[time-nuts] moon bounce for synchronization
jimlux at earthlink.net
Sun Jan 31 01:21:03 UTC 2016
On 1/30/16 4:27 PM, Hal Murray wrote:
> eb4apl at gmail.com said:
>> Back to the Control Room you contact the transmitting station (I think it
>> was DSS12) by voice to insure that they have the station manned and
>> transmitting, and began to operate the "thing". The transmission were
>> specific for each receiving station, because all the complex processing was
>> done at the transmission end: the transmitting equipment accounted for the
>> instantaneous round trip distance between the transmitter and the receiver
>> via the moon and continuously adjusted the modulating code "early" in order
>> to to be received on time. The equipment also introduced a one microsecond
>> shift each second. The receiver had a correlator whose output went to an HP
>> strip chart recorder which draw the correlator output in one channel and a
>> PPS with a minute mark in the second.
> What sort of frequency accuracy were you after?
> Did somebody have to correct for the Doppler due to the rotation of the Earth?
I'm pretty sure they did. Ignacio can say for sure.
We precompensate on transmit to get the signal to arrive at the
spacecraft at its "best lock frequency". Or, actually slightly off, then
we ramp through the BLF so the receiver can acquire the carrier. The
receiver bandwidth might be as narrow as a few 10s of Hz, so you don't
want to be too far off or ramp too fast.
On S-band (2-2.3GHz), the earth rotation Doppler is about 2-3kHz.
(0.5km/sec), depending on latitude and where the Moon is in the sky.
If you're doing the Moon, you put its motion via a SPK file into the
algorithm. In 1970, that would have been a bit more tedious<grin>.
With the moon, it depends on what part of the moon you're aiming at, too.
Last year (March 3rd), I was doing an experiment bouncing a DSN signal
from Goldstone off the moon and receiving it at JPL. As I recall, the
"spot" on the moon was about 800km in diameter. For what it's worth, you
don't need a particularly good receiver when your transmitter is 20
kilowatts into a 34m antenna (DSS 24).
Since our transmit and receive site were pretty close together, the
Doppler of earth motion wasn't much, but there was Lunar Doppler, on top
of that, there's libration.
Chuck Counselman did a bunch of work with VLBI type techniques using
various ground stations around the world to determine where things (like
the lunar rover) were on the surface of the moon.
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