Thursday, November 8, 2007: Day 9, More Drilling and DGPS
We're getting to the end of our drilling rope, just a few flights left. Bob and I wore out a bit going through some very dirty ice, but made progress by chipping the hole out every few minutes to give the bit something to catch on. I think we could chip through the ice all the way if we wanted to (the ice cracks into chunks pretty easily and our chipper bars are sharp and well designed), but nobody else is enthusiastic about this plan.
If you're curious about other types of drilling in Antarctica, including ultrasonic and laser rigs, you might like this article from space.com.
Because we've predrilled our last holes, I took down our local DGPS (differential global positioning system) base station, I'll give you a quick overview of how that works...
First, what we use the GPS system for is to survey dive holes and "transducer holes". Our goal is to locate old experiments on the sea floor very accurately so we can return without scouting around every year. To do this we first survey in the location of our dive hole, where the ROV goes through the ice, then we use an acoustic positioning system underwater to determine when the ROV is in relation to the dive hole. GPS signals don't penetrate water very deep, otherwise the whole job would be much easier! The acoustic positioning system works actually somewhat similar to the GPS system as a whole: The ROV sends out signals and by comparing the receiving times at three different transducer (aka receiver) stations under the ice we can triangulate to the ROV itself. This triangulation depends on knowing the position of the transducers themselves very accurately: a few centimeters off and our positions could be wrong by several meters.
You might wonder why we don't just flag the spots on the ice over the experiments, and why that wasn't done 40 years ago to help us out now. Out here the sea ice moves significantly every year, and every few years it all breaks up and clears out leaving open water.
Regular GPS (global positioning system) technology uses the difference in time signals from a variety of satellites to triangulate the position of the receiver station (usually a handheld or dashboard mounted device). The whole system relies on knowing the locations of the orbiting satellites very accurately: you only know where you are in relation to the satellites. The American GPS satellites slowly transmit data about their orbit corrections and perturbations (they used to "fudge" this data so positions weren't accurate beyond 5 or 10 meters; only the military could decrypt the full accuracy data). The system works very well by taking into account general relativistic corrections, to an accuracy of 2-3 meters. Technically there's almost no limit on the accuracy of the triangulation, but we don't live in a perfect technical environment: ionospheric storms and fluctuations subtly warp the path of the satellite signals, making the path traveled by the signals longer than the actual distance to the satellites. These fluctuations are usually localized to within 10 or 15 kilometers on the Earth's surface, which means they affect the signal's path in roughly the same way over that area.
If we knew exactly how the fluctuations were affecting the signal path lengths, we could compensate for those differences in software and have extremely accurate positioning (within a centimeter or two, or less than an inch). This is exactly the kind of system the friendly guys at UNAVCO supplied us with: a differential GPS base station which calculates the signal error and transmits corrections to a special handheld receiver.
One DGPS technique is to anchor down a receiver to a specific location and collect positions over a very long time (months, years); over time the signal error will wander around, giving positions up to a meter away from day to day, but by averaging over the whole time we get something must closer to the actual position. Once that position is known, we know that the wanders are error and could transmit corrections. The system at McMurdo station uses this method (along with others) because it is a permanent emplacement; unfortunately, the locations we want to get at New Harbor are far enough away that the corrections aren't valid. There's no permanent base station out in the field, and we don't have time to build up a super accurate position, so we use a presurveyed location.
When erecting the base station we carefully positioned it's receiving antenna over an existing cemented in point that has been used on and off over the years and was originally surveyed in using astronomical techniques. This way the base-station immediately knows exactly where it is and can start transmitting local corrections to our mobile handheld.
The receiver antenna (which is carefully positioned) is connected to the "brains" of the base station in this weatherproof box. The yellow computer does the math to determine signal corrections and sends them to the blue radio transmitter, which is attached to a second transmission antenna (not shown here).
We left the base station running for several days straight (it takes too long to hike out and turn the equipment on and off every time we want to take a position fix over a transducer hole), so we needed a big battery. Snow snuck in through the small crack left by the power cable and filled this box up, which was a surprise for me when packing up.
If you've made it this far you've probably got some questions, go a head and leave them as a comment and I'll try to get back to you!
The last couple days have been pretty warm, and the sun heats up dark objects enough to melt snow... these valleys aren't so dry this year!