[bean-weevil]

The GPS project began in 1973. Digital signatures were invented in 1976. I'm guessing adding it on after the fact would have been very expensive.

[toomuchtodo]

There have been progressive improvements as new satellites are brought into service, and those end of life retired. Perhaps backwards compatibility?

https://en.wikipedia.org/wiki/GPS_signals#Legacy_GPS_signals

https://en.wikipedia.org/wiki/GPS_signals#Modernization_and_...