On 3/8/11 11:41 AM, Chris Albertson wrote:
On Tue, Mar 8, 2011 at 9:57 AM, Kevin Watson<time-n...@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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