What's Your Position?
Zodiac, carrying tether, approaches from the Tully.
by Peter Tyson
July 4, 1998
The first piece of news I heard today, Independence Day, was bad, but after
that it was smooth sailing. Literally. The wind and waves subsided
overnight, and the sun came out, making for a stellar day of science and
engineering. Seeing as it's a holiday, I'll leave the bad news for last.
Today was all about navigation, and getting closer to the goal of
retrieving a black smoker chimney or three from the bottom of the ocean. For
those of you who have been closely following this expedition, have you
wondered how the researchers here find their way to the clump of sulfide
chimneys they hope to study and sample? I mean, we're talking 200 miles out
to sea, and then straight down to the seafloor the length of 24 football
fields placed end to end. To complicate matters, there are waves and wind
and currents. And the place they're trying to get to with the tethered robot
and assorted cages lies at the bottom of a steep-sided canyon. Yet these
guys manage to place a "package," as they call any piece of equipment they
send down on a line, within ten meters of where they'd like to have it—and that's without the "eyes" of ROPOS. How do they do it?
With a state-of-the-art system that includes—are you ready for this?—the Global Positioning System, a technique known as dynamic positioning, and
a long baseline acoustic navigation system. The first two, GPS and DP, are
used to position the ship and the third is used to position the package or
to navigate ROPOS. This afternoon I climbed four or five flights of
stairs—the Thompson's bridge is 46 feet above the deck—to see the
phalanx of computers that do the truly stunning task of determining the
ship's exact location (using GPS) and maintaining it (using DP), all in
real-time, all within a few tens of feet.
The GPS unit works by locking onto up to nine satellites simultaneously
and, based on the time difference between its clock and those on each of the
satellites, calculating its precise location. Then DP takes over. To
maintain the ship's position and heading (supplied by GPS and a gyro
compass, respectively), the computers enlist the help of the big bow and
stern thrusters. These engines are powerful. The second mate, a gray-bearded
man named Eric Schoenberg, told me he was on the Thompson when, as a test,
it did a full stop while going 17 knots; the ship was at a standstill one
ship-length later. By being able to push water in any direction, the
thrusters can also turn the ship "on a dime," Schoenberg said.
While I was on the bridge, Schoenberg got a call from the ROPOS room. They
wanted him to move the ship about 30 feet. Half a minute after Schoenberg
punched in the new coordinates, he laughed and said, "And we're there. Did
you hold on tight?" I hadn't felt a thing.
To position a package or navigate ROPOS on the seabed takes one more
step—a long baseline hydroacoustic navigation system. Believe me, it's as
daunting as it sounds. "Long baseline" refers to the distance between
acoustic transponders used in the system. In our case, there are four
transponders suspended 300 to 600 feet off the seafloor about a mile from
the Mothra field. They form an acoustic net that enables "hydroacoustic
navigation"—that is, using the speed of sound underwater to navigate.
For those of you who go in for spherical trigonometry, what happens is that
the cage in which ROPOS sits (also known as the "garage") broadcasts a
signal to the four transponders at nine kilohertz. Each transponder returns
the signal at a different frequency, so the garage's computer can
differentiate between them. The garage then broadcasts a signal at 14
kilohertz to ROPOS, which in turn sends its own signal at nine kilohertz to
the transponders. Finally, the latter transmit back to the garage. The
process takes 10 seconds, but the result is a very precise position for the
garage and ROPOS that is continuously sent up the umbilical to the surface.
If it sounds easy, it isn't. For one thing, the concentration of salt and
heat from chimney plumes changes as one rises through the water column, and
that alters the speed of sound the navigators rely on.
Lifting the tether from the zodiac to the Thompson.
Such minute positioning of the ship, the package, and ROPOS underlay our
successes today. First we placed the line baskets on the seafloor. Each of
these steel cages carries an 8,000-foot line, which will be used to haul the
smokers to the surface. From their maps of the Mothra black smoker field,
John Delaney and
Le Olson knew they wanted to drop the baskets 20 meters
west and south, respectively, of Phang, the first black smoker they plan to
retrieve. Using the expedition's tripartite positioning, they were able to
do that, as was confirmed later during a visit by ROPOS.
The positioning also facilitated the first successful ROPOS dive in days.
Among other accomplishments, ROPOS prepared Phang for removal: placing the
metal frame over the chimney, cinching down the frame's wire cables tightly
around the structure, and cutting it most of the way through with the
modified underwater chainsaw. Tomorrow Delaney's team will try to lift the
chimney free of its subterranean home. Will we shortly see our first black
smoker on the deck of the Tully?
If our luck keeps up the way it did today, then perhaps. The final triumph
of the day was transferring a new ROPOS tether from the Tully, which arrived
back on-site this morning from its good-samaritan mission. The Thompson sent
its zodiac over to pick up the tether, a 1,000-foot line wound around a
wooden drum. As I watched the zodiac slowly cross the half-mile or so
between the two ships, I wondered why it was taking so long. Only after the
Thompson's crew managed to wrestle the thing on board did I learn that the
tether weighed 1,200 pounds.
For the 30 members of the scientific party, this Fourth of July has been a
cause for celebration—all save for the bad news. When I sat down to
breakfast in the mess this morning, my eye caught a terse message scrawled
in black ink on the bulletin board. It concerned the fishing vessel whose
mayday we answered two days ago: "FYI, the F/V Ocean Roamer sank on way to
port. ALL HANDS SAFE." Turns out the Coast Guard had battled all night to
stop the leak, but the sea had won in the end. The boat sank about noon
Peter Tyson is Online Producer of NOVA.
The Tug of the Thompson (June 23)
The ROPOS Guys (June 25)
In the Juan de Fuca Strait (June 27)
Special Report: A Visit To Atlantis (June 29)
Dive 440 (July 1)
Rescue at Sea (July 2)
What's Your Position? (July 4)
Phang! (July 5)
20,000 Pounds of Tension (July 8)
Four for Four (July 11)
Thrown Overboard (July 13)
Was Grandma a Hyperthermophile? (July 15)
Swing of the Yo-Yo (July 18)
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