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foxtrot_xray

CORS Stations?

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Hey guys.. I know alot of ya'll have helped explain other types of marks and such, but I did a quick search, and didn't find any real info on these.. An example of two at a site are: DH3582 and DH3583.

 

I've been to the site, but didn't take pictures. I'll run back there and grab a shot. There's an antenna on the side of a building, and it appears as if it's connected to another antenna-looking thing on a pedestal just nearby.

 

So, my question is, what are these - in simple terms :laughing: - and what is the 'L1' antenna?

 

Cheers!

Mike.

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A Continuously Operating Reference Station (CORS). In simple terms, a CORS continuously records its location based on the GPS satellites. Since the true position of the antenna is know with great accuracy, the difference between the true position and the continously recorded positions detetermined from the satellites is a basis for post-processing GPS observations made by others, includings surveyors using static GPS, in the nearby area. The OPUS website (Online Positioning User Service) is the NGS application for post processing static GPS signals to improve their accuracy using data from the CORS. GPS has two frequency bands, L1 and L2. L1 and L2 are used in combination for higher accuracy in surveying, military applications etc.

 

I have 2 CORS within a mile or two of where I live. The one located at the Arkansas Highway Department facility is a 20 foot tower with an antenna on top, next to a building inside a chain-link fence. Not much to see.

Edited by tosborn

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Mike,

 

CORS refers to Continuously Operating Reference Stations. The NGS has extensive informatio about these very specilized facilities. (In particular, the one you are interested in, in North Georgia, is known as GABG.)

 

Pictures of one CORS installation are here. Others may look different, and rooftop locations seem more common.

 

This topic comes up from time to time, and you might run a search in the Benchmark and NGS forums for more info.

 

-ArtMan-

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Okay, thanks for the info, guys. One last question, if I may.. Many of the CORS stations I read about online have just one PID. How come this one, the GABG (I learned this is 'Georgia Ball Ground') has two?

 

Cheers!

Mike.

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Mike,

If you look at both datasheets you will see that the designations are different. DH3583 is "CHEROKEE CORS L1 PHASE CENTER" and DH3582 is "CHEROKEE CORS ARP". The coordinates of the two stations are also minutely different.

 

I think the answer to your question is in the previously referenced doc GPS Antenna Calibration but when I tried to read it my head exploded. My guess is that there are phase changes to the antennas that have to be dealt with by calibration and that is what DH3583 is, as opposed to DH3582 is the non-calibrated reading. But I could be totally wrong there. Since there are two PIDs that means that both of them are correct, but that begs the question of "correct for what"? What would cause one to be used instead of the other?

 

GPS-savvy surveyors will hopefully speak up and let us know what these things mean in english.

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The L refers to L-band, a radar/radio communications term for frequencies in the 0.4 - 1.6 GHz range (400-1600 MHz).

 

Amplifying upon what tosborn said, and quoting Wikipedia, "The Global Positioning System carriers are in the L band, centered at 1176.45 MHz (L5), 1227.60 MHz (L2), 1381.05 MHz (L3), and 1575.42 MHz (L1) frequencies."

 

The Wikipedia article is a very good overview of the GPS system, its history, limitations and future changes.

 

I work in the field of radio astronomy, and important frequencies in this range (that are starting to get significant interference) are the hydrogen hyperfine transition at 1420 MHz and two OH molecular transition lines at 1665 MHz and 1667 MHz. I'm processing some Very Large Array (VLA) link OH data at the moment, which brought about this ramble.

Edited by BuckBrooke

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The Phase Center of an antenna is the absolute point of the antenna that is being measured to. It is a physical position. It is necessary to know the location of the phase center, usually located relative to the base of the antenna mount, to calculate a proper height of the GPS rod or structure. Surveyors call this base height or rod height.

 

- Kewaneh

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Personally, as a fairly typical (if sometimes a little knowledgeable) amateur benchmark hunter, I have ignored the CORS stations in my area. I see no point in logging them even on GC.com, much less on NGS. The locations are sure well documented. Anyone who needs to know where they are can check on them, realtime on the web if they are so inclined. Just my opinion.

 

Interesting links, though I still want to figure out what a Ball Ground is..... I think.... Thanks.

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The name, BTW, came from the Indians, there used to be a nearby "Ball Playing Field". In case anyone wanted to know. :laughing:

 

So one of the PIDs is the actual antenna, I understand that.. the other PID is... what then? (Again, there are two antenna looking things, I'll have pictures of both this weekend.) One antenna looking thing is like all the others, mounted on a metal building, disk-like in shape. The other is about the size of a brick, square, mounted about 5 feet off the ground. They APPEAR to be connected to each other, and are about 20 feet apart laterally.

 

I think I understand the usage of these (the GABG site was installed and is maintained by egps.net and I went to their site and read up on it) - I guess what I'm asking now is why the two PIDs instead of just one? :laughing:

 

Mike.

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fyi

 

A GPS geodetic solution for a baseline provides the vector between the phase centers of the antennas at either end of the baseline. However, a real antenna does not have a single well-defined phase center. Instead, the location of the phase center is a function of the direction from which the antenna receives a signal. If this variation is ignored, the measured baseline will be between the average phase centers of the two antennas. These average phase center locations are a weighted average of all the individual phase centers for each of the measurements included in the solution. When the antennas at opposite ends of relatively short baselines are identical, these variations should cancel out and no effect is seen. However, different antenna types exhibit different phase variations and baselines with different antenna types will show increasing sensitivity to such things as elevation cutoff angle and the distribution of observations within a solution.

In addition, the phase center is not a physical point that can be accessed with a tape measure by a user who needs to know the connection between a GPS solution and a monument embedded in the ground. However, this kind of connection must be known if a site is ever to be occupied by different antenna types and continuity of positioning is expected. This requires that the vector between the phase center and an external antenna reference point (ARP) on the antenna be known.

 

Antenna Calibration

Edited by Z15

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Buckbrooke,

 

Yeah, we all know that astronomy stuff. No need to drill us on it. My VLA is picking up Pluto, now downgraded from "planet" to "fun sized planet" by the marketing department of NASA. I pick it up on the hodzigene fosdex spectrum, of course, which is being degraded by the use of flamispan phosphate, used in the manufacture of shaving cream. dadgum global warming! (just kidding of course, but I have to admit I didn't understand a word of what you said, even though I caught the gist of it. Any retribution will be countered with a barrage of computerse however).

 

Klemmer--you can't log CORS stations. They don't appear on the NGS site for recoveries. I tried! LOL

 

Z15, the document you referenced was the same as the one I did. My head exploded again, thank you.

 

The question is why there are two PIDs--is one "right" and the other "wrong"? Are they both there to show that correct adjustments have been made? If I am a surveyor, which do I use?

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The question is why there are two PIDs--is one "right" and the other "wrong"? Are they both there to show that correct adjustments have been made? If I am a surveyor, which do I use?

 

Perhaps it would help to consider the L1 PID# as the adjusted horizontal control point and the ARP PID# as the reference mark if the antenna ever needs replacing.

 

John

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The question is why there are two PIDs--is one "right" and the other "wrong"? Are they both there to show that correct adjustments have been made? If I am a surveyor, which do I use?

 

It’s my guess that in calibrating the antennas one PID is the station’s antenna that receives the satellite signal. The other PID is the location of the antenna under test. The two antennas can not be at the same location so there is a need for two PIDs.

 

If that’s not the answer then how about this one: The electrical location of the antenna is dependent on the phase of the signal. If you make a big change in frequency the phase location changes. So one PID is for one frequency the other PID is for a different frequency.

 

I’m betting on the first one.

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<snip>

"Ideally, most of this phase center variation depends on satellite elevation.

Azimuthal effects are only introduced by the local environment around each individual antenna site. These phase center variations affect the antenna offsets that are needed to connect GPS measurements to physical monuments."

 

If I understand this correctly.

The angles of the satts change with respect to the center of the antenna.

 

A slight offset is used or another signal to calculate the difference in the coordinates.

 

As I understand it.

Differences in GPS antenna heights used are calibrated at the point.

 

"GPS users will often find they are using different antenna types within a single baseline as well as within a given network. The use of different antenna types demands that the contribution of the antennas themselves to the geodetic solution be examined."

 

b9e6876f-e3e8-4ed1-961d-eddc89e8b4a7.jpg

 

It goes on to say more of the jest.

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The question is why there are two PIDs--is one "right" and the other "wrong"? Are they both there to show that correct adjustments have been made? If I am a surveyor, which do I use?

 

We use the ARP data. The ARP data sheet is entered into our GPS Baseline processing software along with the Rinex observation and navigation files for one or more CORS stations, for the amount of time that we have been running our bases. These vectors process out and give us good NAD83(CORS) data for any of our stations that we have run long enough to process the data against.

 

As mentioned earlier the phase center is Not a fixed position, it varies for each receiver antenna.

 

CallawayMT

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My VLA is picking up Pluto, now downgraded from "planet" to "fun sized planet" by the marketing department of NASA.

For the record — and in my other life I'm a science journalist and covered this story — the new definition of a planet was made by the International Astronomical Union on August 24 in Prague. Under the new definition, Pluto is a "dwarf planet." Confusingly, dwarf planets are not a subset of planets but are a separate category of object. If you're interested, my report on the decision is here.

 

NASA had nothing to do with this except possibly in a very tangential way that there may be some NASA scientists who are members of the IAU who voted for this resolution.

 

-ArtMan-

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