I'm not sure if this question belongs here or in Amateur Radio, but during the acquisition of signal from New Horizons on New Year's Day, I heard the controllers call out "carrier lock", "symbol lock", "bit lock" and "bit sync" - what exactly does this mean?
First you lock on to energy at (or near) the expected frequency. That’s carrier lock.
Then you start to look for patterns in how the phase changes. The transmitter is coding groups of bits as phase-change “symbols”, and you want to find the time-pattern of those: symbol lock.
But those are not yet bits because the coding works in blocks of bits. Once you find the edges of those blocks, you can decode the bits in them: bit lock.
Finally, you work through those bits to figure out which means what in the data stream, synchronizing your view of the bits meaning with the transmitter’s: bit synch.
There’s more on the New Horizons hardware that does this here.