I'm a beginning GPS user and need some mentoring so that I don't make some major errors. So far, I anticipate only occasional use and have purchased a Promark2 / Ashtech Solutions system. I know it's simple tech.
I download data from UFCORS for a particular station corresponding to occupations with my GPS units. When I look at the data, I find that the Lat, Long, and Elipsoid/Ortho elevations are considerably different from those listed in the NGS Data Sheet for the station mark or ARP of the CORS antenna. I want to use the CORS station as a fixed control, so I understand I have to enter the NGS data into the fixed station.
1. Why is there such a big difference in the elevation from the Data Sheet vs the raw CORS data? A couple of the stations are more than 10 meters different.
2. There are velocity vectors shown in the DS. How do these get incorporated into the data? Years x velocity appled to the data before I put it into the program?
Howdy,
IF I understand your question 1 correctly you are wondering why the position in the RINEX observation file(?) differs from the data sheet for the same site.
First of all, the position in the RINEX observation file should never be considered valid. It is either coded into the RINEX translator (the command string for the program explicitly includes the position) or retrieved from the receiver. In the first scenario it may have been entered incorrectly, not updated or in a different reference frame), in the second it is a pseudo-range (false range) position that does not incorporate needed corrections to improve the site position. It is not significantly better to determine an autonomous position (without corrections based on data from another receiver) than your hiking handheld.
You do not indicate the horizontal difference between the "data" and "NGS Data Sheet." If within a few meters it is within system specifications; accurate heights are considered to be half as accurate as the horizontal. As you indicate it is an ellipsoid height then the geoid model used is not an issue.
Here in Texas the difference between an ITRF 00 ellipsoid height and an NAD 83 ellipsoid height is about 1.2 meters, much less than the 10 meters you mention. Nationwide the differences vary but NOT by ten meters.
In some cases, there are survey monuments under a GPS antenna mounted on a mast. Reading the text of the description and paying careful attention to the site designation is essential. It is especially important that you be consistent when choosing whether to use the L1 phase center or ARP (antenna reference point) as this physical difference between points on an antenna can be a decimeter.
As far as your second question, OPUS applies ITRF velocities to the CORS sites' ITRF positions. It computes in the ITRF (not NAD 83) which is the same coordinate system as the satellite positions in the precise orbit file. Doing this yourself is possible. I'd use HTDP to do so.
If I understand correctly, the unit you are using is single frequency and therefore not suitable for use with OPUS. IF you have access to dual frequency data I suggest that you check your own processing with an OPUS solution. Single frequency data does not include the observations necessary to form an ion-free solution and should be limited to < 10 km baselines.
What do you propose to position? If you intend to use CORS site with your data and use long baselines you will introduce error as you are unable to solve for the large iono error.
If you intend to determine positions with respect to ground monuments and do so in NAD 83 (NSRS 2007) use the published monument position in NAD 83 (NSRS 2007).
NGS still expects that users of its data familiarize themselves with the associated metadata files. Mayhem will ensue when mixing datums and making assumptions about the contents of the files.
I close having introduced too much already. Feel free to ask more questions. Others will undoubtedly provide clearer explanations.
Cheers,
DMM
Thanks for the information DMM. Yes, it is a single frequency system and I will not be able to use OPUS.
I downloaded the mark and CORS receiver system information as well as the information for the period of opservation.
Back to the books. Your comments were helpful.
Tyler
I would recommend you go to Ashtech's site and download GNSS Solutions.
There are a couple of reasons for this. I have found it to be more powerful software than Ashtech Solutions. Since you are learning new software anyway, you'd just as well learn something more current.
One of the main reasons I recommend this is that you are stating you wish to use CORS data with your own data. GNSS Solutions has a cool internet download utility that will allow you to zoom out the map view until you see the CORS site you are interested in getting the data from. Then you can just double click on the CORS site icon on the screen and the download program will pop up. It will automatically go on the internet and select the appropriate location to get the data you need (if available) and set the time span so that it fully overlaps your longest occupation.
When you close the download window after successfully retrieving the required files, it will automatically bring in the control data for the CORS site and set it as a control for your project.
FWIW...
Thanks Jerry. Will do.