A view of Wright Valley, an Antarctic Dry Valley. The laser mapping team visited the valley in person before mapping it from the air. Click to Enlarge .

Shooting the Ground

January 1, 2002

NASA scientist Bill Krabill has been watching the Greenland icecap with a laser altimeter for a decade. By carefully measuring the height of the ice surface to within the width of a human hand (10 cm) he found that over 50 cubic kilometers of ice is vanishing each year. That is enough ice to cover the city of San Francisco to a depth 5 times the height of the towers on the Golden Gate Bridge (a kilometer deep), enough melted ice to raise sea level by the width of two human hairs (0.13 mm). Why is the ice melting? We don't know, but before the measurements by Bill and his ATM, Airborne Topographic Mapping, team we didn't even know that it was melting.

The Antarctic Ice Cap is immensely larger than Greenland's. Is it shrinking, growing, or staying the same? the ATM team is down here to find out. The funny thing is that to find out what's happening to the ice they are measuring the ice free regions known as the Dry Valleys. Antarctica is such a vast continent they could never map it by aircraft, so NASA is sending up a satellite, ICEsat with a laser altimeter to map the continent. ICEsat will calibrate itself by measuring the unchanging topography of the Dry Valleys and then move on to measure changes in the ice.

	Loading the instruments of modern physics into an aircraft using an ancient tool, the inclined plane. Click to enlarge .

The ATM team loaded their laser altimeter onto a Twin Otter aircraft. The laser altimeter, or LIDAR, instrument is like the ones that State police use to catch speeders on the highway, this one shoots foot-long green laser pulses at the ground and times how long it takes the scattered pulse to return to the aircraft. The round trip travel time gives the distance from the aircraft to the ground. (In a similar way if you clap your hands in the mountains you can time the return of the sound echo and find out how far you are from a cliff, 500 feet for every round-trip second) To find out where the ground is, Bill needs to find out where the aircraft is to within a few centimeters! This seemingly impossible job is accomplished with the aid of the Global Positioning Satellites, GPS. A reciever on the aircraft uses the travel time of radio signals from several satellites to figure out its exact location on and above the earth. And finally, the team needs to know which way the beam is pointed, down, forward or to the side, for this they use an inertial navigation unit to sense the orientation of the aircraft. (You have a biological inertial navigation unit in your inner ear which helps you sense which way is down and whether you have changed rotation rate. NASA uses inertial navigation units in spacecraft.) These electronic senses are fed into a computer which calculates the height of the ground. Each second the system makes 5,000 precise measurements, then the computer builds the best topographic map ever made of the land beneath the airplane.

	An all Terrain vehicle sports a GPS receiver antenna as it maps Lake Thomas. Click to Enlarge .

One amazing thing is that we have already mapped Mars by laser altimeter and are only now getting around to our own planet.

The ATM team is setting a fine example of how to do science, they process their data as they take it. This lets them catch any problems and fix them without wasting time. This means that the team has to ride in the planes with the instruments, and the plane ride is a wild one. The pilot has to constantly push the rudder pedals all the way to the floor to keep the plane precisely on course to make the map. The team members report that the views of the Dry Valleys are worth the wild ride.

The ATM team is in an interesting middle position. The ICEsat will use their measurements to calibrate its laser altimeter, while the team itself must acquire ground truth measurements to calibrate their system. To do this they use a flat-surfaced, frozen lake in the Dry Valleys, Lake Thomas. They surveyed Lake Thomas using a GPS system mounted on an All Terrain Vehicle, ATV.

So from bumping across the ice in an ATV, to being thrown around in an aircraft shooting the ground with a laser, to sitting in an office and analyzing data from a satellite, scientists are trying to find out exactly what is happening to the Antarctic Ice sheet. Trying to add this one important piece to the giant jigsaw puzzle that is the earth's climate.

field notes