29OCT08

By Marco Flagg

 

Where is this?

 

 

 

In 1938, Germany dispatched the ship Neuschwabenland to the Queen Maud region of Antarctica. Aboard the vessel were two ‘Dornier Wal’ aircraft equipped with cameras and tasked to survey this portion of the continent then claimed by Germany. These flights collected some 16,000 aerial images, a remarkable feat. Yet, while the expedition had been organized and executed at what surely must have been great expense and effort, the team did not tag the photos with the essential location information! And so, lacking essential latitude and longitude information, this potential treasure trove of images showing the state of a significant portion of Antarctica at the time is still considered useless.

My primary responsibilities on the SCINI team are the underwater positioning and image collection aspects of the robot.  By position tagging all SCINI images, we can use SCINI not only to find ‘what’s there’, but to build maps of the underwater environment and document changes.  Precise positions allow the scientist can confidently re-visit sites either on the same or successive dives or even years. It is this sub-meter level of precision that for example allowed Jeff Miller with NPS, one of our customers in the U.S. Virgin Islands to detect and document the impact of a severe and widespread period of coral bleaching.  It happened after Jeff had studied the site for years and his data base of position tagged reef images of numerous randomly selected sample sites ‘before’ and ‘after’ clearly documented the changes to the reef system (for details, see http://www.nytimes.com/2006/04/04/science/earth/04reef.html).

 

 

 

Preparing Southstar baseline stations for a day of testing

 

But, obtaining precise (< 1m) positions underwater isn’t easy.  Radio waves don’t propagate here and thus GPS cannot be directly used.  And so, high frequency sound waves are used instead.  SCINI’s position is triangulated by first placing ‘baseline stations’ or sonar transducers (acoustic antennas) under the ice, and then measuring the time it takes signals to race between SCINI and each of these baseline stations.

 

The sea ice:  Location of first Southstar navigation tests 

 

While the concept is simple enough, the reality isn’t.  Of course, sonar signals weaken as they propagate from the source – and then need to be distinguished from the noise of creaking ice or seals and in many parts of the world the tiny yet ubiquitous snapping shrimp.  It snaps a claw with extreme violence, momentarily creating a tiny vacuum bubble in the water and generating heat over 1000 degree F (in a very, very small area), then relying on that bubble to collapse onto itself and emitting a crisp snap that stuns nearby pray.  Alas, the snap has energy at all frequencies and sounds like fizzing to divers and sonar systems alike.  Its source was a mystery for many years.  And, it’s not just noise.  Sound waves travel in all directions but then bounce off the sea floor and sea surface.  They create echoes that can easily persist for a hundred times the lengthy of the sonar ping itself while at causing signals to continuously gain strength and then fade again as acoustic paths merge and either cancel or amplify each other.  And just when you thought you were over the hump, you might encounter a total and persistent signal loss across a short path that should clearly work. Congratulations!  You have just become the victim of an unfortunate combination of salinity and temperature layers in the water, refracting all signal energy away from you and creating a shadow zone – the very same type of zones that submariners in the Baltic have famously learned to use for their advantage, hiding undetectable until their pray appears, then pouncing, and quickly retreating to the shadow zone.  

 

Southstar baseline station: A cable connects to the listening device under the ice; the radio antenna relays the signal data to the surface control station.  Precise timing signals from a built-in GPS receiver  synchronizes the clock of all baseline stations to within 0.000001 seconds of each other.  That allows accurate and consistent reporting of signal arrival times at each station. 

 

These experiences can be humbling, and so it should be no surprise that I wasn’t going to take anything for granted and brought two systems.  Our tried-and-true PILOT system which provided SCINI with reasonably although sometimes spotty navigation last year and a new RF-integrated system that combines radio and sonar and GPS technology to at least in theory yield positioning accuracy of better than 1 meter even at SCINI’s deepest exploration depths of 1000 feet.   

 

The Southstar tracking station is really simple:  A PC linked to a radio modem!

It is beyond the scope of this entry to go into much detail, but this much became quickly clear:  Antarctica does hold its challenges.  Some are mundane but nonetheless exhausting – the drilling of holes just to reach the water below.  Others can be hard to pin down – and once found you may realize that danger was lurking all along.  Not only did a lack of earth ground at first introduce a tremendous amount of noise into the sonar system, but we also found ourselves re-wiring SCINI after experiencing electrocution, although mild, and finally composed a rough model of what was happening.  Subsequent conversations in the galley revealed more:  Grounding issues were a major hazard at the South Pose Station where communication systems often fail, and the same issue sometimes caused warning lights to activate in landing aircraft here at McMurdo.    

Navigation plot:  The black trace is SCINI’s path.  The green square is SCINI’s current location.  The grid is 10m x 10m.   This plot of a portion of tech dive #10 demonstrated sub-meter positioning precision for SCINI at a depth of about 100 feet.  It was obtained using the PILOT navigation system. 

And, so I close with this:  The navigation system tests are still in progress, optimizations are still being made.  But, the outlook is good.   We have achieved reliable high-precision navigation on a test dive, but so far only in shallow water of about 100 feet.  Southstar is coming along, and it promises to extend the precision capability to 1000 feet for upcoming SCINI dives.  And yes, ‘navigation monsters’ may still lurking under the ice – but we are prepared to tackle them.

 

 

A reward for a week of hard work:  Hiking up observation hill with a few buddies and enjoying the Antarctic heat.