Modern satellites, including TSX, CSK, ALOS-2, Sentinel-1, and NISAR, have GPS or GNSS receivers onboard to measure the orbits, in addition to other tracking systems. The GNSS data is downlinked with the radar data, but it needs to be processed to calculate precise orbits. The GNSS processing centers have to estimate the precise orbits of the GNSS satellites, which takes some time. We can’t calculate the precise orbits of the SAR satellites until we have the precise orbits of the GNSS satellites.
There is no need for ionospheric corrections of OCN data, and I never heard of any corrections for GRD either.
Ionospheric corrections are highly recommended for InSAR studies with L-band SAR data, such as ALOS, ALOS-2, SAOCOM, and NISAR because the effects are stronger than for the shorter wavelengths.
Ionospheric corrections are recommended for ascending and descending Sentinel-1 InSAR in the tropics and high latitudes, and may help for ascending tracks in middle latitudes. You can see our published paper on ionospheric correction of Sentinel-1 InSAR for more details.
Liang, C., P. Agram, M. Simons, and E. J. Fielding (2019), Ionospheric Correction of InSAR Time Series Analysis of C-band Sentinel-1 TOPS Data, IEEE Transactions on Geoscience and Remote Sensing , 57 (9), 6755-6773, doi:10.1109/TGRS.2019.2908494.
For those who don’t have access to IEEE TGRS, we posted a preprint of the first submitted version on EarthArXiv:
The final published version is only slightly modified to address reviewers’ comments.