Comment by lmc

Typically you will get an image pair for an area every 6-12 days. The phase used in interferometry is massively affected by atmospheric conditions, which can vary a lot in this time, and are difficult to correct for. So, one pair is often not enough for this. But if you look at a bunch of pairs for that area over a longer time period, you might be able to correct for the atmospheric effects and get your differential height map. You can get more accurate elevation models 'out of the box' with different systems, e.g., the SRTM (one of the most well known publicly available global elevation maps) [1] was made with insar but 2 antennas on one craft, and Germany's TanDEM-X [2] is a pair of satellites flying in formation a few hundred meters apart, capturing the same area at the same time.

[1]: https://www.earthdata.nasa.gov/data/instruments/srtm [2]: https://en.wikipedia.org/wiki/TanDEM-X