The satellites follow fixed orbits and the Sentinel satellites have excellent reliability. The SAR (radar) data used is co-registered to align precisely to a single image, this is then geo-referenced using a digital terrain model, which is the reference point for any ground truthing. A Quality Assurance process ensures this alignment is accurate. We estimate the measurements are accurate to within 2-3mm for each data acquisition. Post-processing over longer time periods increases this accuracy to 2mm per year.