[time-nuts] Fundamental limits on performance
Lux, Jim (337C)
james.p.lux at jpl.nasa.gov
Sun Sep 13 17:54:44 UTC 2009
On 9/13/09 10:20 AM, "Magnus Danielson" <magnus at rubidium.dyndns.org> wrote:
> Out of curiosity, what kind of requirements do you guys have? What kind
> of environmental aspects is there? What kind of physical limits would be
The actual environment is often fairly benign (it's in a vacuum in space
after all).. Temperature fluctuations might be 10C or around there.
However, you have to survive launch loads: vibration in the 10grms area;
pyroshock for deployments, etc.
We also test over a very wide temperature range.. -30 to +75 wouldn't be
Power is usually some flavor of 28VDC.
After that it's a matter of negotiating power, mass, thermal loads, etc.
For comparison, a typical flight radio might be 2-3 kg and draw 20-40W,
exclusive of a power amplifier for transmit.
Radiation requirements vary with the mission.. A typical total ionizing dose
requirement might be 25-50kRad (except if you go somewhere like Jupiter,
where MegaRad doses might be the order of the day).. You also need to be
immune (or mitigate) single event effects (SEE) from high energy particles.
A typical requirement is no destructive latchup (SEL) for 75 MeV mg/cm^2
Linear Energy Transfer. There's also Single Event Functional Interrupt
(SEFI), Single Event Gate Rupture (SEGR), Single Event Upset (SEU), etc.
>> So, if I have an orbiter with a good clock around Mars, and I want to
>> discipline a bunch of cheaper oscillators on the surface (or perhaps
>> orbiters), what is the fundamental limit on how good you can do, given the
>> radio link available from orbiter to ground station.
> That depends. You can afford doing bi-directional ranging, as you have
> fairly low amount of space and mars surface nodes. The benefit would be
> that the surface nodes has high stability in position but not as stable
> in longterm, while the space nodes can provide frequency stability.
> Pseudo-ranging aids in orbit tracking and the relative position of the
> surface nodes can be established. Additional space nodes can use the
> resulting pseudolite-satelite constellation for tracking of orbit and
> landing position.
That's the sort of idea.. Consider it as a ensemble system.. But at some
point, the link information capacity becomes the limit on performance.
> I suspect that the weak atmosphere of mars does not call for as advanced
> correction of delays as here on earth, but that could become a research
> field in itself by using two or three frequency rangings.
You bet.. Occultations are a big deal in the radio science world.
> The key to GPS ability to handle low signal strength is the synchronous
> modulation and ranging codes that helps to decorrelate noise.
The same is done with sequential tone ranging on deep space probes.
Ultimately, it's all about carrier phase.
> The surface nodes would benefit from using smaller rubidiums for
> long-term stability.
For some reason, we don't fly Rb sources... (GPS did, but deep space
doesn't.... Very high quality XOs are what we do for those sorts of
JHU APL makes USOs, as does Oscilloquartz in Switzerland.
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