# Problems as a result of clock drift that have occurred in space?

I was reading this question about the clock drift on the Apollo missions and am sure that a drift of 1 second / day on some of the longer missions would likely result in a mission failure due to the nature of orbital mechanics. Is there any mission in our solar system that has entirely failed or had a trajectory deviated from the expected path due to massive (or not so massive) clock drift adding up over time?

For those wondering about clocks, and the drifts that can occur in real-life non-space applications I found a really good resource to explain it in laymans terms. Sort of explains what the every-day objects lose in terms of clock drift such as a wrist watch (20 ppm), it gave me some good scale on the comments.

• 1 second per day, that is 11 ppm. But a carefully selected and temperature compensated XTAL oscillator could be better, about 2 ppm or less. But for very long missions aging of XTALS might be a problem. Instead of a XTAL oscillator an atomic clock may be used. – Uwe Sep 17 '18 at 19:20
• I suspect trajectory measurement errors frequently exceed 10ppm (but I should probably make that a question); midcourse correction maneuvers are a must whether your clock is drifting or not. – Russell Borogove Sep 17 '18 at 19:54
• – Russell Borogove Sep 17 '18 at 20:04

Right now the best sources I can find are:

@MarkAdler's comment below this answer to the question How does Curiosity know how to point and move it's high gain antenna in real time? says

Bottom line: accelerometers give down, Sun gives azimuth. On-board ephemerides of Earth, Mars, and the location of the rover on Mars, all of which are kept up-to-date by the operations team, allow the HGA to be slewed to track Earth in the sky. There is one more critical instrument and calibration required: the current time.

and the collective discussions in various answers and extensive commenting below the question How do launches avoid leap seconds? Why?

Spacecraft have clocks for many reasons. For deep space missions they are important to add time stamps to recorded data and images, and to execute instructions that have previously been sent from the ground.

For say a correction burn or a schedule of images, these instructions are sent ahead of time, and refer to moments in time as defined by the spacecraft's local clock. The ground also carefully monitors errors or drifts in that clock and either makes corrections in the instruction times, or send a delta correction. For different missions and orbits one might be considered better than the other.

For deep space spacecraft, the distances can be measured very accurately using transponders; you send a signal with a long, complex encoded series of pulses, the satellite receives it and rebroadcasts it directly on a different frequency. By measuring the total time delay between the sent and received pulse pattern, you get the round-trip light time. If the returned signal also has encoded on it the spacecraft's clock time, you know how to synchronize it with clocks on the ground, or correct for the difference.

• As an FYI, unless the user has commented @ will not notify or tag them. Thanks for the linked answer about leap seconds, that definitely has some good related discussion. – Magic Octopus Urn Sep 21 '18 at 23:17
• @MagicOctopusUrn oh that explains a lot, thank you! I found more about that in answers to How do comments work? and especially How do comment @replies work? – uhoh Sep 21 '18 at 23:42
• Your dedication to sources, even on meta topics, is commendable haha. Those are interesting reads too, didn't know a few of those tidbits about the SE site. The part about pinging editors is useful. – Magic Octopus Urn Sep 22 '18 at 0:15