[time-nuts] Fundamental limits on performance

Lux, Jim (337C) james.p.lux at jpl.nasa.gov
Sat Sep 19 15:46:09 UTC 2009

On 9/19/09 8:36 AM, "Magnus Danielson" <magnus at rubidium.dyndns.org> wrote:

> Lux, Jim (337C) wrote:
>> On 9/13/09 3:54 PM, "Magnus Danielson" <magnus at rubidium.dyndns.org> wrote:
>>> Jim,
>> As in GPS, for instance, where chip rate is related to carrier frequency.
> Exactly. Very strict integer locked.
> Just realized that the gravity potential detuning needs to be adapted to
> the mars orbit and gravity field. Which also makes an interesting
> side-project to develop gravity maps for mars.
> Is there an established coordinate system of suitable precission for
> mars? Essentially the equalent of WGS 84.

I would assume so.  I seem to recall a definition of a Mars sphereoid, etc.,
in Bulletin 74 from USGS.

Gravity maps in general are important for spacecraft navigators, so there's
probably also good knowledge of that.

>>> Anyway, the questions you are asking have been covered before. It should
>>> come as no suprise that Dr. Simon was (is?) with JPL.
>> Marv Simon is still at JPL..
> As expected. You just don't know what people do today.
> If rolling back to the original question, how would you like the
> fundamental limit to be expressed? There are several parameters relating
> to the mission which can vary. I think orbit solution, transmitt power
> of suitable band(s) and signal structure interconnect. As secondary
> issues is long term stability of frequency sources and stability of
> orbit prediction and hence stability of position. Receiver noise can be
> fairly well estimated.

More along the lines of position knowledge than stability.  I see some sort
of optimal combiner that can build an ensemble estimate.. What goes into the
estimate is a number of "frequency" observables of some sort, with
uncertainties built up from:
-Underlying uncertainty of the source
-Uncertainty due to non-infinite SNR of link
-Uncertainty in length of propagation path (e.g. Relative position
uncertainty, and doppler effects)

> Assuming a decent IMU, receiver aiding can be used. But the IMU aspect
> should not be very new to you guys. :)

Most spacecraft have very small accelerations/jerk.  We use IMUs for things
like entry descent and landing of rovers, but for spacecraft nav, it's all
about things like star trackers and measuring range and range rate.

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