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Accounting for GPS antenna offset

We are working on a high-accuracy geotagging solution that should ultimately be capable of providing the location of the GPS antenna within 2-3cm. 

As I am sure most of us are aware, the GPS antenna is not usually where the camera is located on an aircraft/drone and there will be an offset. This offset will be fixed (relative to the camera coordinate system), so long as the camera is not mounted on a gimbal.

Since our high-accuracy geotagging solution won’t have an IMU on it, we cannot pre-account for the antenna offset before importing the camera positions into Pix4d.

Here are my questions…

  1. Will Pix4D automatically figure out that there is a constant offset, account for it, and still output a mosaic that has a georeferencing accuracy close to the 2-3cm range?

  2. If the software can’t automatically determine/account for the constant offset, would it be possible for a user to simply type in the offsets (X offset: 5cm, Y offset: 10cm, Z offset: 20cm, for example) relative the camera coordinate systems since Pix4d effectively determines the orientation of each camera during its initial processing anyway?

I am also interested in it. Since there are multiple GNSS units available (AsteRx-m UAS) , or will be available (Emlid Reach) which is able to have RTK or PPK solution with precise geotagging via camera shutter synchronisation this issue is very interesting for us. Trimble UAS Master has the capability to enter fixed camera-GNSS antenna offset, so I think Pix4D should have it also!


Great question and without more information I’m thinking that you’ll need to spatially know at a minimum: 1) the Phase Center Offset locations for your specific antenna [L1 and L2], 2) its specific calibration data including Phase Center Variations, and 3) the image sensor’s centroid and its relative relationship to the PCOs. I’d also guess that none of this matters to Pix4D as it becomes relative. For calibration data, check with Geo++. If not there, check with NGS - they might have the calibration data for your antenna model. Both places test, but Geo++ charges for their tests and NGS doesn’t for antenna manufacturers. The Geo++ tests are more rigorous than tests conducted by NGS, I believe.

This also brings up something that has not been overly clear in the metadata that I’ve examined; namely, making an unambiguous distinction between altitude and the integral barometrically derived value tagged versus and an honest GNSS RTK 3-D fixed height; ellipsoidal or whatever.