[time-nuts] Fundamental limits on performance
magnus at rubidium.dyndns.org
Sun Sep 13 22:54:32 UTC 2009
Magnus Danielson wrote:
>>> 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.
> Sure. But consider that we are talking about space-loss for a MMO system
> and assuming a T of half a sidereal day a quick calculation would give
> an orbit radius of about 12869 km. That is about half that of GPS. The
> maximum distance would be about 13309 km. This is the worst-case space
> loss needed to be handled. Selecting a lower orbit would significantly
> lower those numbers.
> The coding gain is of importance. Recall that the GPS uses very short
> C/A code of 1023 chips, achieving about 30 dB of coding gain. The P code
> however achives a much higher coding gain, 127,9 dB. The modern L2C and
> L5 signals use alternative approach to the C/A and P lock-in mechanism.
> The C/A code requires 21 W where as L2 P code only requires 6 W being
> fed to the antenna. You can lower those values by counting in the 3 dB
> gain of lower space loss. The high C/A power is needed since the coding
> gain isn't stellar.
> There are many parameters to play around with, but improved coding gain
> would be one way of getting better performance for a limited wattage.
> If the limitation of "half-space" antennas can be removed for the
> benefit of a directed antenna... considerable antenna gain could be
> made. The downside would be dependence on working mechanics which I
> would assume should be avioded for all kinds of reasons.
The point of this exercise was to compare the systems and see to what
degree a scaled system could very well meet the needs within power
budgets without inventing the wheel from the beginning. I.e. look at
previous experience and see what it says just to see if something
similar would be reasnoble or not.
Looking into "Telecommunication Systems Engineering" by Lindsay and
Simon would be the perfect companion as it discusses Shannonesque
treatment of the problem, includes deep space issues, covers both
sinusoidal and P-N ranging receivers.
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.
Anyway, the questions you are asking have been covered before. It should
come as no suprise that Dr. Simon was (is?) with JPL.
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