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.

  • $\begingroup$ speaking of bits and blocks, any thoughts on what is or isn't considered "spread-spectrum" Have deep-space spacecraft always used some form of spread-spectrum for data downlink? $\endgroup$ – uhoh Jan 2 at 7:47
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    $\begingroup$ Thank you - this clears things up. But I'm still curious as to what constitutes the "lock". Is the mere detection of a carrier on the expected frequency enough to call it "lock"? I suppose bits pours out of the decoder regardless, so at what point can you declare bit lock? $\endgroup$ – OZ1SEJ Jan 2 at 10:07
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    $\begingroup$ @OZ1SEJ Many radio receivers use some variant on a phase-locked loop - en.wikipedia.org/wiki/Phase-locked_loop - which use a feedback mechanism to synchronise the broadcast radio signal to a signal generated within the receiver. A PLL has a lock when that synchronisation has been achieved. $\endgroup$ – mjt Jan 2 at 11:32
  • $\begingroup$ Something to keep in mind here--the signal coming back from New Horizons is extremely weak, it takes all of NASA's technological wizardry to pluck it out of the noise. It's not like the radio in your car where you can simply turn the dial and clearly hear the stations. $\endgroup$ – Loren Pechtel Jan 3 at 3:49
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    $\begingroup$ Spread spectrum in the form of hopping the carrier around by a bunch is good for reducing interference (which only occupies some of the frequency slots), but does nothing for noise (which is equally annoying across the band). I don't know of a hopping deep-space link. Spreading the frequency out via modulation can be helpful, and is (usually? often?) done in poor SNR links. That's what the "symbolization" does: Use as much of the available amplitude, frequency and phase variation as possible to spread those symbols characterizations out and make them distinctive over the noise. $\endgroup$ – Bob Jacobsen Jan 3 at 8:00

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