[time-nuts] The 5MHz Sweet Spot

[Jim Lux]

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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