Anyone know of a Paper or Article somewhere that spells out (for a 60+ old transit/chain Surveyor) how to incorporate CORS DATA into a LOCUS Static survey, A station just became close enough for me to use, and I would like to avail myself of it. Tnaks in advance. JS

Could you be a bit more specific? The NOAA CORS website is pretty user-friendly. In Solutions, you just select the CORS file along with your Locus raw data, then process it.

I am not an expert, but I can give you a rough outline.

1) Take a look at your session and write down the day and time it started and the duration of the session in hours as well as what time zone this data is in UTC or local or daylight, etc.

2) log into http://www.ngs.noaa.gov and select the CORS option under the banner logo at the top.

3) find the 4 character moniker for the station near you from the map, but do not go to the station data itself, or if you do back out to the cors page.

4) Select the button which says UFCORS (user friendly cors) and you will be presented with a form to fill out the session data you collected in 1).. then hit the button to submit that data and move on to the next form

5) The next form in UFCORS has some additional options to set as follows:

a) the station name per 2) (if it doesn't show then there is no data from that station concurrent with your session and you're out of luck)

b) Data interval (I suggest set to 10 seconds, but you can probably leave "As is" also)

c) Say you want the data sheet also.

d) Say no to IGS orbits I think that is precise empheris and AS doesn't use them so it isn't useful to download. Also they are often not available as soon as the other data anyway)

e) Compression Suggest you set to PKZIP some of the other formats are real hard to find extractors for.

f) Hit submit button and wait for the few minutes it takes to compile your data. When it is ready you will be presented with your browsers save file dialog box, and you can save the zip file to your sessions project directory.

6) Extract the files (you should be able to find an unzipper if you don't have one.)

7) Among the files will be NGS data sheets (.DS) and another sheet with coordinates (.POS). You can probably use either one. But I open write or wordpad with the .POS extension and go down to the NAD83 coordinate set, [[!!**Be really sure not to use the ITRF values.**!!]] Print it out or keep a window open in your editor

8) In your session in AS go to control and enter the stations ID and position data from the data sheet. The elevation should be the L1 phase center and it is going to be ellipsoid height. This is the easiest to do from the .POS file rather than mess with station height, and then try to figure out the antenna height and phase center height above that using the .DS data. In essence they are giving you the actual elevation of the L1 phase center in the .POS file so your antenna height will thus be zero (0) when you enter it below. Choose fix horizontal or hor/vert if you aren't holding other benchmarks for vertical.

9) Go to files pulldown and import GPS data from a file and you should see files that came out of the archive in the list in your project directory in the form Bname001.DAY where name is the name of your cors and day is the julian day number 1-366

10) On the observations tab you set antenna height to zero and vertical and antenna unknown.

11) You should now be able to process and get vectors to any station with obs data simultaneous to it.

You may get an AS warning telling you it is going to only be able to do an L1 processing since you don't have hardware lock. Just click through that.

12) My preference is to click on the vector and set elev mask to 15 degrees since it is L1 only.

Evaluate, adjust, etc.

jerry
Modified By jerry wahl on 9/29/2001 at 11:19 AM

Why set the antenna to "unknown"? The logfile for the station contains the antenna type.

As I said I am not an expert, but here was my logic.

1) the type of antenna doesn't matter if you get the coordinates of the L1 phase center, which is what you get.

2) AOS With AOS I got headaches trying to get all the heights right, i.e. height of station, height of antenna reference point, offset to L1 phase center and if you already have the data to the phase center why bother. (sort of same answer as 1)

Am I missing something?

Jerry Wahl wrote:

Am I missing something?

Maybe, maybe not. The fact is, I'm not sophisticated enough to provide a definitive answer.

I do know that an antenna's phase center isn't a single well-defined point, but "moves" with respect to signal direction. Whether or not one's software correctly applies the antenna model is another question I cannot answer, nor can I state with confidence that it really makes that much of a difference in the baseline solutions. However, I figure the more error sources that might be eliminated, the better, so I always specify the antenna type when possible.

GPS Antenna Calibration at the National Geodetic Survey provides a good introduction to phase center matters.

In any case I would need to correct my outline to specify use of the L1 phase center values from the .POS file since the ones I am looking at have first the values for the ARP (antenna reference point) and second the values for the L1 phase center.

As far as I can see the info contained in an antenna definition is the height above the antenna reference point of the L1 and L2 phase centers and the radius of the antenna for reduction of slope HI measurement. I guess they assume the L1 phase center is fixed or average as I doubt there is any capability of the software to model shifts.

When using dual frequency equipment you might want to use both relative to that ARP position. There is no need to worry about where the ground is under the station.

My initial experiences with CORS left me quite confused as there seemed to be conflicting data on the antenna parameters at the cors sites. Perhaps with UFCORS the data is more clear.

- jlw

PS one reason I think they use the choke ring antenna's is to minimize the horizontal shifting problems but I don't have a source for that supposition at hand.
Modified By jerry wahl on 9/29/2001 at 7:17 PM

I like Jerry Wahl's step-by-step. If you are working with the Locus you may as well just use the L1 phase center and set antenna height to zero and antenna to unknown for that CORS.

If you want to mix and match receivers then you want to create (or select) an antenna type based on the information in the files and the data sheets.

I have found one case where the data file listed a different antenna from the data sheet. An email to the NGS got me a fast answer that somebody had forgotten to edit the data sheet when they changed out an antenna. Hey, the NGS is people too. The people who run those receivers are probably pulling their pants on the same way.

Check out http://www.ngs.noaa.gov/CORS/information4/ for a little light reading on the subject of the CORS.

Read the information from the NGS carefully!!! How you use the data can make a difference in the results you get from the data processing.

There are lots of documents on the NGS web page about the CORS. It is worth reading that stuff if you want good answers.

Try it out on something where you already know the answer. That is the best way to sort out your procedures.

Thanks much Jerry, your response was just what I needed, Jim