Question: Does anyone have any advice on how one might georeference/orthorectify oblique imagery taken over the open ocean from a manned aircraft? The aircraft will have an RTK-enabled GNSS/INS onboard. I was thinking we could use the timestamps to link the position of the aircraft with the imagery. However, this is only one piece of the puzzle, as we would need to stretch the image to fit it into its position on the ground.

Since we will know the position, heading and height of the aircraft and the angle at which the images are being taken, surely we have all the information we need, but how would it work in practice?

Answer: 
Orthorectification consists of two image adjustments corresponding to the sensor model and the terrain relief. Over the ocean you should be able to ignore the latter because the ocean is defined as 'sea level' so ellipsoidal height is sufficient.

[OSSIM has a utility](https://trac.osgeo.org/ossim/wiki/orthorectification) to produce orthos which allows for ignoring DTM elevation relief, so with the right inputs this should be able to create orthos over the ocean.

Two issues will be:

* creating a sensor model to supply the correct inputs, and
* validating the orthophoto mosaic results
First you will have to calculate external orientations for the images using the GNSS/INS. Normally you would do aerial triangulation to further improve the EO so that your ortho rectified images would fit together even better and also include ground control points to assess accuracy.

Since you have mainly open water. You can forget about doing an AT with automatic tie point collection. It will find false tie points that will mess up the AT result. So you can only rely on the GNSS/INS.