[time-nuts] Spacecraft Timekeeping

Magnus Danielson magnus at rubidium.dyndns.org
Tue Mar 8 21:45:33 UTC 2011


Kevin,

On 03/08/2011 06:57 PM, Kevin Watson 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.

First thing to consider is that standard GPSes will not meet your needs, 
since they have to obey the height and speed limits for export rules.

The side-effect is that doppler frequencies may be much higher and both 
tracking and acquisition needs to include these more extremer doppler 
frequencies.

Use of PTP within a rocket or spacecraft may or may not be a good thing. 
NMEA + PPS may suffice and be less power-hungry. IRIG may also be an option.

If timing and frequency needs strict correlation with ground, relativity 
comes into play due to altitude alone shifts frequency.

Cheers,
Magnus




More information about the Time-nuts_lists.febo.com mailing list