Synthetic Aperture Radar principle

This page contains basics of synthetic aperture radars (SAR), I did my best to keep as simple as possible on the main stream, more scientific details can be found as linked detail pages, or as linked articles. The idea is to make you understand how one use radio-waves to image Earth (or other planets)

Few remember, but the word RADAR is an acronym for "Radio-wave Aircraft Detection And Ranging". Forgot about the "aircraft" because our ancestors first used radars from the ground up towards the sky (and we do the opposite). The first function of a radar is range objects (i.e. measure distances from you to the object). This is how it works...

Ground based surveillance radars steer their antenna to scan an area hence producing a two dimensional (range × direction) image. This is the familiar rotating aerial on can see in the vicinity of airports and on the ship poles. "Synthetic aperture" is a technique allowing using a large virtual antenna made up by the successive positions of a small antenna (by chance aircraft must move to stay in flight!). This virtual antenna can even be steered "by calculus" to scan a wide area.

With a second antenna (either a real one carried simultaneously by the aircraft, or a virtual one corresponding to a second flight over the same area) we can measure terrain elevation, making 3D maps. This technique is called SAR interferometry...

The synthetic aperture technique assume we know the successive positions of the real antenna, but if an object is moving, its successive positions relative to the real antenna do not match that of the relative landscape. Here are shown the techniques allowing detection, velocity and position determination for these moving objects...

Unlike sea waves, radio-waves (as visible light indeed) have a "vibrating direction" feature called polarisation. Though our eyes are insensitive to polarisation, we sometimes use polarising glasses (e.g. because sunlight reflected from the sea surface is strongly polarised, skippers all wear polarising sunglasses. Perhaps also have you seen on of these few 3D flicks... Whew Grace Kelly in Dial M for murder...). Radar can use this feature to retrieve subtle information on the reflecting media such as moisture of ground, water pollution a.s.o.