Correct height rtk and post process with Rtklib

giulio

Member
For rtk surveys, the height to be entered is the one read by the rover pole (if no other adapters are mounted), for RTKLIB static surveys at this read height I should also add that of the phase shift of the phase center which, if I'm not mistaken, is 11 cm, right
Cattura arp.JPG
Cattura arp 2.JPG
 

Eric Tweet

Active Member
I believe you don't need to manually add the phase center offsets as long as you select the correct antenna type in the options. This will take the values of the phase center offsets from the igs14.atx file and apply them. So all you need to do is enter the rod height in the appropriate "Delta U" field of the Positions tab.
 

giulio

Member
Right I inserted the JAVTRIUMPH_LSA_NONE.atx file and the ellipsoidal dimension has been corrected. On the other hand, the rtk quota is offset by about 11 cm as if this correction were not applied
rtk lib rtk.JPG
 

Eric Tweet

Active Member
A few thoughts...

First, I've only used RTKLib a few times so I'm no expert on any of it's usage. With that being the case, I'm not entirely certain what your screenshot is showing, I'm afraid. Is your spreadsheet showing multiple measurements of the same point?

Next, how did you determine it's off by 11cm? Is that by comparing your measured position with a published datasheet, or perhaps by measurements made by other people or other equipment?

Then, one thing to check to see if the offsets are being applied from the .atx file is to change the setting for antenna type. You may be able to set it to a NULL antenna, or even manually open the .atx file and set the offsets to zero. Then when you re-process the file, you should see the effects of removing the offsets to verify that they were in fact being applied correctly in the first place.
 

giulio

Member
Yes in rtklib I checked and the antenna corrections are applied correctly, the statically elaborated point is the c1002 which in rtk and rtpk provides a share of 446.40 in rtklib with antenna application file JAVTRIUMPH_LSA.atx provides a share of 446.246 (which is the right one for the point in question) without atx file 446.367 not correct, but closer to that rtk and rtpk for this I have the doubt in rtk.
 

Eric Tweet

Active Member
Ok, that makes sense, and I understand what your screenshot is describing now.
The only other things I can think of at the moment are:
- During static processing, ensure your base file also has the correct antenna calibration file and Delta U chosen.
- Is the "U_m" value ellipsoid height or orthometric? I'm just wondering if there may be an effect by some combination of using a Geoid for RTK but not during post-processing...
- Another thought is whether RTK and post processing are using the same ellipsoid (for example both WGS84 or NAD83(GRS80))

I can't help but wonder if the issue may be more related to the base station than the rover. For example, if your base is a RTN or CORS station, it may be transmitting the ARP location when using it for RTK, but the file for that base station when used in post processing would be the location of the phase center and still require consideration of antenna calibration file and height of the ARP above it's reference coordinates (if any). Some topics like that may be ruled out if you are using your own base station, and configuring it in the field using the LS.

I'm mostly making some educated guesses at the issue, considering my lack of knowledge of RTKLib and the fact that I don't own an LS receiver (yet)... :(
 
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