[time-nuts] Fundamental limits on performance
Magnus Danielson
magnus at rubidium.dyndns.org
Sat Sep 19 15:36:21 UTC 2009
Lux, Jim (337C) wrote:
>
>
> On 9/13/09 3:54 PM, "Magnus Danielson" <magnus at rubidium.dyndns.org> wrote:
>
>> Jim,
>>
>> Magnus Danielson wrote:
>>> Jim,
>>
>> One aspect of why synchronous data and carrier may be worth pointing
>> out. If the carrier and modulation is asynchronous, then the carrier
>> tracking and data reception needs to recover their respective clocks
>> independently. However, by transmitting them in a synchronous fashion
>> and making use of this fact at the receiver, then the carrier tracking
>> can aid the code tracking in which case the code tracking only need to
>> retain the phase, which leaves more margin to propperly decoding the
>> message. Thus, a better BER is achieved for the same S/N or for that
>> matter, a worse S/N can be tolerated for the same achived BER compared
>> to the asynchronous modulation technique.
>
>
> 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.
>> 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.
So start from receiver levels, coding gains to suitable correlation
signal strength, then calculate typical receiver path losses. In the
other end, calculate typical space losses from orbit, and expected gain
structure at the transmitter. I think the critical parameters for the
mission would resolve themselfs fairly quickly. MMO gives long
observation times. An alternative would be going back to TIMEX style and
just use doppler observations, but that worked best for navy and land
apps with restricted dynamics (no dropping down from the sky dynamics).
Bi-directional ranging could be used, but it does not really change the
fundamental limits, it just change the mechanisms for orbit predictions
somewhat and convert pseudo-ranges into ranges for improved
time-transfer and orbit prediction before full constellation exists.
With a single node (sat or surface) can bi-directional ranging resolve
time difference completely and place the node in a sphear of known
radius from the node. A second node would reduce the position to be
placed on the intersecting circle between those nodes and a third would
provide the point estimate, which infact is a small space due to
uncertainties. The GPS requires 4 observable nodes to achive the same,
as it's pseudoranges is unidirectional and thus can not by themselfs be
reduced to real ranges. Another aspect is that the range equations
should be much quicker and easier to resolve as the time-error would be
much less than initial for a GPS equalent case. But it doesn't shift the
receiver limits by much.
Assuming a decent IMU, receiver aiding can be used. But the IMU aspect
should not be very new to you guys. :)
Cheers,
Magnus
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