Tag Archives: navigation

P–7 days: Navigating Pluto

As of the first instant of this morning, July 7th on the US east coast, the little spacecraft was about 8.7 million kilometers from Pluto, getting 1.2 kilometers closer each day.

We’re almost completely recovered from the hiccup that happened July 4th when New Horizons took on a little too much multitasking trying to simultaneously compress data for downlink and load up the sequence of activities for the flyby at the same time. This caused the equivalent of a brain freeze, and the spacecraft went into a defensive fetal position, spinning up to 5 rpm to provide stability for its earth-pointed dish antenna, then dropping off into a light midsummer night’s snooze. We have to wonder, when spacecraft sleep do they dream of electric sheep? (With a nod to Philip K. Dick.)

We lost some tracking and science data, but it’s not a vital loss that won’t be made up in the following days with much more powerful information on where we are and what Pluto looks like. Pluto currently spans about one percent of the field of view of our long-range camera, covering about 55 pixels. You can’t see a lot of detail in an image only 55 pixels across, you can only be taunted and tantalized by vague shapes and shadows. Those shapeless, shifty features will begin to stabilize and fill out in exquisite detail over the next few days.

Our last maneuver is behind us—several days ago—and we have delivered the spacecraft from our capable hands into those of Misters Newton and Einstein whose inviolable laws of physics will guide New Horizons the rest of the way in. Nothing more can be done about where it is and what time it gets there, but we can do a series of Knowledge Updates to better inform us on where to point the cameras and other instruments. That’s what the Navigators get into today, delayed by the little snooze, as the first new tracking data comes down from the now revived and presumably refreshed, spacecraft. Here is where the rubber meets the road in a series of tightly coordinated and time limited interactions between the Navigators and the rest of the project teams. The spacecraft may get its snoozes now and then, but the Navigators are headed into some sleepless days and nights.

 

I don’t know if I’ll be able to continue these posts while things get hectic, so for interim entertainment here’s some filler about Voyager’s encounter with Neptune in 1989 from a Navigator’s perspective. There are a lot of similarities with the current Pluto adventure, and differences too. The excerpt is from the chapter titled “Neptune” from my novel The Darkest Side of Saturn. The spacecraft is named Nomad, but don’t be fooled, it’s really Voyager. There are some fictionalized versions of real events that happened during that wondrous time, but the rest is full of lies. Your task, dear reader, is to separate the wheat from the chaff.

Navigating Neptune

The party started in the Navigation Team’s operations area an hour before midnight, two hours before the Neptune closest approach time. They fished drinks out of a metal washtub filled with ice, beer, juice, and sodas. They cut slices of brie and slabs of pâté from paper plates between the computer terminals lining long tables on both sides of the room. They spread their fare on stout crackers and delicate wafers, popped olives into their mouths, and poured wine from bottles and jugs into plastic cups.

They were very happy, all nineteen members belonging to the Navigation Team functional groups: Radiometric Data Conditioning, Maneuver and Trajectory, Optical Navigation, and Orbit Determination. Harris, the newest of the new and lowest of the low on the navigation totem pole, was perhaps happiest of all. After working nearly twenty-four hours straight, their job was finished. Nomad was nearly on course. The new camera pointing commands had been uplinked across four hours and seventeen minutes of light travel time to the spacecraft. Now the laws of physics ruled supreme. There was nothing more to do, nothing more that could be done. Eat, drink, and be merry, for in a few more hours we either bathe in glory or go splat!

They pitched darts at Neptune. On the far wall of the navigation operations area was a large bulletin board. Tacked onto the center of it was a square of graph paper three feet on each side with tick marks annotating every ten kilometers of the divisions. This was the navigation target plane for Neptune, although Neptune was not on it. At this scale, Neptune would have been represented as an enormous circle whose closest edge was a few feet down and to the right—off the chart.

Dozens of small multicolored paste-on dots sprinkled the chart—yellow, blue, green, brown, black, and red ones. A red one near the center, larger than the others, was labeled “TCM20 aimpoint.” Because of the inevitable maneuver errors, this was where Nomad certainly would not go, although it shouldn’t be far away. The smaller colored dots, representing solutions vying with each other to show where Nomad was actually headed, had meandered away from the aimpoint as new data came in after the maneuver.

They were throws of the navigator’s darts, almost literally. Each dot represented an experiment to test the sensitivity of the trajectory to the ubiquitous errors in every source of information. No two solutions were exactly the same except that they tended to cluster in places. There was a cluster of green dots for solutions that used optical data of Triton and a blue grouping about thirty kilometers away accounting for data that included optical observations of the temporarily named N1—first of the new satellites discovered by Nomad.

For the latest solution—they were loathe to call it final; there was always just one more data point to include, just one more computer run to make—they’d placed a red dot and labeled it NAV3, the name for their ultimate delivery of knowledge. Nevertheless, it was final, the last delivery that could affect the success or failure of the encounter, delivered to the sequence designers so that Nomad’s camera angles could undergo a last-minute shift and she would send back pictures of things like satellites and cloud features instead of nothings in empty space.

Knowledge and control were the two halves of the navigator’s function. Gather the data to determine Where are we and where are we going?, then use the knowledge to apply the control: Burn the engine! Change the trajectory! Finally, when it’s too late in the game to burn the engine, when it’s too late to control the trajectory, all you can do is improve the knowledge to point the instruments, whose purpose is . . . to gain more knowledge.

Job done, the party navigators pitched darts while the rest of the Nomad engineers and scientists labored away over hot computers in rooms upstairs. They watched the Lab TV news report as Nomad approached the ring plane crossing at midnight, or rather, as the downlink data containing that event approached Earth, because the event had already happened over four hours earlier. Either Nomad had already splattered against a random rock just outside the visible ring, in which case the signal would come to an abrupt halt in a few minutes, or it had not. The navigators, including Harris, were well into some serious beer drinking.

At six minutes after one o’clock in the morning, as Nomad skimmed over the top of Neptune and Carl Sagan’s face graced the TV, Harris ambled to one of the several phones scattered over the operations tables and punched a number.

“Hello?” Diana answered.

“Hi. Whatcha doin’?”

“Oh, nothing much right now.” She seemed distracted.

“Why don’t you-all come on down here to our party?”

“Why thanks, Harris. I believe I will.”

Minutes after they talked, as Harris lifted his fifth can of beer for the evening, Diana walked through the door of the navigation area trailing a TV crew carrying lights, sound equipment, and a camera. They taped her even as she walked. As one of the few female scientists on the project, and a good-looking one at that, she was in heavy media demand.

Uh oh. Harris discreetly parked the beer can on the table behind a stack of printouts as she walked straight up to him. Drinking at the Lab was forbidden; getting caught on television was certain career death. He tried not to wobble as she shook his hand.

“Congratulations,” she said formally for the camera. “Looks like it’s going to be successful.”

“Congrad . . . grajlations to you too. It’s a good encounter, huh? Good stuff, you know what I mean? I mean we’re doin’ good.”

“Loosen up,” the director said. “You’re like statues.”

At the director’s suggestion they linked elbows—beautiful young female scientist and intrepid male engineer—and faced the camera with silly grins on their faces.

“No no. Too posed. How about a toast.”

Somebody brought them plastic glasses filled with orange juice. Harris and Diana held them high and clicked them together.

“Here’s to science,” Diana toasted and smiled.

“Here’s to engineering,” Harris answered.

“Here’s to knowledge,” she said.

“And here’s to control and a dual occultation.”

“May we be so lucky.” Diana’s smile began to crack. “Here’s to wisdom.”

“And good ol’ ingenuity.”

They chugged their orange juices, the navigators clapped, and the director was mollified. A few minutes later another network TV crew stole Diana away for an interview.

saturn_0.5inch

P–11 days: Navigating Pluto

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Part of the Project Nav Team: Jeremy, Bobby, Chris, Derek, Dale, Coralie, Tony. (Photo credit: Dale Stanbridge)

As of midnight beginning the morning of July 3rd on the U. S. east coast, our intrepid New Horizons spacecraft—the long-distance eyes and ears of the human race and our ambassador to the outer reaches of the traditional solar system—cruised along at 13.5 million kilometers from Pluto, looming closer by 1.2 million kilometers every day.

The Plutonian system of satellite orbits now swells rapidly in our telescopic eye: the camera named LORRI (LOng-Range Reconnaissance Imager). If abstract orbits were visible, the outermost one—Hydra’s—is about twice as large as the one-third degree field-of-view of the camera. That means the system is only a little larger in New Horizons’ black sky than the full moon seen from your back yard.

Even the orbit of Styx, the innermost of the small satellites, overflows the image boundary by about 20 percent. Only Pluto and its large companion, Charon, still fit comfortably within a camera frame. From this point onward, the spacecraft will have to “slew” to get OpNav images of satellites from one side of the system to the other.

The day before yesterday we had a close encounter with Styx. The band! Not the satellite.

Rock stars and around a hundred-or-so space cadets mingled in close proximity, taking pictures and signing autographs. A photo-op is a photo-op, for rockers to mingle with Lords of the Solar System, and space cadets to schmooze with Lords of Rock. Who was most impressed? I’d say the space cadets.

Which raises the question: I assume the band was named for the mythological river, but was the satellite named for the river or the band? Well, ostensibly for the river of course, but was there a hard-core rock fan lurking amongst the namers of names? And I don’t mean a geologist.

The Navigators’ DTpD (Dreaded Target-plane Drift) seems to have drifted to a dead-end, coming to rest about 25 kilometers from the aim-point, that point—just outside Charon’s orbit in the big Plutonian dart board—which is most desired by scientists and engineers alike of the project. The solution and its attendant cloud of possible errors is comfortably inside the box of acceptability, a 300 x 200 kilometer region where the photo-ops are best and all the other instruments stay happy. Now, if the DTpD is really dead and stays dead, not coming zombie-like back across the river Styx, we’ll have only one more thing to worry about. (And it’s a good thing, since there are no more maneuvers planned and therefore no way to change the trajectory. We have what we have.)

That one last thing to worry about is the arrival time, which is still uncertain by a little more than a minute earlier or later. We don’t know the distance to Pluto precisely, so we’re not sure when we’ll get there. We’ll find out in the last few days by closely watching how the satellites spiral outwards as their orbits expand in LORRI’s point-of-view. When we know how far we are and when we’ll get there, we’ll tweak our already-loaded sequence of events by the right amount of seconds, and all will be well.

That will be a very intense time for the Navigators. But we’ll cross that river when we get to it.

New_Horizons_-_Logo2_big

Added:

Conspicuously missing from the KinetX Project Nav Team image are Fred, Ken and Philip. Also not shown but very appreciated are the ones who provide reality checks to keep us honest—the JPL Independent Nav Team: Paul, Shyam, Dylan, Steve, Gerhard, Bill, and Mike.

P–15 days: Navigating Pluto

As of today, Monday June 29th, I find myself airborne headed for the Applied Physics Laboratory in Maryland while 13 hours earlier, at midnight leading into this morning on the east coast, New Horizons found itself 18.5 million kilometers from Pluto and moving 1.2 million kilometers closer every day.

The Dreaded Target-plane Drift has come and gone. For about a week, the solutions drifted downward and leftward on the target plane—that big dart board in the sky—each time more tracking data was added, until finally it departed the bottom of the acceptable target box, a 300 x 200 kilometer rectangle centered about 14,000 kilometers down (Ecliptic south) and left (eastward) of Pluto.

The cause of the Dreaded Target-plane Drift is not yet completely understood, although it’s almost certainly related to earlier errors in locating the centers of Pluto and Charon, and this was most likely caused by inadequate knowledge of the albedos of those bodies, the dark and light patterns covering their surfaces. It’s hard to model the surface brightness of a body you’ve never seen up close before.

The DTpD, while it was in play, was a very worrisome thing because the Navigators didn’t know if it was real or—much worse—caused by a hidden problem, maybe serious dynamic mis-modeling that’s been overlooked to this point or even an unknown bug in the software. This caused some very restless activity and sleepless nights for several days in a row. It now appears that the drift has stopped and the current position, just outside the box, is correct and the earlier positions were not. There was probably not a dynamic mis-modeling problem or software bug. The moving finger of Navigation writes, and having writ, moves on.

In any event, the Dreaded Target-plane Drift may be a thing of the past as the Navigators rely more on the pin-prick images of Styx, Nix, Kerberos, and Hydra. Those bodies are small enough that no appreciable diameters are yet seen and the albedos of their surfaces doesn’t even enter the problem. So … while the DTpD may never rear its ugly head again, the Navigation Team will not hold its collective breath. Eternal vigilance is the norm.

At this red hot moment, if the current dart in the dart board is the correct one, surrounded by it’s little cloud of Gaussian-shaped probability, then the science observations won’t be completely optimal at closest-approach time, and the intended occultation of the Sun and Earth by Charon may not happen.

Thus in the wee hours of tomorrow morning the penultimate maneuver, TCM17B1, will execute and move the drifted solution back into the box, the happy return of the prodigal son. It will cost a few centimeters per second change in velocity, and then all will be well.

We hope.

P–21 days: Navigating Pluto

New_Horizons_-_Logo2_big

Continuing the countdown, focusing on optical navigation using images of Pluto and the satellites.

As of midnight this morning of June 23rd, Eastern time, we’re 25.4 million kilometers from Pluto and moving 1.2 million kilometers closer every day.

The whole thing comes in a rush; time is not a leisurely independent quantity here—it’s relentlessly urgent! Another maneuver approaches fast: TCM17B1 executes in seven days, the next-to-last opportunity to correct errors before the flyby, and Navigation needs to come up with the “correct” trajectory ahead of that to support its design and implementation, a process that will take most of that seven days.

They need answers now! Both Navigation Teams—PNav (Project Nav) and INav (Independent Nav)—are busting their butts to figure out what’s going to happen in the target plane—that big Plutonian dart board in the sky. The error ellipses shrink down slowly as we get closer, but the solutions wander around, some of them inside the previous error ellipses, some out, depending on what combinations of optical data are used and a host of other assumptions, all in the form of experiments to figure out the most realistic answer. How many toad tongues and bat wings do we need to add for a brew that is true? (With apologies to Danny Kaye and The Court Jester; I couldn’t resist.)

Ideally new solutions wander around inside all the older ellipses as the newer ones shrink down around them. Lately INav has been holding fairly steady, but PNav dithers around, sometimes in, sometimes out of the ellipses computed only a day or so earlier. The game right now is to figure out which OpNav (Optical Navigation) modeling is correct, and which misleading. Pluto itself seems to be the problem.

Images of Pluto, that is. Since we’ve never seen this Honorary Planet up close before, extracting an accurate estimate of its center from fuzzy optical data is as much art as science at the moment, and nobody will know what the real answer was until we actually get there and know all the Plutonian “blemishes” in detail; the craters and icy plains and mountains and scarps, and whatever else we find.

However, at this red-hot-moment, they are only just beginning to pop out in sharper detail, the dark regions and the light, and in order to realistically extract the center of a 2400 kilometer wide body to within a few tens of kilometers, you have to model the blemishes with an accurate “albedo map.” The problem is, the details pop out faster than you can model them, so you play a catch-up game with the Pluto images, and if the current albedo map is behind the times, so is your estimate of where we’re

Plutonian Albedo Map

Plutonian Albedo Map

going in the target plane. There are other models for center-finding too, and they’re also in the mix of experiments, all of them throws-of-the-dart to see where we come up relative to Pluto—all of them to see which ones we believe.

INav seems to have avoided the problem up to now by the simple expedient of not using Pluto images at all and relying on images of its companion, Charon. Because Charon is smaller, it has smaller center-finding errors. PNav is coming around to the same conclusion: Don’t trust Pluto!

Fortunately, the smaller satellites Nix and Hydra are beginning to pop out of the optical wood-works, and because they are little more than specs in the images, mere pin-pricks of light, they will be much better located against the starry sky. After a while tinier Styx and Kerberos should become usable too. Ironically, the best information of where we’re going relative to Pluto is going to come from those satellite pin-pricks in the images, and not from Pluto itself.

Meanwhile, I depart for Maryland Monday to become a small part of this Plutonian melee with whatever humble capacity I can bring—it’s been a long time since I’ve done this kind of interplanetary navigation, and the learning curve is extremely steep and precipitous.

And meanwhile, nobody can stop the relentless march of time as Pluto expands in our vision and the activities grow more urgent. The pressure is on.

May_12_Pluto

P–24 days: Navigating Pluto

 

NavTools

Tools of the Navigation Trade

… wherein we continue the countdown with talk about target-planes, TCMs, trajectory errors, cabbages, and kings.

As of the stroke of midnight early this morning beginning Saturday, June 20th (U.S. Eastern time), New Horizons was about 29 million kilometers from Pluto and closing at 13.8 kilometers per second, or 1.2 million kilometers every day. Since the entire civilized world is metric (excluding the United States, Liberia, and Myanmar), I’ll let the units stand, but if you absolutely have to, you can convert to miles using a factor of 1.6 kilometers per mile.

News Flash: TCM17, which was scheduled to execute next Wednesday, June 24th at P–20 days, is cancelled. The velocity change of 7 centimeters per second, providing competition for a fast-moving turtle, would have made too little difference in the arrival of the spacecraft compared to the current size of the known errors. That small velocity change will be deferred to TCM17B1 at P–15 days when it will have grown a little and the errors have shrunk. At least some people on the operations teams will get the weekend off, but the Navigators continue to add new data and run new solutions today.

News Flash 2: The Dreaded Target-plane Drift, bane of previous Voyager flybys of Uranus and Neptune, has not reared its ugly head as yet, and shows indications of perhaps never doing so. The trajectory solutions remain relatively steady with only a little dithering while the associated error ellipses (more on that below) shrink down around them. However, I’m not holding my breath yet.

End of News Flashes

Use your imagination to float in space beside New Horizons. The Plutonian system, encompassed by the orbit of its most distant known satellite, Hydra, spans less than a quarter of a degree in your unaided vision, less than half the size of the Moon seen from your house. In the telescopic view of the LORRI (Long Range Reconnaissance Imager) instrument—the prime camera for Navigation—it now fills most of the image and daily grows larger.

With images from LORRI, as of yesterday the Navigators predicted our arrival error in the target plane to within an ellipse of about 90 x 50 kilometers, “1-sigma”. That’ll continue to improve as we get closer. Statistically the 1-sigma means our actual arrival would be within that ellipse about 39% of the time. (For math purists the 39% is for a 2-dimensional Gaussian distribution; for a 1-dimensional distribution it’s the familiar 68%.) That size error from a distance of 29 million kilometers ain’t bad shootin’, and we owe it to the magic of OpNav (Optical Navigation) and the collective expertise of the Navigation Teams, including all ten members of the PNav Team (Project Navigation of KinetX Aerospace) and seven members of the INav Team (Independent Navigation of JPL). The large size of the Navigation effort attests to its importance to the success of the mission.

Think of the target plane as an enormous dart-board centered on Pluto, with our target-point about 12,600 kilometers down and to the left of Pluto, just outside the circular orbit of it’s biggest companion, Charon. Pluto is about 2400 kilometers across (not a big body, only two-thirds the size of the Moon), so our target-point is about five-and-a-half diameters away. You’re a giant, pitching darts from 39 million kilometers out. Some of them hit that small ellipse, others fall outside, but they’re constrained to a tight grouping that, if the ellipse were 3 times larger, would fall inside 99% of the time.

The biggest problem for the mission is not that small 90 x 50 kilometer error ellipse in the target plane; it’s the much larger error in the predicted distance to go, in the neighborhood of plus or minus 1000 kilometers uncertainty in the distance of Pluto from the Sun and Earth.

Why don’t we know it much better than that? After all, we’ve been tracking Pluto for 85 years since discovery by Clyde Tombaugh in 1930.

We don’t know it because 85 years is about one-third of the orbital period of 248 years, and in order to pin-down the heliocentric distance to a much smaller error, astronomers and the scientists/engineers at JPL who publish planetary orbits world-wide (and specifically for New Horizons) would need most of a full orbit of Pluto tracking behind them.

Unfortunately the Navigators, wizards that they are, can’t do much about this arrival time quandary at the moment. The OpNav images they use, taken against a background of stars, are very effective in telling us where we’re going in the plane of the image, which is basically parallel to the target plane, but it’s hard to squeeze information out of an image in the perpendicular direction.

This is just like in your picture of Aunt Molly about 29 feet away against a backdrop of mountains; it’s easy to measure her position in the up-down, left-right plane of the picture relative to features on the mountains (assuming you know all the camera parameters like field of view, size of the pixel array, etc., which you do), but hard to determine exactly how far away she is—the in-out direction—within a couple of feet unless you have the scale, meaning knowing exactly how wide and tall she is (which you don’t, since Aunt Molly is not a particularly cuddly person and you’ve never been up close for an opportunity to measure her).

We can’t do anything to adjust the arrival time to better than 70 seconds or so because our last scheduled maneuver, TBM17B2, is 10 days out, but the knowledge in the timing won’t improve significantly until 3 days out, much too late for a maneuver. If the project didn’t do something about that, New Horizons would fly by Pluto clicking off pictures at the wrong times, perhaps over a minute too early or late, which, at 13.8 kilometers per second, means a total error—early to late—spanning 2000 kilometers more or less.

Fortunately, the project can do something about it. Even though the trajectory can’t be easily adjusted after P–10 days without a big risk of something going wrong and totally blowing the mission, the knowledge continues to improve with continued OpNavs (because we start getting the scale by watching how the system expands in the images), and the Navigators deliver a “knowledge update” late in the game that’s considerably more accurate in the arrival time. With that, the sequence team tweaks the timing of the already-uploaded sequence of events for the cameras and other instruments, and all turns out well. We get pictures of the things we want to see.

We hope. Nothing is ever guaranteed in this world or in deep space beyond death and taxes. However, rest assured and be comforted that the Navigators “almost always get you there!”

NavMethods

Methodology of the Navigation Trade (Thanks to Douglas Adams)

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P–30 days: Navigating Pluto

 

Goldstone_DSN_antenna

Goldstone 70 meter antenna / NASA / Wikipedia

2015/06/14. Sunday
The third of a series of posts about navigating to Pluto

Distance: About 36 million kilometers from Pluto at 5:30 am Arizona time as I write.
Velocity: A not-much-changing 13.8 kilometers per second toward Pluto and away from Home. Not-much-changing because the pull of Sun’s gravity isn’t very strong out here.

I think I’ll do nuts and bolts today: how do the Navigators do what they do? That means I need to talk about the DSN (Deep Space Network) and navigation data types and other technical what-not. Let’s try to keep it light and entertaining, shall we? If you’re not technically inclined, you may be excused from today’s session, but be warned you will be tested on the material in the final exam. Attendance at the next session is mandatory.

But first, News Flash! The maneuver this morning was successful as I see in an email from Alice, the mission operations leader: “Initial TCM assessments are showing a nominal burn within the expected parameters.” Chris, one of our Navigators at the Mission Operations Center at APL (Applied Physics Laboratory) in Maryland says “The Doppler residual came up right around 1.06 Hz … indicating very likely a nominal burn.” Woo-hoo! We’re walking a little under two feet per second slower toward Pluto so that we’ll get there about 83 seconds later, close to the intended arrival time.

Back to the Deep Space Network. What a romantic, adventurous, ambitious name! I can hardly believe it’s real; the term evokes wonder every time I give it a little thought. Especially since there wasn’t any Space when I was growing up, much less Deep Space. I mean, most people didn’t think we could ever go into Space, and if you thought differently you were a Space Cadet.

I was a Space Cadet. All the way up to Sputnik in 1957. Then Space became IN. I thought I’d become IN, too, from Space Cadet to Visionary in a single launch, but no, that didn’t happen. It’s hard to project the necessary gravitas at age fifteen. Since then, I’ve navigated spacecraft to every planet in the solar system. From zero to sixty in only seventy-two years! (And I’m still a Space Cadet. When does the gravitas kick in?)

Where was I? Oh yes, Deep Space: a term that never fails to evoke romance, mystery, and adventure. Are we earthlings actually tracking things in Deep Space? Yes, we are! And someday, if we’re good and don’t kill ourselves first, we may even go out there ourselves.

The Deep Space Network: There are three locations almost evenly spaced around the world where NASA operates antennas that track New Horizons. The locations ensure that we (New Horizons) will be in sight of at least one of them all the time. There are several antennas at Goldstone, California; several near Canberra, Australia; and about an equal number not far from Madrid, Spain. We call those sites Goldstone, Canberra, and Madrid for short, but more commonly we talk about the antennas themselves, like DSS-14 (Deep Space Station 14, a 70 meter wide antenna at Goldstone) and DSS-65 (one of the 35 meter antennas at Madrid.)

The DSN antennas collect four different tracking “data-types” from New Horizons and funnels them to the Navigators through various channels that aren’t important to this discussion. Here they are (bear with me, it gets a little thick for a few sentences here and there):

(1) Doppler data: A DSN station sends up a radio frequency signal. It travels 4.5 hours to the spacecraft. When the spacecraft gets that uplink, it sends back a downlink signal that’s in “harmony” with the uplink. That means that in the process of turning the signal around and retransmitting it, New Horizons accounts for the frequency of the uplink on a cycle-by-cycle basis. When it gets back to Earth 4.5 hours later, it’s compared to the frequency that went up. If it’s the same, then nothing has changed, and that’s evidence that we live in a static universe in which nothing moves! That doesn’t seem to be the case. We always see a shifted frequency, pretty firmly establishing that we live in a non-static universe!

eeEOoo

Doppler effect defined

The received signal is different from what went up. Why? Ta-da, the Doppler effect of course, named in honor of Christian, its discoverer. That’s the eee-ooo sound you hear when a train goes by, but radio waves behave the same. And when you analyze that shift, you get clues about how the spacecraft moves, and just as important, how the tracking station moves because of the earth’s rotation, and when you put all that information together over the course of a tracking pass that lasts several hours, you can figure out not only how fast New Horizons is moving away from the Earth, but also you can determine its location in the sky: the Right ascension and Declination (reverting to astronomy-speak for a moment). The Doppler data is incredibly powerful in navigation, at least in the radial direction, and a change in New Horizon’s radial velocity of only 1 millimeter per second will look like a big signal.

So there—we’re through the hardest part of the discussion I think.

But wait, there’s more!

(2) Ranging data: If you time the signal (and the DSN has extraordinarily good timers, accurate to a gnat’s ass (another technical term), and know the speed of light (which we do) you can get the distance to the spacecraft to an accuracy of much less than a kilometer out of 4.7 billion of them. Now there’s a truly astounding accuracy). There are a lot of complications, of course, but that pretty well sums up the big picture for ranging data.

But wait, there’s more!

(3) DDOR data: Sometimes we use two of the DSN stations simultaneously, like Goldstone and Canberra and—over the course of about an hour—alternately track the spacecraft and then a quasar near it in the sky. Quasars are conveniently loud at radio frequencies, and they’re so far away that they don’t budge over the course of many, many years,

Delta-DOR defined

Delta-DOR defined

so they provide a very nice fixed reference system (thanks, Mother Nature!) for figuring out the direction of the spacecraft to jaw-dropping accuracy, about one-millionth of a degree. This data type has the gawky-gangling name of Delta-Differential One-Way-Range, which we usually shorten by calling it Delta-DOR, or writing it “DDOR”.

But wait … (oh, never mind). There’s one more.

The three types above—Doppler, ranging, and DDOR—are so-called radiometric data types. They’re all “centered” at Earth, so-to-speak, so the farther away New Horizons gets, the less accurate they are. To add to the uncertainty, we don’t know the distance from Earth to Pluto very well yet, so even though we might know the distance from Earth to New Horizons to that gnat’s ass, we don’t know New Horizon’s distance to Pluto to better than, very roughly, 1000 kilometers. That leads us into:

(4) OpNav data: The 4th data type, optical navigation data, or OpNav in the vernacular, is based on pictures taken from New Horizons of the things out in front of it, namely Pluto and his retinue of satellites, downlinked to the DSN and thence to Navigation. The OpNav team, a subset of the larger Navigation team, is led by Coralie. She and her team pick the locations of the tiny blobs of Pluto, Charon, Nix, and Hydra out of the noise of the images (a very difficult and tedious process with all kinds of complications) and compare them to locations of stars in the same images. This “pins” the spacecraft down against the stellar background, and since the locations of the stars in the sky are well know, so is the location of New Horizons. (The remaining two known satellites, Styx and Kerberos, aren’t used for the OpNav process because they’re too small and hard to see.) The location of the stars, Pluto, and satellites in the images constitute the data passed to the Orbit Determination team led by Fred.

So, that’s the end of the data descriptions. Not so bad, eh, if you’re still with me.

Since the OpNav data is spacecraft “centered”, it gets more powerful as we get closer to Pluto, until finally there comes a time when it overwhelms the accuracy of the radiometric data and tells us where we’re located relative to Pluto rather than Earth. From then on, OpNav data is the prima donna of the navigation show except for complications (always complications!) like when the spacecraft gets really close to Pluto and the Doppler data begins to “feel” the gravity. This doesn’t happen until the last day in the flyby because Pluto is so NOT massive (at least as compared to the planets).

It’s the job of Fred and his minions (of which I may be one), to boil all of this big slurry of data down (remember that big black witches’ pot in the first post?), scads and scads of it (another technical term), until there’s nothing left but a tarry black residue. That’s what we call the solution. It tells us where we are and where we’re going.

But wait … It’s not cut and dried with just one solution. There are a lot of unknown parameters that go into the witches’ brew, and the Navigators have to make assumptions about their values and how much they trust those assumptions. There are also a host of other variables, too much to go into, so just call them toad tongues and minced-spiced bat wings. They all affect the solutions to varying degrees.

The Navigators boil down many, many pots of witches’ brew, stirring vigorously, tasting occasionally and adding different amounts of toad tongues and bat wings to suit, so there are finally a lot of those tarry residues at the end of the process of orbit determination, each with a different taste (read trajectory). It’s the Navigators’ job to assess the flavors of all these and decide which one meets the reality of a successful Plutonian encounter, then deliver that nugget of information—a predicted trajectory—to the other teams of the mission so they can act on it.

So there it is in a nutshell.

There is no more. Today.

 

P–33 days: Navigating Pluto

Here’s the second of a series of posts I hope to write from now until P day. Ideally, you should read them chronologically, but alas, that’s not the way blog posts post. Maybe someday I’ll reorder them into an immensely long single article, if I write many more. That shouldn’t be too hard, since there can’t possibly be more than 33, right?

 

Position: About 39 million kilometers from Pluto.
Velocity: 13.8 kilometers per second toward that destination as of this red-hot moment (red-hot moment being a technical term for right now, but don’t let this engineering-speak throw you; it’s a simple concept.)

Right now is 2015/06/12 00:18 GMT, which is June 11th at 8:18 pm for those of you on the east coast, 5:18 pm for the west coasters, and sometime in between for those somewhere in the middle (unless I’m writing this tomorrow instead of today, but then … never mind). Thirty-nine million kilometers is about 24 million miles for the metrically challenged, and 13.8 kilometers/second is a zippy 8.6 miles per second using the tried and true rule-of-thumb conversion factor of 1.6. I’m too lazy and time-challenged to look up the umpteen-decimal-digit conversion, so that’ll have to do.

Whatever the numbers, the arrival time—the big moment—the instant the rubber meets the road—is at the Pluto closest approach time, July 14th, 7:50 am Eastern if all goes as planned. At that red-hot moment, New Horizons, the-little-spacecraft-that-could, will pass about 14 thousand kilometers from the center of Pluto, give or take a tad (another technical term), a lonely 4.8 billion kilometers from home.

Distances in miles and kilometers are pretty meaningless when the numbers are this big, so let’s boil them down by expressing them in terms of the distance from the Sun to the Earth. That’ll be in terms of AU, meaning Astronomical Units. At the moment, we (New Horizons) are 31.7 AU from home, and only 0.3 AU from Pluto. Only one percent left to go. There, isn’t that better?

At this distance, it takes four-and-a-half hours to send a message up to the spacecraft, and another four-and-a-half hours to get the reply back. Not a particularly robust mode of communication if you’re exchanging jokes or having an argument. Even worse if you’re trying to drive a spacecraft going 12 times faster than a speedy bullet. There’s no room in a laboratory somewhere with a big steering wheel and blinking lights in front of an Enterprise style view screen with the spacecraft spring-loaded to make instant changes, like a race car responding to the driver’s twitch of the wrist. No, it’s all done by computer and planned days, months, and sometimes even years in advance.

Yesterday we discussed a maneuver. I use the term we loosely, because basically I listened in on a mission telephone conference between mission engineers and scientists to decide whether or not to do a trajectory correction maneuver this Sunday. I say “listened” and not “participated” because I don’t know enough yet to participate without making a jackass of myself. It’s been nine years since I paid much attention to this mission, being wrapped up in another mission to Mercury—that’s a lot of miles traveled and water under the bridge. Catching up to what the Navigators are doing and how they do it is pretty much like drinking a river: there are so many technical perplexities evolved since launch, it takes weeks to get the brain around them (if ever).

It was a lively discussion on whether to correct the accumulated trajectory errors this Sunday or defer them to a later scheduled time after they’ve grown a bit more—the pros, the cons, and in-betweens—the yeas and nays and furthermores. Reminded me a little of Congress, but without the rancor. After all, we are civilized and rational.

Considering whether to do the maneuver or not, being an old airplane pilot (with de-emphasis on old) I’m philosophically inclined to use an airplane metaphor. When you’re on a long final approach to touchdown, say seven miles out, do you maintain your altitude for as long as you can and then dive for the runway? No! You enter the glide slope and make as many corrections as needed on the way down. Waiting for the inevitable inevitably means you save up all your mistakes for the end, and wind up over-correcting or under-correcting and getting into a PIO, technically known as a Pilot Induced Oscillation to us pilots, quite possibly leading to a PIC, otherwise known as a Pilot Induced Crash.

So much for metaphors. But it’s apt, I think, because I’m inclined to never give up the opportunity of correcting a small known error at this red-hot moment in exchange for a larger error later on, during such a critical final approach to a runway we’ll not have another opportunity to land on in this lifetime.

Anyhow, the maneuver, named TCM16B2 (Trajectory Correction Maneuver umpty-ump) will be performed in the wee hours of Sunday morning, just after midnight. The size is pretty small as these things go, just under two feet per second, slower than you can walk, but it makes up about 760 kilometers error at the target point, and about 84 seconds at the target time.

Beyond this maneuver, there are more opportunities to correct further developing errors at P–20, P–14, and P–10 days. After that … the end. Not enough time to plan and execute more maneuvers. The best that can be done is to continue taking tracking data right up to the last day and use the continually improving knowledge of where we are (and where we’re going) to uplink tweaks to the spacecraft: where to point the cameras and other instruments—and when to snap the pictures.

Will we be successful and take pictures of real things on and about Pluto, or will we get back the equivalent of a thumb over the camera lens: a lot of high-definition pictures of empty space? It will all become clear in 33 days.

PlutoDog2