Monday, August 6, 2012

Keep Calm and Carry On: From MER to MSL

Watching Paint Dry

I always liked the “watching grass grow” analogy to boredom better. Seemed a little more tangible, a little more organic. Plus, paint smells. A team I used to work with back in Colorado actually started growing grass in the hours upon hours of boredom that the spacecraft was affording us. 

1 day, 9 hours, 27 minutes, and 30 seconds.

Press just came into the CMSA. I’m inundated with questions like, “Why the peanuts?” Because of reasons, is why.

1 day, 8 hours, 48 minutes, and 45 seconds.

Just wrapped up a DDOR*. Telemetry coming back in whatever round trip light time is.

Oh yeah, what’s cool is that I bought one of those AeroPress things. Stuff makes quality ‘joe.

Well, wait, that’s not the coolest part. I guess the coolest part is that I’m sitting at a console in the Cruise Mission Support Area (CMSA) for the Mars Science Lab. I'm watching telemetry flow as we approach Newton’s pull from the body of Mars. I’ve been in this room roughly once a week since I started doing uplink operations – and subsequently rolled off of the MER project – and it’s not a whole lot different today than it was on those days. The downlink rate of the MSL spacecraft is just low enough to be annoying. It’s mostly an insanely unfair function of Earth-to-spacecraft range, and that range is [insert big frickin’ number]. With low data rates, we don’t get snapshots of the spacecraft’s state of health as often as we like, so we deal with the reality of watching paint dry.

Watching paint dry, by the way, isn’t all that fascinating. People say they want my job, but they don’t really know what it can be like. It can suck the life out of you. Maybe a telemetry channel will update here, maybe we’ll get a message from the spacecraft there. Not a whole lot. She flies herself, really. 

But in about 30 hours this room will be full of people like me, all of them brimming with anticipation as we watch a machine attached to another machine that just ejected from another machine… land itself on Mars.

1 day, 8 hours, 28 minutes, and 35 seconds.

I still don’t believe that this is actually happening. The cool factor is a little subdued at the moment.

“(Let’s Stop) Reinventing the Wheel”

With the paint drying ever so slowly, there’s time to reflect.

Spacecraft operations teams function mostly on reality. Engineering is done with numbers, and analysis without numbers is only an opinion. The doses of reality come in sobering and inconsistent waves, juxtaposed by the tunnel vision that is, itself, the spawn of equal parts innovation, paranoia, and adrenaline. 

When you start diving deep into the caverns of a spacecraft system, you start to see heritage: this thing or that process or this guideline or that scenario is a particular way because somebody did it on a previous mission the same way and it worked well. There was no need to reinvent a wheel because the operational regime of said thing hasn’t changed in years, if not decades. Time – and its evil brother, Money – is saved when an engineer takes these perfectly acceptable shortcuts in design and process. Efficiency and incremental improvements are nice-to-haves; effectiveness is indispensable.

The need to invent or create is insatiable for an engineer. It gives you a sense of reality and a connection to what you do. It makes it… real. But the desire to create is in exact opposition to reality that there is probably already something or someone out there that can do the same job.

Went to bed – came back again in the morning.

12 hours, 42 minutes, and 42 seconds.

Our flight director just instructed our ACE – the person who clicks the buttons to send the actual ones and zeroes to the spacecraft – to “turn off command modulation.” This is, nominally, the last thing we’ll send to the spacecraft until we’re on the surface.

When a new, big flight project comes along at JPL, this desire to invent and create is magnified. We delude ourselves into the notion that since we’re new and big and bad, everything we make needs to be newer and bigger and badder. It’s a serious problem when we put those blinders on and act like we’re the first in the business to do this business.

On one hand, the inheritances of many pieces that make MSL happen are clear and obvious. But on the other hand, there are many new inventions. The contrast in these two categories is stark.

The trouble comes in the apparent middle ground, where it seems that you can’t place a particular problem that needs to be solved into a well-defined spot in the spectrum from “inheritance” to “invention.” You subconsciously activate your tunnel vision, removing any ability to see what else is around you, and start reinventing things. You work literal days and literal weeks on this new problem to be solved, and then at the end of it someone more astute or removed from the process intelligently points out that your problem had already been solved. If you had simply asked the right question to the right people, you would have been done with this problem weeks ago. 

The truth hurts. Your need to feel connected, to feel reality, was too much, and you cracked.

What’s worse than the subconscious blinders are the blinders of outright denial, the blinders that you artificially place in your view because you refuse to accept that there’s no room (or time, or money) for you to invent something. 

We at JPL – and everyone at every abode to engineering and science in the world – have a problem with these blinders. We must collectively reset ourselves and remove our blinders. Turns out, we’re pretty good at pressing our own reset buttons, which brings me to my point: We learned how to press that reset button because of the MER experience.

10 hours, 48 minutes, and 38 seconds.

My Mission Manager mentioned my glorious “good luck” muttonchops (remember these?) at the 9:30am press conference. They are officially famous, but not Bobak Ferdowsi kind of famous.

The CMSA is quiet. The EDL Manager said that we’re “rationally confident and emotionally terrified.” I only agree with the first half of that. I do have stake in the success of the Sky Crane – that stake being a job – but I’m not so emotionally attached to it. Maybe it’s in my head, maybe I’m just going through that stage of denial. Maybe the acceptance will occur before this countdown clock hits zero.

One thing, though, is clear: MER paved this path. In between every thought about MSL, this one squeezes itself in. You’d be hard-pressed to find someone in this room that would disagree – but only if you pressured them into saying it. It’s easy, though, to find people in this room that don’t dwell on this notion as much as I do.

Then I walk across the street back to Building 264. I hop in the elevator and ride it up to floor 5 – wait, no, sorry, floor 6, old habits. The old Callas Palace has been spliced into two rooms and is now outfitted with sleeker, shinier machines running sleeker, shinier software. The shades are drawn. And yet, with all the differences, it still screams MER. Come Monday morning, we’re going to pick up where we still haven’t left off with Opportunity: tactical daily planning so that we may explore Mars. It's a monument to renewal and reinvention to inject ourselves with a sense of reality.

We hit Curiosity’s Sol 0 at 13:50:00 UTC (06:50:00 PDT) this morning. Breezed by without a blink from the team, most of who are sleeping anyways. Until Opportunity conks out, MSL will be perpetually behind the 3000 sols that she has under her belt already. It’s an astounding wealth of experience that we’re about to start all over again.

So, to my team I silently say, “Let’s please stop inventing the wheel.” We know how to do this. The game has not changed in any fundamental way. We’re the only people in the world that can drive cars on Mars, and we’ve done it all before. This one’s newer, bigger, badder, but it’s still just a machine on six wheels.

Unreserved and unabashed, we have to remind ourselves: “Keep calm and carry on exploring.”

Literally Unbelievable

10 hours, 23 minutes, and 28 seconds.

Most of the team is out to lunch. We’re spying on the media from afar, trying to enjoy our last lunch before what may be the biggest moment JPL has ever experienced.

The word “unbelievable” is, to me, the epitome of hyperbole**. And yet, I feel the need to actually apply it to my experience at JPL.

Interestingly, the enthusiasm and desire to start this blog was intensely opposed by this mental block of mine that causes me to refuse to accept the reality of what it is that I do. Occasionally I’ll reconcile the reality: “Holy crap, I make 1’s and 0’s that get turned into a series of ups and downs in a radio signal that’s leaping across space to be interpreted by a computer on another solid body millions of miles away.” But it's ephemeral, and I forget it immediately. The light that clicks on and reveals the truth is now off again.

It’s just weird. Plain weird.

So now I’m involved with the newer, bigger, badder machine – our wonderful MSL, tucked tight behind an impenetrable heatshield, wrapped from the topside by a metal cage with rockets – and I just want to sit there and invent. I want to create. I want to have a level of involvement, something that can validate the reality that is staring me right in the face. Instead, I feel an extraordinarily benign — if there can be such a descriptor — disconnect from this reality. 

I literally don’t believe that there is a car on Mars, exploring the rim of a crater we call Endeavour. I literally don’t believe that this car gave inspiration and birth to another, bigger, badder, more complex car that is hours away from landing an astronomer’s stone’s throw from this first car. My brain isn’t processing this data.

This feels like a problem to me.

6 hours, 0 minutes, and 0 seconds.

Just finished up the last Cruise Ops Status Tagup meeting. Our Flight Director dropped the last marble into the "days until landing" jar. There was applause, lots and lots of applause. Also just did the last "Playcall" for EDL Parameter Update #4 — everyone agreed we were no-go. She would fly out that 500-meter error.

It hasn't gotten any more real. I can't just sit here and watch. I can't just sit here and enjoy the moment.

1 hour, 10 minutes, and 13 seconds.

The Odyssey spacecraft is rolling to its off-nadir position to point its UHF antenna where the MSL landing site will be in an hour or so. She is out of contact with the DSN, and this is worrisome because it recently had reaction wheel issues. Hopefully she comes back for us when we expect her.

The room I'm in and the rooms surrounding it have filled up to capacity. There are celebrities, big-shot JPL'ers, family, friends. And they all believe. Why can't I? What switch do I need to flip?

50 minutes, 20 seconds.

Odyssey is back in contact with the DSN. The roll maneuver went off without a hitch. She is ready to support the bent-pipe relay for MSL EDL.

This disbelief makes me nervous. My hands are getting cold — maybe it's the air conditioning vent right above me — and I'm pacing. 

13 minutes, 51 seconds.

MSL is either dead or alive. Nothing in between.

The room is growing silent; it's about to happen. The make or break moment. I start recording video with my iPhone.

7 minutes, 0 seconds.

Atmospheric entry. The donut-shaped Cruise stage caused a couple brief blackouts in the radio signal as it passed in between the descent stage's low-gain antenna and Earth. She'll burn up in the atmosphere. She was good to us.

I'm shaking. 

0 minutes, 5 seconds.

Tango-delta nominal. Bridle cut. GNC done. Pyro fire. Closed-loop flyaway controller active. 

It's dead in here.

0 seconds.

We're on the surface of Mars. "Flight, spacecraft mode is SURFACE-NOMINAL"


I believe. That just happened. It really just actually happened. 

While people are debating whether or not you're a bigot based on your desire to consume a chicken sandwich, we got busy landing a robot on Mars. 

I believe. That just happened. It really just actually happened.

While people are going through places of entertainment or worship and opening fire with semi-automatic weapons, we got busy celebrating a multi-country achievement dedicated to all of humanity.

I believe. That just happened. It really just actually happened. 

While people were complaining about $2.5 billion dollars wasted on thinking metal, we got busy looking at pictures of actual wheels of an actual car on an actual other planet.

Yup. There are those pictures, but on the big screen in the CMSA. It hit me like a concrete wall: I believe. I see it, and I believe it. It's there.

And now I can also believe with an intense satisfaction that what I — we —  did and still do with MER is real, too. It may have taken an overpowering experience of seeing a real achievement happen in real time to get me to believe this, but at least something did it. They say that you'll always remember the day you got married, the day your kid was born, and your first Mars landing. I'm not married, I don't have kids, but I now have a Mars landing under my belt. One day later, the nerves still aren't gone. I will sure as hell remember this day. 

Those pictures we look at every day — and boy, do I still peruse the MER pictures every day — are real. The dust-covered, low-resolution images from MSL's landing make this all the more apparent. Those images are really not even on this planet. That's really a car with wheels and solar panels and a crippled arm. It's real.

Now, I must keep calm, and I must carry on exploring. 

We have a new robot that's also doing real things, for real, on another real planet. A real robot with a real older cousin with 3000 real sols under its belt, with no desire or need or, might I say, ability to quit on its own. 

Here's to us. Here's to the people that believe in us.


*DDOR = Delta Differential One-Way Ranging. Gives us velocity of the spacecraft in a particular direction with incredible precision.
**The irony of this statement is not lost on me.

Sunday, August 5, 2012

Stunning and Comprehensive MSL Summary

Hey folks!

If you’re not aware that MSL is landing tonight, then here’s your announcement:


A colleague sent a very impressive summary of the landing system and sequence of events.

Check it out!

I will be watching right from the mission control center at JPL. I know you’re jealous.

Saturday, June 30, 2012

New and updates — and stirring the pot of rumors

How is Opportunity doing? Well… good. Prett-ay good.

Daily Solar Power Is Off the Hizzay

While I idly watch, MER-B's power is shooting through the roof: 600 Watt-hours* predicted. As a reference, we were soaking in about 250 W-hours at the dead of Martian winter (middle of March here on Earth). Tau — a measure of atmospheric opacity — is at an all-time mission low (which means clear skies), and we've had several significant dust cleaning events (which means clean solar arrays). This increase in power is all in spite of the now low northerly tilts that Opportunity is at — you know, the thing that kept her alive in the depths of winter.

If I were so inclined to get in trouble, I'd post the watt-hours plot our Power team puts together. But I'm not so inclined! For that, we'll have to follow our glorious mission manager updates.

Opportunity's Location, Recent Activities, and Future Plans

Opportunity has been skirting the rim of the Cape York geological feature since she departed from her winter parking in the northern havens:

There's been a discovery of new Gypsum (errr, presumably Gypsum) veins:

And we've found a nice juicy spot for a RAT hole — a target named "Grasberg":

Unfortunately, the Mars Odyssey (ODY) spacecraft went into "Safe Mode" a few weeks ago. MER has always been incredibly reliant on ODY for data downlink via the relay chain, and when ODY goes down, MER has to stand down for a few sols. MRO also provides good relay support, but much more rarely than ODY. We have to schedule "passes" with these spacecraft strategically, i.e. weeks ahead of time, so that their teams can piece together their multi-week set of commands appropriately to handle the relay supports. Changing them tactically, i.e., the day of the relay pass or otherwise shortly before then, is a difficult process and things like geometry and spacecraft sequence engineering constraints can bite us.

There is a lot of so-called unsent data on board Opportunity that prevents the team from scheduling data-intense activities (like hi-res Pancams, MIs, or mobility data). As such the work on Grasberg and the surrounding territory is slow.

For the future: We plan to cross the skirt of Cape York to the north to look at the transitional layers (do I sound like a geologist yet?) thereon. Then, southward back from whence we came — to the first entrance into the Endeavour Crater on the south side of Cape York, and then further sound to Cape Tribulation. Betcha we get there before next Martian winter — those RPs are itching to drive the heck out of Oppy, and they're damned good at it, too.

A Mission Milestone -- Sol 3000 
According to my highly technical (read: I cheated and used SPICE again) analysis, Opportunity will hit Sol 3000 at 2012 JUL 02 01:51:32.572 UTC, or 2012 JUL 01 18:51:32.572 PDT.

Holy… what? Seems about right: I started work on the MER project nearabouts sol 2350, which was nearabouts July 2010. 

At the moment I'm not grasping what this really means… I haven't had the time to let it soak in yet. It's just unreal

Rest assured, the MER team is having a big get-together brunch this weekend to celebrate.

Allow me to stir the rumor mill!

I've received several emails asking me about my rumored move away from MER to the Mars Science Lab (MSL) project at JPL. Let me settle all the rumors now: Confirmed. In fact, I left the MER project about two months ago.

Much as I loved working on the MER program, opportunity (pun not intended) awaited me on MSL. It was not a decision that I made without a lot of careful thought — I sat around for a few months working up to actually making the decision. 

There are several good bits to come out of this move, however:

1) I have no plans to discontinue this blog. I never did! MER impacted me in a way that I'll never forget. I'm always going to be plugged into Oppy's updates. She is quickly approaching the off-earth traverse distance record and I want to be blogging about it. No question about it.

2) A large portion of the MSL tactical uplink team comes from MER. ("Gee, you know, it would be prudent of us to hire people who have done all this before…") I see a lot of familiar names and I'm finally meeting some past "MER Legends." Granted, MSL is a new kind of beast, very very unlike MER, but the overarching process of getting 1's and 0's to her is the same. Experience counts.

3) I don't have to move offices! MSL owns floors 4 and 6 of building 264 at JPL, and MER (along with Odyssey and MRO) takes up floor 5. So I still get the chance to swing down to say hello to the MER folks every now and then. In fact I still use the coffee machine in their break room, because if I'm ever going to drink anybody's office-coffee-club coffee, I'm going to drink MER's. **

And several unfortunate ones:

1) I have less time to stay focused on generating consistent, reliable posts here. I'm working crazy hours and will soon be transitioning to "Mars time" shift schedules when we land on Sunday August 5th. POWER THROUGH!

2) I can't blog about MSL, at least not in the capacity that I'm granted here by the PR folks at JPL. 


I've received some good, insightful comments about the Radio Science posts (here and here). I responded with a little too much brevity; and the comments pointed out that I lacked a sense of clarity in the posts! Good catches all a round.

I plan on some supplemental pieces on Radio Science whenever the results go fully public.


*Watt-hours = Energy. Watts are energy (specifically, Joules) per unit time; multiply by a time unit and you get Joules back. You may be asking, "Why not just say 'joules', then?" If we were to express it as Joules — Watt-seconds — we would have these huge numbers that are hard to grasp. Watt-hours are a convenient (though inconsistent!) way of bringing a numb back down to something the human brain to contextualize. It's a colloquial engineering term.
**They've got this old school diner coffee machine that is meant to stay on 24 hours a day; oh boy have I been using it. Late nights on MSL have caused me to leave these silly notes next to the MER coffee machine — something to the effect of "LEAVE ON FOR MATT, 06/29/12 evening" — so nobody turns off my caffeine supply. Sometimes, on top of my note someone from MER will leave me donuts. I love these people…

Sunday, June 10, 2012

Happy 3000th, Spirit

About 3 hours and 32 (Mars) minutes ago, it was zero hundred of Sol 3000 for the Spirit rover. (This means Opportunity's 3000th sol is only a few weeks away.)

Wonder how many sols more it will be until a human stops by and designates the final resting place of Spirit as a historic monument of humanity?

Saturday, June 9, 2012

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


Really, this took much longer to put together than I had imagined. There were a few technical snags — most of which are explained by the fact that I'm a bad programmer — but we finally have something worth discussing.

In Part 1, I talked about the basic science behind the Radio Science campaign that Opportunity undertook from January through the middle of May. Now, we have the results from that. Where in the sky was the Earth when Oppy was doing the experiments? Why does it matter? How did that change over time? How do the various variables relate? How can we easily represent all of this?

I've boiled it down to three plots. First, a few disclaimers and notes:

1) Since I'm not allowed to use operations data or products to create these plots, I have to play stupid and guess at which times of the Martian day to look at the data. As always, I used the SPICE toolkit for the geometric computational heavy lifting. Based on this data and what I remember from all those tactical shifts, I could piece together the relative time of Martian of day at which these happened. I looked at the orientation data of the High Gain Antenna (HGA) to see when it was moving and then — by hand! by hand! — picked out the ones that were "obviously" RS-DTE experiments rather than just the regular DTE/DFE's (see Part 1) when the HGA is also moving. The RS-DTE's are easy to pick out simply by time of day.
2) No durations shown here, just middle of the guessed window. Close enough! Most windows were about half an hour in length.
3) All Martian times of day (TOD) are Hybrid Local Solar Time (HSLT; we also just say "LST"), which is consistent with the rover's flight software.**
4) The plots of position of Earth in the sky (azimuth, elevation) are just showing where the Earth is, not where the antenna was actually pointing. I've done this for simplicity, and because backing out the real position of the HGA difficult. There are ~5 degrees of error here, which, for the purposes of this blog, is well under my "do I care?" radar.

The Evolution of Earth Elevation

We like to schedule DFE/DTE communications with our rover at the same time of Martian day. This gives us predictability, consistency, and flexibility in the plans from sol to sol. However, even if we could get the time on the DSN to do that (nobody can, it's a shared resource), geometry plays a big role. Both planetary geometry and the relationship of Earth and Mars time (i.e., the length of their day) have effects on when we can schedule communications passes — and remember, RS-DTEs are simply communications passes without much data going between the rover and Earth. 

First, as you may know, the Mars day is not equivalent to the Earth day — though it is very close. It's roughly 24 hours and 40 minutes, depending on what you mean by "day". Are we talking solar day? Mean solar day? Hybrid solar day? Then we get different values. The variance of this couples with the fact that, simply due to the relationship of Earth's position to Mars' position over time as they move through the solar system, the Earth will be at different parts of the sky at different times of the year. 

Put it all in a big confusing pot, and baby you got a stew goin'.

Right. So. How we best show this? The plot below shows this basic relationship: I've plotted elevation of the Earth from the local-level horizon at Opportunity's position on Mars at the same time LST (hybrid! hybrid!) from Sol 2800 to Sol 2971, which corresponds roughly to the times of the Radio Science experiments.

I could have picked any arbitrary time of day. The point is to show how the same time of day doesn't give you the same Earth position. We can see that sometime in the middle of February the elevation peaked. This pattern would repeat over and over if I had simply extended the time back. 

The lesson here is this: Earth elevation and Martian time of day only approximate proxies for one another.

Right, so, we see that elevation drifts. How did the RS-DTEs take account for this, or take advantage of this?

The RS-DTE Observations

One of the key characteristics of good RS-DTEs is a low Earth elevation angle. This exaggerates the Doppler-shifted signal, reducing the noise and clarifying the meaning of the data. We know that the Earth will probably be at low elevation angles in the morning and late afternoon, so it would be prudent to schedule our RS-DTEs there. When you slap on operational constraints — can we get that time with the DSN? do we have the solar energy to do it at that time of day? are we doing other things on the rover that preclude HGA movement? etc. — you get large variation from sol to sol in the time of day, and consequently the elevation angle of Earth, for each observation. 

The plot below shows all of the measurements that I gleaned from the SPICE kernels (see above). Details:
1) Left axis (blue): Elevation angle of Earth in degrees during the observation
2) Right axis (green): HLST Time of Day of the observation
3) Bottom axis: Time (duh)

Here are my observations:
> We tried for a lot of elevations under 30 degrees. Notice how we were getting those early on — then the power got too low, and we had to sacrifice the quality of the data for the survival of the rover, so we scheduled the RS-DTEs at an earlier time of day when there was more sun on the arrays.
> Another driver for the earlier and earlier times of day of the observation is found in the first plot from above: The Earth was lower and lower in the sky at the same time of day, and we wanted to hit particular windows of elevation angles, we ended up moving observation times to get the elevation angles right.
> The earlier the measurements got, the more we (presumably) doubled these tracking passes with the DSN as uplink passes to load the next sol's set of plans on board. Win win!

Finally, where was the Earth in the azimuthal direction relative to North? Here we have a plot of azimuth angle clockwise from North and elevation in concentric circles,

North is up (0 degrees azimuth); East is to the right (90 degrees azimuth). For each of the observations, the azimuth was roughly the same; this has mostly to do with the orbits of Earth and Mars, less to do with local solar time. Of course, you can't tell the direction of time in this plot, but that's not the point!

The End of RS-DTEs

There is an incredible amount of input going into these observations; a hundred constraints and a hundred desires. Now we're driving and hunting down veins, leaving radio science to next Martian winter — about an Earth year and a half from now. 

I hope this was educational!


*No, I don't have a Twitter account. No, I don't want one. hashtag PleaseLeaveMeAloneAboutIt
**Fun fact: MSL will be using "mean" solar time — "LMST". It matters.

Sunday, May 13, 2012

[insert song about getting on the road again]

This actually came sooner than I expected: Opportunity drove for the first time since Sol 2795. From Phil Stooke's post:

Paolo the RP wandered into the UMSF forums the other week to announce things like planned (and now executed) "drive direction imaging" and a "turn for comm." The words couldn't have fallen more welcome on our ears.

For now, it'll be step-by-step kind of drives. We still need those recharge sols, and I'm not exactly clear* on the possible continuation of Radio Science. (Speaking of: Part 2 of 2 of this in the works!)

Winter was a challenge — but obviously one we could manage. Next up? Sniff out Morris Hill, maybe do some drive-by IDD'ing of the other potential juicy bits of Cape York, and then it's off to Cape Tribulation. Funny to think that Cape York was this big fat mystery to us when we arrived; it marked, and has since represented in spirit, the arrival of the Opportunity rover at the massive Endeavour Crater. Now, we've done our business and are ready to move on to climb some mountains. It's another new adventure, and this time we don't have to trek tens of kilometers to get there in the first place.


*Not exactly around a lot in MER town these days. Sad.

Friday, May 4, 2012

First Paper about Endeavour is out! (paywall'd)

Here it is! Unfortunately, paywall-blocked. I guess it got published a month ago, but just now showed up on Slashdot.

But hey, it's out there… and it's been in the works for quite a while now.

Sunday, April 29, 2012

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...
Don't make it boring.

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). 


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

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


*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. 

Saturday, March 31, 2012

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.



*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.