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.)
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