I haul a large box antenna, a laptop, and a software refined radio (the receiver) over the bumpy ice surface, trudging off to a radar transmitter stationed about a kilometer south of camp. I jump over melt streams and weave my way through the maze of crevasses. Eventually I spot a small black flag on a bamboo pole waving in the distance and I gradually make my way in that direction. Interestingly, black is actually the easiest color to spot against the stark white surroundings.
When I arrive, I open up the box containing the ApRES radar – a phase sensitive, low power, light and compact instrument. From the outside, it’s just a yellow waterproof box that resembles an oversize briefcase. Inside, there’s a circuit board and a few cables to connect antennas, a laptop, and a power supply.
Working with my teammate from Stanford, I set up our transect. We turn on the radar transmitter, sending electromagnetic waves into the ice. As we walk away carrying the antenna and receiver, we hope that it will be able to detect the signal after the wave has bounced off the bed of the ice sheet.
These invisible electromagnetic waves are very powerful. While we can’t see or hear anything, these waves are propagating down through the ice. When they reach the transition between ice and bed rock, they reflect and travel back up through the ice to the surface. By recording how much time has elapsed while the electromagnetic wave travels down to the bed and back up to the receiver, we can calculate the exact thickness of the ice in the particular location. In this case, the ice-bed interface is over 1 km (0.6 miles) below us.
We can also use this electromagnetic signal to recover properties of the ice column, like what the temperature of the ice is and how it varies spatially. This is important information for understanding the evolution and stability of outlet glaciers and how they may contribute to sea level rise.