InSAR: how radar measures millimetres from space
About this app
The radar satellite Sentinel-1 passes twice over a sinking city. The echo comes back shifted because the wave travels down and back: a phase shift of one full turn equals 2.8 cm. The interferogram shows colour rings above the subsidence bowl; in the section the colour jumps every 27.7 mm.
With explain mode: an AI tutor walks you through the app, points out what matters in the scene and answers your questions.
How radar interferometry works
A radar satellite such as Sentinel-1 sends microwaves with a 5.55 cm wavelength sideways to the Earth and receives the echo, even at night and through clouds. Besides the strength of the echo it records its phase. When the satellite flies the same orbit 12 days later, the phases of both images are compared. If the ground has moved, the path is longer or shorter. Because the wave travels down and back, half a wavelength of movement, about 2.8 cm, already changes the phase by a full turn.
What the app shows
The side view shows two passes over a sinking city, the two echoes one above the other and a phase dial. On the right the interferogram builds up: every pixel is coloured by its phase shift, and colour rings appear above the subsidence bowl. A section through the bowl shows that the colour starts over every 27.7 mm. The side view is exaggerated and not to scale.
Things to try
Run both passes and set 14 mm of subsidence: the echo is shifted by half a wave. At 27.7 mm the dial makes a full turn and the echo looks as before. Pick the example "Mexico 1 year" with 217 mm and count the rings. Then switch to "subsidence in mm": the rings turn into the bowl.
Frequently asked questions
- What is InSAR?
- InSAR stands for interferometric synthetic aperture radar. Two radar images of the same area taken at different times are compared. The phase shift of the echoes reveals how far the ground has moved along the satellite's line of sight, often to the millimetre.
- Why does one fringe equal 2.8 cm?
- Sentinel-1 uses a wavelength of about 5.55 cm. The wave travels to the ground and back, so every centimetre of subsidence lengthens the path by two centimetres. A full phase turn, one colour cycle in the interferogram, therefore equals half a wavelength, about 2.8 cm along the line of sight.
- How do you know how many turns there were?
- The phase alone is ambiguous: 0° and 360° look the same. So the fringes are counted inwards from the stable edge of the bowl and added up. This unwrapping gives the total movement, as long as neighbouring pixels do not differ too much.
- Does the app use real satellite data?
- No. The app uses the real parameters of Sentinel-1, wavelength and repeat time, but a model bowl. The Mexico City example of 217 mm per year comes from a radar study of 465 large cities (Nature Cities 2026).
Subject: Geography & earth science
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