppler shift in the coherent downlink signal. Most of these are, again, due to the relative velocity of the observer and source. There are other effects that aren't important to us. With all of these systematics removed from the signal, you'll still see variance in the downlink signal. Why? 

Because the planet is wobbling, not just spinning, is why.

The rotation of any object, planet or otherwise, will always be imperfect. It will "precess" and "nutate" over long periods of time:

The exact amount of precession and nutation tells you a lot of things, such as the internal structure of the planet. For instance, if the planet is solid through and through, it won't "slosh" around a lot and will be more stable — and the doppler shift of a stationary thing on the planet will reflect this. If a planet has a liquid core, then it will precess and nutate about its spin axis in a particular way — and the doppler shift of a stationary thing on the planet will reflect this. Measurements of these small movements allow you to lock down, but not fully determine, this structure. Therefore, all that extra downlink signal variance can be derived into the structure of the planet.

Way. Effing. Cool.

That wraps up this part. Up next: The results of Opportunity's winter Radio Science campaign.

The Radio Science Campaign, Part 2/2 — The Results

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*Track = pass = communication session. "Track" is the most widely used term at JPL, insofar as my ears can tell. I'll use these terms liberally at the expense of the sanity of people that think there's a difference between them. Also, "track" is not really used as a term for relay sessions with the orbiter, even if the orbiter is a "bent-pipe" and sends Oppy's data immediately back to Earth via a track that it, the orbiter, has scheduled with the DSN. I mention this because we also maintain the capability to change particular parameters with these relay sessions with the orbiter.

**The "beep" actually doesn't always tell you anything about whether or not the uplink was successful. It may, but only by implication, and not in every instance. What it REALLY means is more technical and not worth delving into. Ah, engineering subtleties…

***It's actually really hard. 

I'm gonna go with… Mars Quake. Because I can.

(Alternate title: "I have eighteen theories regarding the Opportunity phantom movement anomaly, and unfortunately none of them include aliens...")

Sometime between sols 2894 and 2899, the rover moved. Which is cool and all, since that's what a rover is supposed to do. Except when it's not supposed to.

Subtle; notice that it was only the left wheel:

Courtesy user elakdawalla from UMSF -- thanks!

This is significant mostly because it's a mystery why it happened at all. It is not significant from a rover health perspective. Put a two dozen engineers in a room and tell them that only some nearly indeterminably small thing happened, and you'll suddenly have two dozen engineers picking each other's brains for hours. The thing happened, the thing wasn't dangerous… but my god, that thing has an explanation somewhere. It bugs us when we can't explain something.

Of course, when you're not expecting anomalies with the rover, you're not taking a whole lot of pictures and you're not collecting a whole lot of data. We certainly didn't expect any kind of movement and therefore the number of pictures at our disposal to catalog the sequence of events is rather low. 

We know a few things: 

> It occurred sometime over a range of a few sols

> It occurred around IDD activity close to the surface

> It is possible that the right rear wheel moved as well

> It is possible that there was additional movement on sol 2900 or 2901

We don't know some other things:

> Our exact attitude (orientation) before the movement — more on this in a bit

> When in the possible sequence of events the movement happened

> Why the movement happened

Now, not only do we not expect the movement of the rover at all, we don't want it. Although the change in orientation is on the order of a few hundredths of a degree, and although it doesn't affects the amount of sunlight hitting the arrays because the change in orientation is below our level to detect the corresponding change in solar power reaching the rover, and although we can easily adjust for such a small movement for high-precision IDD work, there is one thing for which this matters:

Radiooooo Ssssscience

(Say each word with a pause, imagining that you're both a) Neil deGrasse Tyson and b) in a big room. Say it out loud, too, no matter where you are.)

Alas! Radio Science! 

The collection of the radio science data is pretty much invisible to the amateur MER followers. It doesn't produce pictures, it doesn't move the rover, and it doesn't have that wow factor… Well, at least at first glance. To the MER team, radio science has been this thorn in our side because Opportunity simply didn't have the power levels to execute as many as we wanted. We wanted all those radio science tracks to get in, but it's just so damned hard. Sometimes you win, sometimes Mars wins.

A few weeks back, the guy in charge of the radio science campaign gave a presentation to the MER tactical team regarding the data he had collected so far. Simply stunning. The science all boils down to measuring the doppler shift*, subtracting out the components of this shift that are due to known sources, and seeing what's left. What's left is the signature of the planet's wobble. Right, so, why does such a small movement of the rover — on the order of millimeters, as are as we can tell — matter?

The signature of the Mars' wobble, like that of Earth, is very small. Very, very small. The changes it induces in the shift of the radio signal are on the order of… well, something really small. When that is mapped into a velocity, it's under a millimeter per second. The exact position of the rover doesn't matter for the collection of the radio science data, as long as it doesn't move; if it does move, the movements need to be known down to below the centimeter level of precision. (We can measure that, by the way, using the rover's VisOdom software; we just chose not to move due to power levels this winter. Two birds, one stone.) If the rover moves, and we don't know exactly how or where, this has a magnified effect on the quality of the radio science data.

Right. So. We shouldn't move.

I think I can speak for the MER team when I say that our current best theory for the movement is that this wheel is perched on some crumbly rocks. All the math and mechanics and discussion we throw at it boil down to that: just crumbly rocks. It's a decent theory and it explains most of what we've seen. (Ahem… most.) 

In the coming sols, we'll be taking lots of orientation data and lots of new pictures to get a high-fidelity feel for our current state, so if it happens again, we have something against which we can take a difference. We've got roughly a month left until we will start moving again — we have to wait for the sun to get higher in the sky** and we want to collect another months' worth of radio science to get those error bars down.

Onward!

——————

*Yes, I'm still working on that promised Radio Science post. It will include all activity up to now, a brief overview of the basics of the science, and how we chose to squeeze it into Oppy's daily activities. Big deal!

**Happy Solstice! 03/31/12, winter solstice hit the southern hemisphere of Mars. Recall that minimum solar insolation was a few weeks before that.

A Squyres Sighting

It was one of those days: Eight different people wanting nineteen different things at four different times on three different floors. (Hey, at least they were all in the same building.) I was running all over the place and making phone calls and sending emails in the elevator.

I'm not in my office very much; my boss, two doors down from my cube, is always surprised to see me. On this one day in particular, before the madness started, I had spent 3 consecutive hours in my office — a veritable record for the last two and a half months, no doubt. Two days before I was on as TAP/SIE for Opportunity, and she had just had that little dust cleaning. Gave us a few watt-hours:

Courtesy user "dilo" at UMSF

It was late-morning by now, meaning it was time for the APAM (Activity Plan Approval Meeting) where the power analysis is shown for the sol(s) that the team is planning that day. I had to drop by a co-worker's office on the same floor for another task so I hurried up to the MER sequencing room to get that tag-up out of the way to make it to the MER meeting. I opted for the stairs; my building's elevators are notoriously and commonly out of order, so I didn't want to take my chances. As I made it up to the fifth floor, I could hear someone coming down from the floor above. This guy opened the door from the stairwell to the halls of the fifth floor and held it open for me. He was shorter than me and walked with a lot more intent; cowboy boots, to boot. As soon as he stepped into the hallway, he stopped dead in his tracks, apparently lost. Being in the hurry I was, I had no patience to wait for him to make a move. I walked right around him and headed straight for my destination. The best shortcut happened to be through the MER area of the fifth floor, and as I took off around this guy in that direction, I noticed that he was following me. I held the door open for him and noticed something very familiar about him — did I know him? 

Meh, I was too busy to turn around.

After my quick tagup around the corner, I came storming into the MER sequencing room to see if I had missed today's power analysis report. Mike the Mission Manager was on duty, and he told me: "There's been a Squyres sighting! He's here for the MSL Surface ORT [Operational Readiness Test] and decided he'd stop by to, 'Drive a real rover on the surface of Mars'."

My eyes shot to the top of my head as they do when I'm thinking back in time: Hey, that son-of-a-gun that looked lost was Dr. Squyres! If only I had turned around to see where he wanted to go, I could have had some good face time* with the man who so boldly leads us into the vast unknown of Mars. Dr. Squyres knows that there is a person named Matt Lenda, and that this Matt Lenda does TUL and TAP/SIE work for the MER project. Dr. Squyres does not know if Matt Lenda has a face at all, much less a face that he would recognize. So even if I had turned around, he would have thought I was some nice stranger.

Damn. Well, there will always be more opportunities to run into Dr. Squyres by happenstance in the building 264 stairwell.*

Right, so, why the heck was Squyres at JPL? As my bracketed insert above tells us, the MSL project had a surface operations test that week. In these tests, the team simulates real-time data going to and coming from a rover clone down the street at JPL; it exercises our processes, our tools, and our teamwork. (I also participated, but several days later.) Squyres was there as a co-investigator. He couldn't ignore his loyal MER team and decided to walk down a flight of stairs to say hello. He brought with him an announcement: He had prepared a presentation especially for the MER team, to be given the next morning.

After hearing this news, I looked at my calendar: I was triple-booked at the time Squyres had proposed. I quickly emailed three people: "Cancel my 10am with you. I've got cooler things to do."

The next day, I got to the room twenty minutes before the presentation was going to start. (The only person to beat me to the room was Scott Maxwell.) Fifteen minutes later, the room — small given the expected crowd — was filled to the brim. People had brought chairs form offices and placed them in the hallway; standing room only in the main room. We were all excited to hear Steve just… talk.

We all welcomed him and Project Manager John Callas gave the room to Steve — to uproarious applause. We needed this invigoration after three months of frustrating winter ops, and we let Steve know it. He was all ear-to-ear smiles, excited, jumpy, ready to unleash all sorts of awesome upon the willing**.

[insert Steve's talk here]

Well, I can't spill the beans and scoop the science team. A lot of the stuff in Steve's talk was not news to us or the rest of the world; what was new was the story that Steve and the rest of the science team had put together. We had all these pieces to the puzzle of Cape York and now we have a full story to tell about this place. It's some of the most fascinating science you can imagine when you start hearing Steve describe, well, rocks. Just rocks. That's all. The team has submitted a paper about the science so far at Cape York to the journal Science. And when we see that paper drop, we can celebrate. But only briefly, because there's work to do. We're over the winter hump and we're ready to boogey.

Steve ended his talk clocking in at about an hour and a half (including questions from the team), and then he was off again.*** Despite the stay being so short, the team felt juiced, pumped, rocking and rearing to go.

Now that's a leader — a guy who walks into the room, grabs its attention without a moment's hesitation, and says, "Here's what up…"

———————————————————————

*This is blatantly untrue.

** = us. In case that wasn't obvious. Which it was.

***There was one last bit to Steve's talk I didn't mention: "What's next?" Simple: Although we see evidence of the phyllosilicate clays at Cape York, we don't expect to get that lucky. We'll stick around CY for a bit then head south to Cape Tribulation as soon as we can — giving us the chance to climb a mountain.

Over the hump!

It's been over a month since my last post. Upwards of a dozen people have asked me, "When are you posting again?"

Well, right now.

But the point of their question was, really, "Where have you been?" A few things combined chaotically into a fully-functioning ecosystem of busy:

1) MSL is a hard bird to fly. Long days, long nights, lots of time away from MER.

2) Oppy's been quiet. Filling out the Greeley Pan (part 45 to be taken tonight) has been the biggest consumer of bits, maybe second only to the MI stacks of Amboy. The MI mosaic is something like 17Lx2Wx5D and we've got maybe one-third of it captured. The Mossbauer has been accumulating hours of integration time, trying to sharpen up the resolution of the composition curves the scientists derive from its data. We're somewhere north of 50 hours by now, with 100 being the target. We're playing the power game sol-to-sol balancing the activities. Nothing new has really happened for about… 2 months?

3) I've been working on a Radio Science post. Being roughly halfway through the Radio Science season (ahem, winter), we've collected a huge amount of data to get those error bars down. This post is still in the works and is a bit difficult to put to together; it's driven by some cool geometric simulations that are easy to understand but hard to create. A friend works for the NAIF[link=http://naif.jpl.nasa.gov/naif/] folks across the street from my office building at JPL and she's been debugging the issues with me. On a scale of 1 to way-cool-awesome, it's about a 12. Stand by for that bad boy.

4) Got a new sound system for my vinyl records. Wasted a lot of time listening to it — sounds gorgeous. Much like the Mossbauer, I'm accumulating hours on it: it's tube-driven and the tubes take, incidentally, about 50 hours to be broken in. I find this all sorts of meaningful. It's like my own Mossbauer. In my room. Making loud noises.

So there you have it.

The big piece of news this week was that there was a small cleaning event[link=http://marsrover.nasa.gov/mission/status.html]. It bought us a handful of watt-hours that made all the difference. This comes at a time when we're approaching minimum solar insolation (power from the sun) — to hit sometime this week — after which it's pretty much only going to get better. We're over the hump! It's the Wednesday of the winter season.

In other major news, there was a Steve Squyres spotting this week. That'll get its own post because the experience served as a reconstitution of the MER spirit (pun not intended) for the whole project .

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".

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

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*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."

"8 more years, starting tomorrow."

Tonight at about 9pm Pacific (roughly 13:15 Local Solar Time at Meridiani), Opportunity will have spent 8 years on the surface.


Our SOWG chair put it best: 

After having spent 8 years doing surface operations, we need a recharge sol.

This is, of course, in reference to our current ambitious science campaign and trying to do it with a very dusty rover in a very cold winter. More on this in Part 8 of the previously-promised-I'd-have-this-done series of our time on Cape York.

(djellison @ UMSF)

100 Sols on Cape York — An Absurdly Brief (Read: Long) Summary (Part 7/8)

Part 7: Tryptophan Tilts at Turkey Haven

Sols

2772 to Present

"tl;dr" Description

We left Homestake for a haven or two.

Highlights from Matt's Notebook

Sol 2778-2779:

> The DSN is helping look for Phobos-Grunt. Might steal our uplink. &^$*#&$#^%

Sols 2783-2784:

> RPs wanted to do something fancy. TUL said "no." RPs said, "Ok."

Sols 2792-2794:

> Secret Woids: boggle / inbred / accost. I want better ones.

Sol 2800:

> AGU Conference today. Broke the news on Homestake. Can finally finish that blog entry.

With Homestake finally out of the way, we had about a week before the Mars Science Lab launched and had great potential to steal our time with the DSN for a good while. (A thing that, incidentally, never occurred. We couldn't have asked for a better launch from MSL.) Without time on the DSN, we can't uplink; without uplink, the rover can't move; without movement, she'd be stuck in a region with poor northerly tilts. We needed more than 10 degrees of northerly tilt to be safe and still perform science observations over the winter; 15 would be better.

We sequenced a nice set of drives (with a dogleg around the icky — although did that icky ever turn out to be icky?) to get ourselves to a standoff/scout position where we could image two spots where we knew there were good northerly tilts according to orbital imagery and subsequent digital elevation maps. We needed the ground imagery from this standoff position to resolve the orbital measurements and provide some much-needed precision.

(Sol 2777)

We found Turkey Haven to our south, with tilts a little above 10 degrees in the northern direction and bits of outcrop, to boot. Here's an image looking south on the last leg of the drive indicated in the image above:

From both Turkey Haven and the previous standoff position, we could see another bit of northerly tilts to the north: Winter Haven. We had two places to sit, each of which had good tilts and perhaps some good science targets. We also knew that these were the best two sites for the tilts because the ground-based imagery validated the orbital imagery and gave us confidence that the sites the orbital data showed were indeed the sites we wanted. No need to turn 'round the northern corner of Cape York to see if there was anything else that the orbital images missed. To get an idea of the tilts, here's a great virtual overlay of Oppy from Stu:

With the image-mapping of our winter sites complete before we felt like they had started, we had some time to ourselves: we selected a smooth part of the outcrop under our feet at Turkey Haven to poke with the IDD. We called it Transvaal — and here's an approach+Hazcam shot of them:

(Sol 2785)

"Hey, whaddya know, the power went up a few percent with that tilt," we told ourselves when we bumped up onto Turkey Haven. Hey, you know, that's how reality works.

We poked and Transvaal and the surrounding outcrop for roughly a week. The results? Nothing juicy. Same rock, different sol. Gave us no pieces for the Martian water puzzle. With more targets at the other Winter Haven site, a few meters to our north, and with better northerly tilts there compared to Turkey Haven, there was no reason to stay and give Transvaal what it had coming. Time to move on to Winter Haven. 

Somewhere along this whole trip, time... disappeared. Just went away. Magically, we were at another 100-sol boundary. In the sequencing room at JPL, we post a sign at the door that tells everyone for which sol(s) we're building commands today. I may or may not have stolen one of them:

Not stolen.

I dibbs'd it (a dibbs is valid even if nobody hears you!) and took it. Felt like it was cool to have one of these. Felt like I'd be keeping it around, even if 2800 is just… a number. I wonder who'll get sol 3000? (5000? 10000?)

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This is, really, the end of the first 100 sols at Cape York at the edge of Endeavour Crater. As of the time of writing, we've tacked on another 50 like it ain't no thang. The final part of the promised 8-part series will be a summary of what has happened since then — and where we plan on going once the winter is over.