[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.
More information about the time-nuts
mailing list