Situation: I have merged six Landsat 8 image tiles to one big tile for the bands 4,3 and 2. These are Landsat level 2 products, so already corrected to surface reflectance and need no further processing. I have separately downloaded six band 8 image tiles for the same region. They are of a different date.merged those to one tile too. All the raster images are 16 bit. What options do I have to perform pansharpening on the 432 band combination? Grass Gis apparently only takes 8 bit. Semiautomatic classifications plugin in QGIS requires a metadata file. Of course no one single metadata file applies once the tiles are merged to one. Also, the band 8 is of a different date than the other bands. please advise.
Since you have QGIS you'll have access to Python and the various GDAL tools and scripts. As of GDAL 2.1 there is a pansharpening Python script available called
gdal_pansharpen.py. If you don't have it already it is avaliable here. You can view the arguments and their descriptions at the official documentation page along with several examples. Additionally, the script is used in this article from Planet.
Another option is Orfeo Toolbox which is an open-source remote sensing toolbox. It contains various tools that can be used from the command line and through QGIS. OTB has its own pansharpening tool which is documented in the OTB CookBook. OTB setup for QGIS is covered here.