[time-nuts] Water on Enceladus - What does this imply about NASA'a ability to measure frequency?
jimlux at earthlink.net
Fri Apr 4 09:32:55 EDT 2014
On 4/3/14 11:17 PM, Chuck Forsberg WA7KGX wrote:
> One needs to know the carrier frequency. Must be a high quality reference
> for the Cassini transmitter.
Two way measurements are most likely here (although Cassini does carry a
USO). So the downlink is locked to the uplink which comes from a maser.
The design of the transmit and receive system to hold that performance
all the way through the chain is a challenge.
Interestingly, since the light time to Cassini is hours and hours, you
need a *really* good clock on the ground, because you're comparing the
phase of a signal you radiated several hours ago with the phase of the
signal you are receiving now.
To put this in perspective.. at a range of a bit more than billion km
(one way) the light time is a bit less than 5000 seconds,or 10,000
seconds round trip. So there are 4.2E13 cycles of the down link signal
between Saturn and Earth. The Allan Deviation is on the order of 1E-15,
so we are typically measuring the phase of that signal to a few degrees.
So, things like the physical temperature of the 70 meter DSN antenna
make a difference. If the optical path is, say, 100 meters, and the
temperature of the dish changes 1 degree C, what's the phase change?
Well, steel has a CTE of about 13ppm/degree, and 13 ppm of 100 meters is
about 1mm, which is about 10 degrees phase shift at 8.4 GHz (lambda = 36mm)
Solid Earth tides also feature into this.
I will point out that *measuring* the performance of the radio on the
ground in the lab is quite a chore. You can easily see the air
conditioner cycling on and off (that bump in Adev at 1000 seconds) and
diurnal cycles in temperature. most of this affects things like the
cables and connectors in the test setup.
And woe to the rookie engineer who thinks they can make the measurements
with any old signal generator from loan pool.
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