What are the key takeaways from “GPS Hidden Messages - Computerphile” on Computerphile?

What is this episode about?

GPS satellites transmit more than just location data; they broadcast mysterious, encrypted messages at the discretion of military command. Traffic analysis of these signals reveals a shift in 2011 that correlates with the implementation of over-the-air rekeying, suggesting these hidden messages serve as the mechanism for distributing military encryption keys globally.