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.

 

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