Given that service is over the air, what kind of security is used to prevent man-in-the-middle and other attacks?

Sail uses advanced encryption technology for all wirelessly transmitted data that can potentially be intercepted. 

For securing its microwave point to multipoint wireless links, Sail is using the Wi-Fi Protected Access 2 (WPA2) Enterprise protocol. This protocol uses a strong encryption method called AES-CCMP to encrypt data transmitted over the air. Point to multipoint wireless links operating in mmWave bands are encrypted using 128-bit AES. 

PtP wireless links by Sail operating in the microwave band use 128-bit AES encryption. PtP wireless links operating in the mmWave bands require a very clear line-of-sight and use very-high-gain antennas, which result in very narrow beam-widths (3 degrees or less). Signals sent over such links are practically impossible to intercept because any attempt to introduce a receiving antenna to intercept the signal would disrupt the PtP link itself.