[time-nuts] Fundamental limits on performance

Magnus Danielson magnus at rubidium.dyndns.org
Sun Sep 13 17:30:59 UTC 2009

Lux, Jim (337C) wrote:
> On 9/13/09 5:02 AM, "Magnus Danielson" <magnus at rubidium.dyndns.org> wrote:
>> I doubt it. Looks like another antenna and set of receiver electronics.
>> Unless someone does something smart, it looks like it can be ruled out
>> from power and weigth constraints. Pulsars are there thought and if
>> several can be observed their beat frequency would be fairly low such
>> that phase-relationships can act like a positioning in space and time.
>> It may be good to have redundancy such that gravitational potential can
>> be resolved separately.
>> Space-ship to space-ship or earth station needs to rely on the link
>> between them. Synchronous carrier helps alot and stable clocks in each
>> end require less dynamics for steering.
> The way we do this now is to lock the local reference oscillator (on the
> spacecraft) to the uplink signal, which is derived from a Hydrogen Maser.
> But that requires that the uplink be there, and there's really only about
> half a dozen places in the world that can radiate the signal, and there are
> multiple other consumers of the resource.

Which seems quite reasonable for your normal deep space probes, but for 
a Mars Positional System (MPS) it needs to be fairly autonomous from 
earth. However, you should consider that your normal approach may very 
well be used for routine calibration of long-term drifts. You may 
actually survive on rubidiums for long-term drift. The hold-over 
effectively becomes a constellation (sats and surface nodes) averaging, 
which could use the constellation control algorithms as already 
developed for timing labs and TAI/UTC coordination.

> Starting with a good quality clock helps (after all, no amount of fancy
> control system can stiffen a wet noodle)

Certainly. But if they become too heavy artifacts to sling-shot to a 
neighbouring planet...


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