We were not measuring frequency but clock offset. The frequency offset
was estimated from the daily clock measurements.
If I remember correctly we got .1 uS accuracy.
We had several Rubidium, Cesium and later a H-maser frequency standards
and we kept continuous recordings (strip charts) of the phase differences.
The transmitter precompensation kept care of combined Doppler (Moon
motion and Earth rotation, which is the main component). We operated at
X band, the transmitting antenna had 85 ft or maybe less, I don't know
exactly and the transmitter probably had 20 Kw.
By contrast the receiving antenna was small, I found at a picture
(http://www.douglasvanbossuyt.com/2009/07/19/goldstone-dsn-complex-tour/) of
one that looks the same, at least the dish and pedestal are almost
identical.
I forgot to say that this system was abandoned in favor of VLBI which
gave more accuracy and a lot more useful info. Both systems had the
drawback that the monitoring was not continuous and needed a lot of
resources at both ends. By contrast Loran-C and GPS used existing
infrastructure that was continuously available. BTW, I don't know how
Loran-C stations were synchronized, this came granted by the Navy.
We also used once a traveling clock, it was an HP cesium, the one with a
Patek-Philipe clock on the front. The crew who operated it told us funny
stories about the clock traveling on commercial jet seats and afraid
passengers asking about this bomb looking device and being told that it
was some kind of "atomic" thing.
Regards,
Ignacio
On 31/01/2016 a las 2:21, jimlux wrote:
On 1/30/16 4:27 PM, Hal Murray wrote:
[email protected] 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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