[time-nuts] Spacecraft Timekeeping

jimlux jimlux at earthlink.net
Wed Mar 9 14:02:03 UTC 2011


On 3/8/11 11:41 AM, Chris Albertson wrote:
> On Tue, Mar 8, 2011 at 9:57 AM, Kevin Watson<time-nuts at enuuf.com>  wrote:
>> Hi Jim,
>>
>> As part of my research into keeping time on rockets and spacecraft, I joined
>> this list to see what I could learn from the masters. Of course I'm a
>> knuckle-head for not assuming that you'd be one of the resident masters
>> <grin>. Anyway, as my accuracy needs are modest (~10uS across many onboard
>> computers), have access to GPS most of the time and don't really need to
>> worry about relativistic effects (yet, anyway<grin>) or radiation effects
>> (due to redundancy), I thought I'd use a GPSDO that can handle a decent
>> amount of holdover and then use PTP to distribute time across the vehicle.
>> Do you, or anyone else, have a recomendation for the GPSDO? Jackson Labs'
>> (http://jackson-labs.com/) DROR seems like it might work, but I wonder if
>> there might be better alternatives.
>
> Off hand I'd worry a little about vibration.  How do crystals work
> when being shaken with huge amount of mechanical and acoustic energy
> during launch?

Oh, I'm sure that's just a matter of ordering the right oscillator, or 
packaging.  People launch optical payloads that can only tolerate a few 
Gs, so that's doable.  Unless you're doing something like mounting the 
oscillator on the engine <grin>...

The oscillator in your wristwatch or in a manpack radio probably sees a 
tougher environment, vibration wise, than most spacecraft gear.

There's already crystal controlled radios on rockets for things like 
range safety and telemetry, and I don't think they do anything 
particularly exotic.  On the other hand, they also probably don't try to 
do timing to nanoseconds, either.



>
> Cooling.  Almost all commercial off the shelf gear depends on air and
> has a maximum altitude at which it will operate.  Off gassing might be
> a problem too if there is flux left on the PCB or even fingerprint oil
> leftover from assembly.

yes and no.  Cooling of most leaded components is through the leads to 
the PC board. Cleaning takes care of outgassing, etc.   The Mars 
Pathfinder rovers used essentially off the shelf commercial radio modems 
that were cleaned by hand.



>
> PTP is a new and not so mature technology so you will need to
> characterize it on your own and likey port it so the specialized
> processors you use your self

Well.. it's "new" compared to IRIG and NTP, but it has been around a 
while. However, given that the 1588 spec has had a couple significant 
revs, I suspect that "interoperability" might be dicey.

But the basic concept is pretty straightforward, and if you're 
implementing the ethernet PHY so you can get the hooks to the timing 
info, then you're in good shape.

>
>




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