Tag Archives: trajectory

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

Epoch State: What’s it all about?

author

Epoch state is spacecraft-navigation-speak for the beginning, the thing that kicks everything off, specifically a trajectory. It’s the position and velocity that initializes the problem Where are we and where are we going? The spacecraft trajectory propagates downstream from there, subject to all the gravitational and non-gravitational accelerations acting on the spacecraft—such as the pull of gravity from the sun and all the planets as well as the firing of thrusters that maintain its attitude. There’s always a fair amount of uncertainty in the trajectory because you never know the epoch state perfectly and there are always errors in your model of the accelerations. Think of a trajectory as an animal path through the jungle: It’s a bit wide and fuzzy because you don’t know it exactly, but if you wait long enough you’ll see an animal come along, by-and-by.

Well, that’s a little bit like what this initial post is supposed to do: kick off a trajectory of blogs subject to the forces of reality and non-reality (i.e., imagination) as they impinge on my consciousness, as well as dollops of whimsy bubbling up from the subconscious and elsewhere (e.g., from you). And like errors in a spacecraft trajectory that grow and grow after you depart the epoch state, I expect these blogs to also wander considerably. They’ll start off oriented towards my last novel, The Darkest Side of Saturn, but after a while you can expect to see them drift farther and farther from that fuzzy animal path.

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Voyager at Neptune

This OpEd ran in the Sunday L. A. Times, 1989/08/06

Neptune! Almost 3 billion miles from earth. It’s a cold, impersonal place, a speck of a planet through even the largest telescope, but an enormous gas giant full of mysterious wonders when seen up close.

In a few more weeks we’re going to experience it up close, you and I. We will see it through the eyes of a solar diplomat: a traveler, explorer, adventurer, and representative of the human race; a large metal, plastic, silicon representative named Voyager.

When we arrive, what should we expect? The unexpected! That’s the lesson learned from previous visits to unexplored planets. Are there rings at Neptune like those at Jupiter, Saturn and Uranus? Possibly, but in incomplete arcs, unlike rings anywhere else. Are there other satellites, besides the already known Triton and Nereid? Yes! One has already been discovered, and there’s tantalizing evidence of more—perhaps many more, maybe even clouds of them — lurking just beneath the fuzz in the images painting the screens of video terminals at the Jet Propulsion Laboratory. In just days, now, they could rise to visibility above the electronic background noise.

Neptune has its own internal heat source, and radiates more energy than it receives from the sun. Why? Is this what drives its turbulent atmosphere, generating the large spots already found? Soon, we may know.

And Triton? We’ll find whether it has an atmosphere, and whether we can see through to the surface. Will there be pools of liquid nitrogen, or will gasses be frozen in slabs littering a desolate landscape of craters and mountains?

Among the answers to the questions we know to ask will be more questions we’ve not imagined—the unexpected!

When Voyager arrives at Neptune—on August 25th—it will be the first time since creation that anything human-made has been to that planet. We should enjoy, appreciate, and celebrate the event, since it will also likely be the last time it will happen during our lives.

That’s because a very special arrangement of the solar system, one that occurs only about every 175 years, was required to allow Voyager to make the trip in “only” twelve years. It had to go by Jupiter first, making a hard left turn in that planet’s gravity to pick up energy in a crack-the-whip fashion to go on to the next planet. That was Saturn, which turned it left again, gave it more energy, and pointed it towards Uranus. At Uranus, in 1986, it picked up still more energy and made course for Neptune. Since then it has been “cruising” at ten miles per second toward the planet. At Neptune it will skim over the north pole three thousand miles above the atmosphere, turn downward so that it’s headed south out of the solar system, and make a final encounter with Neptune’s largest satellite, Triton, before beginning a larger interstellar voyage.

The science at Neptune is important, but think also about the voyage, the adventure. Knowledge is good for the human mind, but travel is food for the psyche. And Voyager’s travels have been and will be prodigious. It has been on its way from earth since 1977, wending a crooked path through the outer solar system. Now, in a handful of days, it makes its final rendezvous, a close brush and embrace with Neptune, before flying out of the solar system to begin an odyssey through the Milky Way galaxy; an unattended, lonely voyage that may last from millions to billions of years.

Towards the beginning of that longer journey, a mere few hundred thousand years in the future, our sun will have become a faint, uninteresting star in Voyager’s eternally night sky. But no one will be with the spacecraft to appreciate that fact, and our ambassador will slowly tumble—sightless, senseless, and alone —in an immensely empty void.

A fellow engineer on the navigation team claims that the spacecraft will be on display in the Smithsonian Museum 200 years from now. He thinks that by then we’ll have both the technology and wherewithal to go out, find and catch Voyager, and bring it back. I’d like to think we would be able to do that, but if I’m still around I’ll vote to leave it alone. There’s something wonderful about the thought that a piece of ourselves is somewhere out there on a winding journey between the stars on its way to eternity. It’s like having immortal children.

So this is an adventure, and we’re all on board. The solar system is our playground, and after that—the stars! There are hazards ahead—for example, unseen ring particles orbiting Neptune could smack into us, prematurely ending Voyager’s life—but we’ll probably make it through to see the wonders of Neptune and Triton.

Then will begin the grander voyage—the one that requires us to be romantics instead of realists; dreamers rather than schemers: Even though Voyager will go blind and deaf after a few tens of years; even though it will die an electronic death, it will still have the germ of human creativity and daring incorporated into its very structure. It carries two messages—an explicit one in the form of a golden record, and an implicit one stated by its profoundly improbable existence. And both messages will say to the finder, in essence, “I am from the planet earth. I am of the human race. We are small and insignificant, but our souls are large because we have set out on a journey to know the universe.”

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