Consider a vehicle like the space shuttle. There must have been many subsystems with timed tasks dependent on a common clock. So you could have a single clock feeding time to all the subsystems with timed tasks. But you could also have multiple clocks synchronized with each other or with a main clock.

So I'm wondering: Does a rocket normally have a single common "source" of time? Or does it have multiple sources of time synchronized with each other? If they are synchronized with each other, is there a main clock that serves as the ultimate reference for all other clocks?

Big thanks if you can clarify!

  • 4
    $\begingroup$ We know at least one pretty famous and highly-visible case where different clocks disagreed: Boeing Starliner CST-100 Orbital Flight Test-1. $\endgroup$ May 15 at 19:49
  • $\begingroup$ Oh interesting! What went wrong and why? Any reference to news/articles with more detail? $\endgroup$
    – user39728
    May 15 at 19:51
  • $\begingroup$ There is no one universal answer. That said, the concept of a device (or redundant devices) that send out a one Hertz signal for synchronization is so common that there's an acronym for them: 1PPS device, or one pulse per second device. $\endgroup$ May 15 at 23:38
  • $\begingroup$ Great! Thanks, David! OK, between those two comments I think I know everything I hoped to know. If anyone writes this up as an answer, I'll accept :) $\endgroup$
    – user39728
    May 16 at 0:44

So you could have a single clock feeding time to all the subsystems with timed tasks. But you could also have multiple clocks synchronized with each other or with a main clock.

For shuttle it was kind of both; there was a Master Timing unit, but the onboard computers had their own clocks as well.

Master Timing Unit

The GPC complex requires a stable, accurate time source because its software uses Greenwich mean time (GMT) to schedule processing. Each GPC uses the master timing unit (MTU) to update its internal clock. The MTU provides precise frequency outputs for various timing and synchronization purposes to the GPC complex and many other orbiter subsystems. Its three time accumulators provide GMT and mission elapsed time (MET), which can be updated by external control. The accumulator's timing is in days, hours, minutes, seconds, and milliseconds up to 1 year.

The MTU is a stable, crystal-controlled frequency source that uses two oscillators for redundancy. The signals from one of the two oscillators are passed through signal shapers and frequency drivers to the three GMT/MET accumulators.

The MTU outputs serial digital time data (GMT/MET) on demand to the GPCs through the accumulators. The GPCs use this information for reference time and indirectly for time- tagging GNC and systems management processing. The MTU also provides continuous digital timing outputs to drive the four digital timers in the crew compartment: two mission timers and two event timers. In addition, the MTU provides signals to the PCMMUs, COMSECs, payload signal processor, and FM signal processor, as well as various payloads.

Source: Shuttle Crew Operations Manual p. 2.6-17


  • COMSEC Communication Security
  • FM Frequency Modulation
  • GPC General Purpose Computer
  • MET Mission Elapsed Time
  • PCMMU Pulse Code Master Modulation Unit

Very precise clocks are also very heavy (the best fill entire rooms, and also require specially-built structures to keep them vibration isolated). Therefore, unless you are making a navigation satellite, whose entire purpose is to provide atomic-grade accuracy clock signals to the rest of the world, or you have some extreme redundancy requirements to satisfy, there is not enough benefit to carrying such a thing on a spacecraft, because lifting more weight is very expensive and needs to be allocated to more crucial things. If your orbit lets you get enough signal from it, just carry a GPS or equivalent receiver, and use that as your clock reference. If that doesn't work for some reason, you can keep a very large, very expensive, very precise ensemble of atomic clocks in your satellite control ground station, use that to very finely control the carrier frequency of your command signal, and synch the local oscillators onboard the spacecraft to that uplink.


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