Both of our flight opprotunities to Siple Dome were cancelled. The first one was scheduled for Friday. The second one for Saturday. There are no flights on Sunday and ANITA would be at Siple Dome by Monday. So we cancelled any and all trips to Siple Dome this season.
It's sad, but okay, because the flight path was headed right for WAIS Divide and predicted to only get to 400 km from Siple. So we focused on calling Ben and Nan at WAIS Divide to give them instructions on the ground pulsing.
Our ground stations are simple. They consist of a high-voltage pulse generator, a GPS receiver, and an antenna. The pulse generator makes a signal similar to what we would expect from a neutrino or a cosmic ray, a bright spark that lasts less than a billionth of a second. The GPS receiver syncs the pulses so that when the clock on the payload turns over to the next second, so does the lclock on the ground station. The antenna is just like the ANITA antennas: an antenna in the shape of a horn that amplifies the signal in the direction it's pointed. We can adjust the strength of the signal by attenuating the signal and by ``moving'' ANITA closer to the station. A weaker signal corresponds to a lower energy or more distant neutrino.
When I call Nan and Ben at WAIS Divide, I give them the coordinates (latitude, longitude, altitude) of the payload. Given that we know the exact coordinates of the pulsing station, we can use that to figure out where to point the antenna. We called and repointed the antenna that once an hour for the entire time that ANITA was close enough to WAIS to register pulses from the ground station.
How far away is that? Well, ANITA flies at 37 km above the ice, which gives a horizon distance of 690 km above sea level. If your antenna is up on a glacier 1800 m above sea leve like it is at WAIS divide, then you should see out further On the way out, we placed bets on what the furthest distance was that we recorded pulses from WAIS. It was 714 km. I lost.
We were in range of the ground station for 2.5 days. We were pulsing twice a second continuously during that time, so I estimated that we recorded 147600 events on the on-board hard disks assuming that we only triggered on about half of them. We did several studies with weaker and stronger events, so that's a reasonable estimate.
At one point the payload flew within 70 km of WAIS divide. Nan and Ben could point the antenna by eye at that point. Keep in mind that the balloon inflates to the size of a football field at float, so it's pretty easy to spot.
In the end, ANITA ended up flying over Siple Dome. The station manager called to tell us that he saw it. But it's okay, because we got such great calibration data at WAIS Divide, that we won't need any more.