Understanding line-of-sight displacement

A radar satellite measures distance changes along one direction: the line from the satellite to the ground, which is tilted — typically 30–45 degrees from vertical — and points sideways from the orbit. Every InSAR measurement is the projection of the true three-dimensional ground movement onto that single line. A point subsiding vertically by 10 mm appears as somewhat less than 10 mm of line-of-sight movement; a point moving horizontally may appear strongly, weakly, or not at all, depending on direction.

Satellites pass on ascending (south-to-north) and descending (north-to-south) tracks, viewing the ground from opposing sides. Comparing the two geometries over the same area helps separate vertical from east–west movement and is standard practice in careful analyses. North–south horizontal movement, however, remains poorly observed by either geometry — a genuine blind spot that honest reporting states explicitly.

A trustworthy InSAR report will name the geometry used, state whether values are line-of-sight or projected to vertical (and under what assumption), and note where the geometry limits interpretation. If a report presents 'subsidence' values without mentioning line of sight at all, ask questions.