On 09/01/2012 05:53 PM, Jim Lux wrote:
On 9/1/12 8:32 AM, Bob Camp wrote:
Hi
Observing a curve and being able to compensate it are often two
different things. Hysteresis is one very obvious example. Another is
simple sensor lag. A some what less obvious one is that the
temperature performance is also influenced by the rate of change in
temperature.
Here's another thing to consider:
If your crystal is running 3 ppm / C, and you are after 3.0 x 10^-11
stability at one second - You will need to either have a rate of
change at ~ 1x10^-5 C/sec (0.6 mC / min) or you will need to
compensate for some pretty small changes. That of course makes a bunch
of assumptions ….
In this application, the requirement for frequency accuracy has to do
with initial acquisition.. that is, you want the signal (or receiver
tuning) to be within some few hundred Hz of where it's expected to be
(because the receiver is narrow band).
The ground station typically has a Doppler predict based on orbit
knowledge, that predict has some uncertainty. Added to the radio
frequency uncertainty. (SNUG - Space Network User Guide, has more info)
Once you've acquired, the receiver and ground station will track (i.e.
the ground station puts in the estimated Doppler, so all you're really
tracking is the variation in the local oscillator). (for a LEO satellite
at 2.3 GHz, the 7km/s orbital velocity already puts tens of kHz
variation on it)
(and this completely neglects that a modern radio could use something
like an FFT for acquisition)
Temperature changes are pretty slow.. I'm seeing 5-10 degree cyclical
variation over 90-100 minutes. Actually, the bigger change is during the
warm up transient, going from off and cold to on and warm over 10
minutes or so.
In other applications, where you're not going in and out of the sun
every revolution (i.e. deep space, rather than LEO) and you were
interested in Allan deviation type measurements for gravity science
(where we're looking for 1E-13 over 100 sec sort of performance), what
we'd probably do is warm up early.. Turn it on, compensate based on the
measured temperature, and then hold the compensation fixed during the
measurement, letting the ground worry about the apparent frequency
change due to Doppler. We'd have a high quality narrow band signal, just
at an unknown (but reasonably stable) frequency. What the science team
is usually interested in is small relative changes in phase &
amplitude(occultations) or in small changes in frequency (Doppler, for
gravity science).
(we regularly measure velocity to cm/sec precision for outer planet
orbiters like Cassini, Juno, etc.)
If you can make reasonable predictions of the heating and cooling
profile, you could use that and hopefully gain a decade or so, and any
slew in detectors and would mostly affect the remaining error.
You could also make use of uplink carrier and GPS to improve the model
state of your crystal. That way the predicted and feed forward values
could be kept fairly well adapted. Naturally, you would want a fall-back
scenario to back out for wider offset search if you failed to maintain
the model. You don't want to loose your bird due to temporary system
failure. Another alternative would be to allow for the downlink
frequency to be a hint for the uplink where the frequency is such that
you can "pull in" the bird if need to.
Cheers,
Magnus
_______________________________________________
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
