[time-nuts] Fundamental limits on performance
magnus at rubidium.dyndns.org
Sun Sep 13 20:33:47 UTC 2009
Lux, Jim (337C) wrote:
> On 9/13/09 10:20 AM, "Magnus Danielson" <magnus at rubidium.dyndns.org> wrote:
>> Out of curiosity, what kind of requirements do you guys have? What kind
>> of environmental aspects is there? What kind of physical limits would be
> The actual environment is often fairly benign (it's in a vacuum in space
> after all).. Temperature fluctuations might be 10C or around there.
Temperature fluctuations can be a downer. Just look at the lunar
exclipse season effect on GPS satellites and how that rendered refined
atom standard control. Naturally the rubidiums was more affected than
> However, you have to survive launch loads: vibration in the 10grms area;
> pyroshock for deployments, etc.
Naturally. There should be more shaker tables around... I have seen so
many faults occuring on gear where vibration is never really discussed,
but the actual gear needs to survive it never the less... and it is not
launches that I am thinking about.
> We also test over a very wide temperature range.. -30 to +75 wouldn't be
A bit out of normal "commercial" temperatures.
> Power is usually some flavor of 28VDC.
> After that it's a matter of negotiating power, mass, thermal loads, etc.
Which is what I expect for any space mission.
> For comparison, a typical flight radio might be 2-3 kg and draw 20-40W,
> exclusive of a power amplifier for transmit.
Typical volume? Half a liter?
> Radiation requirements vary with the mission.. A typical total ionizing dose
> requirement might be 25-50kRad (except if you go somewhere like Jupiter,
> where MegaRad doses might be the order of the day).. You also need to be
> immune (or mitigate) single event effects (SEE) from high energy particles.
> A typical requirement is no destructive latchup (SEL) for 75 MeV mg/cm^2
> Linear Energy Transfer. There's also Single Event Functional Interrupt
> (SEFI), Single Event Gate Rupture (SEGR), Single Event Upset (SEU), etc.
I assume there are a bunch of strategies to handle those.
>> 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.
>> I suspect that the weak atmosphere of mars does not call for as advanced
>> correction of delays as here on earth, but that could become a research
>> field in itself by using two or three frequency rangings.
> You bet.. Occultations are a big deal in the radio science world.
Well then, that is settled then. :)
An alternative to the continous dual frequency approaches of GPS and
GLONASS is to time-share them. While the RF section becomes slightly
more complex, you save the weight and power.
>> The key to GPS ability to handle low signal strength is the synchronous
>> modulation and ranging codes that helps to decorrelate noise.
> The same is done with sequential tone ranging on deep space probes.
> Ultimately, it's all about carrier phase.
Exactly... and long observation times with predictable signal.
Sequential tone ranging may fit better for deep space while multiple
parallel tones (i.e. code) fit better for intra planetary positioning.
>> The surface nodes would benefit from using smaller rubidiums for
>> long-term stability.
> For some reason, we don't fly Rb sources... (GPS did, but deep space
> doesn't.... Very high quality XOs are what we do for those sorts of
> JHU APL makes USOs, as does Oscilloquartz in Switzerland.
JHU APL wasn't on my map, but they do not show up on the normal
commerical map. Oscilloquartz is... do you use both?
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