On 5/28/14, 2:11 PM, Tom Holmes wrote:
Thanks Jim.
So if, just for fun since this is time-nuts after all, I wanted to make a
similar measurement in my back yard here in the relatively stable Ohio,
would I be able rig something up to monitor the position changes? Obviously
a lot of averaging of GPS position data would be needed but I'm not sure my
Z3801 or any of my navigation receivers have the necessary resolution to see
even 10 mm.
T
Sure it can.. you don't do it by averaging lat/lons, though...
What you need is to program your receiver to give you the raw
observables (someone on the list can point you to how you do this), and
then you need to get them in what's called RINEX file format. You
submit your RINEX formatted files to JPL's web based GIPSY processing
site (wait at least a week or two after you collect your data, so they
have the precise GPS ephemeris figured out), and get your geodetic data
back.
As I recall, given your position, they have post hoc ionospheric
calculations from recordings made at high precision receivers near you
which they can use to compensate your measurements, etc.
I'm oversimplifying a lot of the above process out of ignorance, but
basically that's how it's done.
I have been told that it's pretty easy to see things like solid earth
tides with a fairly simple system. Mind you "fairly simple" to these
guys might mean a high quality choke ring on a geodetic pedestal with a
precision invar rod driven into the bedrock within a steel protective
casing. but it could also mean a helibowl antenna with the helix wound
on a plastic solo cup stuck inside an aluminum kitchen stove burner liner.
One of the longest continuously running geodetic receiver stations is at
JPL and is basically a D&M choke ring sitting on the ground on the edge
of a hill. (this is referred to as the Aries Site*, for some reason)
http://goo.gl/maps/mXTcK
in the lower right corner of the dirt area south of the parking lot
area, you'll see a dark dot in the middle of a lighter round thing:
that's the antenna.
The trailer at the top of the image is where the receiver is, and the
coax is running along the east side of the parking lot.
Contrary to the copyright date at the bottom, this image is at least 2
years old, because they've moved the trailer away, and the receiver is
now in a little building across the street (on the west side of the image).
*It might well be that initial testing of ARIES (a VLBI experiment)
http://ipnpr.jpl.nasa.gov/progress_report2/42-26/26G.PDF
was done up there. There's a couple surveyed concrete pads with steel
mounts and benchmarks. The DSN report was from 1975, so it's
substantially before my time at the lab (as well as most people around),
although there are some familiar names in that paper (Spitzmesser has
dozens of GPS related pubs)
https://ia600505.us.archive.org/3/items/nasa_techdoc_19840071052/19840071052.pdf
has a picture of the smaller 4 meter system (it's more portable than the
9 meter system, and claims better performance because of the liquid
helium cooled maser, portable cesium clock, etc.) I guess "portable" is
relative, especially when you talk to DSN folks who think in terms of 70
meter antennas. (For scale..the building in the background of the
picture is Bldg 180 at JPL, the main admin building, and is 9 stories
tall. A 70 meter DSN antenna is taller than that building)
what's fascinating is that today, you can buy, for less than $20, a low
noise block converter for 12 GHz (Ku-band) with a noise temperature <20K.
But back in the early 70s, "pre-GPS" days, you had to be a serious
time-nut...
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