In an FCC filing, Google has told the US government that it believes its secret airborne network won't interfere with any existing networks and won't harm any people or animals. Google has been hoping to perform a "two-year nationwide test" of the network and recently addressed some concerns people had raised about it.
In the filing, Google only calls the project a "nationwide testing of airborne and terrestrial transmitters in the 71-76 and 81-86 GHz bands (collectively, the E-band)." It wants to keep the project a secret, but all signs point to it being for Project Loon, Google's airborne network of balloons which it has primarily tested in New Zealand. The application is signed by Astro Teller, the head of Google's "X" division, which houses Project Loon.
The "E-Band" that Google says it will use is often deployed as a wireless backhaul option for network providers. Fiber is, of course, preferable, but Fiber is expensive and sometimes—like in the case of Project Loon—you just can't use a wire. Most E-Band applications use highly-directional antennas and are capable of multi-gigabit speeds over a mile or two. Google notes that it will have both terrestrial antennas that "will be pointed upward" along with airborne transmitters. In Project Loon, this would suggest the E-Band would be used for balloon-to-ground and balloon-to-balloon communication.