Across-track InSAR (XT-InSAR) is a special mode of interferometric SAR (InSAR), where the individual SAR images have been acquired from slightly different look directions. The measured phase difference contains information about the elevation of the targets on the ground, but it can also be affected by temporal changes between the individual SAR images. XT-InSAR can be implemented using one or more SAR systems in both single-pass and repeat-pass configurations. To mitigate temporal change between acquisitions, the XT-InSAR configuration is selected based on the intended application and frequency used by the system. If a single SAR sensor is used in the repeat-pass mode, temporal stability can be achieved either by a selecting a lower frequency and focussing on the larger, more stable targets (e.g., P-band, 435 MHz InSAR in forests) or by selecting a higher frequency and focussing on already stable environments (e.g., X-band, 9.65 GHz XT-InSAR in urban environments). Using two or more SAR sensors in a single-pass, tandem configuration, it is possible to measure elevation of temporally instable targets using higher frequencies, as demonstrated by the SRTM and TanDEM-X systems over vegetated areas and ocean.
References: bamler/hartl, one on SRTM or TDM for DEM, one on BIOMASS for forestry, one on Sentinel-1 for urban areas, one on TDM on vegetation