On 11/3/13 4:27 AM, Magnus Danielson wrote:
Hi Jim,
Ground-based GPS-like transmitters could be a nice option to consider.
Well, that's basically what we do today. We transmit a very stable
carrier with some tones or a PN sequence modulated onto it. The
spacecraft recovers it, generates a return signal locked to it, which we
get on earth and analyze to determine the range and range rate (or if
we've got two stations, we can do some cross range with Delta DOR or VLBI).
This is known as "two way" ranging.
Having a onboard high quality source means we can do "one way" ranging,
so we can range without an uplink, or conceivably, the spacecraft can
figure where it is by looking only at the uplink.
A high quality onboard source also can be used to provide navigation
signals from an orbiter to a lander that's not visible from earth 12
hours a day, there's no direct to earth link to something on the surface
of Mars. An even bigger deal to something like the far side of the
moon, although various and sundry schemes have been proposed (a relay
satellite at L2, for instance)
Would also provide a frequency reference for steering/compensation of
that oscillator. Either that avoids deep space ranging, or it is only
needed for verification.
Onboard, so far not many people need precise time and frequency. It's
almost exclusively for navigation, and low rate communications (if
you're receiving 8 bps at -160dBm, the phase noise of your LO has to be
pretty good).
One of the hopes of the DSAC (Deep Space Atomic Clock) is that with
"easy" high performance, people will come up with science measurements
that depend on it. Interferometry among a constellation of satellites
is one.
We do a lot of gravity science by measuring the orbits of spacecraft (it
is, after all, how we know the earth is pear shaped). More recently,
GRACE and GRAIL used two spacecraft in the same orbit with very accurate
ranging between them to accurately map the gravitational field of the
Earth and Moon, respectively. Once you know the gravity, you can infer
the internal structure: is it layered, or uniform, or lumpy, etc.
A small very quiet clock would make putting a constellation of little
spacecraft around something like Europa possible.
When the entire spacecraft is a liter or two, you can't burn another
liter on a USO, and even 100cc is a big chunk of volume.
I thought the important part of the mission was to keep stable phase and
frequency? :D
We in the space telecom business KNOW that the whole reason we launch
satellites with science instruments is to provide data other than random
PRBS for our telecom system, so that we have something interesting to
look at. Solar panels exist to provide power for our telecom system. The
attitude control is to properly orient our antennas.
Cheers,
Magnus
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