On January 22 Google filed an experimental radio application at the FCC. The company has requested confidential treatment of the application, so significant portions aren’t publicly available.
As part of the filing, Google filed a request for confidentiality, which is public. It contains a few technical details. Two separate transmitter types are identified, both operating at low power in the range 76-77 GHz, and using FM and BPSK modulation. The 76-77 GHz band is used for short-range vehicular radar and, knowing Google’s interest in vehicles, it’s reasonable to assume that is what the experiment involves.