Do the Voyager space probes perform time synchronization regularly?

Question

According to JPL: http://voyager.jpl.nasa.gov/mission/weekly-reports/

Voyager 1 travels at a velocity relative to the Sun of 19.03 km/s (or 0.000063478 c, or a time difference of ~ 0.0005 seconds per year), and Voyager 2 travels at 15.428 km/s.

According to the factsheet, Voyagers do not have a time synchronization system on board, atomic clock or something like that.

Do they need to synchronize the time with the earth? Or do deep space exploration probes need this in general?

If so, how do we do it?

Answer 1

According to JPL's Voyager FAQ:

There is no clock chip, as such, in the spacecraft. The "clock" is really a counter, based on one of several electronically generated frequencies. These frequencies, based on a reference, generated by a very stable oscillator, are converted and fed to different locations in the spacecraft as synchronization signals, timers, counters, etc. The "clock" signal is part of the information telemetered to the ground and it is with ground software that we convert to day of year, time of day Greenwich Mean Time.

This means that any data it still collects (it wouldn't be much, according to this answer) is accompanied by values of Voyager's internal counter, and this counter data later converted to time here on the good old Earth (where, I imagine, any such time difference due to Voyager's velocity relative to Sun will be accounted for, whenever necessary). I also can not think of a single instrument onboard Voyager that would require clock synchronisation with some timekeeper on Earth, and they can easily rely on the internal clocks, relative to which Voyager makes observations.

To put it differently, if Voyager needs to take measurements of something every 60 seconds, it will inspect its internal counter and take another measurement after its internal counter says 60 seconds have passed, not when those 60 seconds would have passed for us on Earth (which might take an extremely tiny amount longer).

Answer 2

I'll only give a small overview if this:

Or do deep space exploration probes need this in general?

Inherently, they don't - but often they do perform time synchronization, or clock re-calibration.

The clock on spacecrafts is a counter, often maintained in software, derived from a hardware oscillator. An absolute(1) (earth) time can be derived from that counter when we know when it started and the period of which it is incremented. So time on the spacecraft is a relative time, just a counter from when it was started. (This is conceptually the same as how time is kept on a normal PC).

Now, all oscillators drift, the counter wraps around, and there's other problems with this kind of timekeeping, but carrying atomic clock for timekeeping is normally infeasible on spacecraft.

However, as long as one knows the value of the onboard clock (the counter), when it was started, how much that clock has drifted, all data is available to calculate the time.

Calculating the absolute (earth) time can be done by ground systems derived from the relative timestamps of data from the spacecraft clock to earth time. Commands sent to the spacecraft can be translated the other way, from the absolute time used on earth to the future relative time used on the spacecraft.

Ofcourse, the translating to/from earth time to the spacecraft time can be done onboard the spacecraft as well instead of on ground systems - in such cases periodical time synchronization is needed so correct time can be maintained. Correct time in this case can conceptually just be to transmit the current absolute time to the spacecraft, and have it maintain the difference between that time and its onboard counter, so it can just add an offset from the onboard clock to get the absolute time.

Whether absolute time is kept on the spacecraft or the ground system depends on the spacecraft and mission - often both are done in order to ease and prevent errors in timekeeping from coming up. In either case telemetry from the spacecraft about the onboard clock, its value, whether it has been reset, overflowed, and other parameters are very important.

Some references:

• Spacecraft Clock Maintenance for MESSENGER
• The Effects of Clock Drift on the Mars Exploration Rovers
• Basics of space flight chapter 11 in particular
• Core Flight Executive source code and accompanying documents

(1) Ofcourse, the "absolute" time we use is really also a relative time from an epoch and according to some more or less arbitrary calendar.