[amsat-bb] Theoretical Question

This isn't a question regarding amateur satellites but it is an interesting theoretical one to pose here and the same laws of physics apply. I know that the higher the frequency that is transmitted from a satellite the larger the Doppler shift will be. The Iridium satellites transmit at around 1.5gHZ. At that frequency I know the Doppler is large. How do they manage to keep the portable handsets on the ground locked to the signal? Are the handsets frequency agile to an extent? How do they achieve frequency lock so that the digital signals are decoded?

Dave Marthouse N2AAM dmarthouse@gmail.com