A single InSAR measurement describes movement along one direction: the satellite's line of sight. That number can include a mix of vertical, east-west and north-south motion. By combining several viewing geometries, and sometimes ground measurements, analysts can build a much fuller picture of how the ground is actually moving.
Why the line of sight matters
Most Earth observation SAR satellites look sideways rather than straight down. The displacement they record is the projection of the real movement onto that viewing direction. A point can appear to move away from the satellite because it subsided, shifted sideways or did both.
Japan's Geospatial Information Authority explains this in its InSAR technical Q&A, along with why ascending and descending passes provide valuable extra information.
What two viewing directions add
Ascending and descending acquisitions typically look from opposite sides. With suitable observations and consistent references and time periods, they can be combined to separate vertical and east-west movement. This is often called decomposition (see the InSAR glossary). Because standard near-polar orbits are less sensitive to north-south movement, that component is usually assumed to be small or constrained with other data.
GNSS, leveling and other observations can help complete a three-dimensional estimate. How far that estimate can be taken depends on geometry, data density, uncertainty and the movement mechanism. Read more in our article on ascending and descending orbits.
Why this matters to engineers
A bridge manager may need vertical settlement, a mine's geotechnical team may care about movement perpendicular to a slope, and a reservoir specialist may want to separate uplift from sideways movement. Stating the quantity you need before the analysis begins ensures the right data and method are chosen.
SkyGeo's introduction to InSAR explains how the measurement is calculated. A good report labels measured line-of-sight values and derived movement components separately, so users always know what they are looking at.
Frequently asked questions
-
It gives two of the three components, typically vertical and east-west. The third, north-south, usually needs an assumption or additional observations.
-
Yes. With suitable geometry and assumptions, or with additional measurements, InSAR is widely used to estimate vertical movement such as subsidence.
-
It contributes only weakly to most observations from near-polar orbits, so it is generally hard to isolate without other data.
Get the movement direction you need
SkyGeo can design an analysis that delivers the movement components your decision depends on. Contact SkyGeo.
