Radar (RAdio Detection And Ranging) systems

To make a radar image, the instrument “lights up” the target – even a whole planet – with a short radio pulse, much like a camera flash. It then records the echoes that come back.

A photograph taken with a white flash or in sunlight shows the surface in many colours. A radar image shows it at a single wavelength only: in a single “colour”.

A radar image is built from two main properties of the echo: its intensity (brightness) and its arrival time (phase). An echo from farther away comes back later, so it also lies farther along in the image. The Doppler shift tells on which side of the planet’s centre the echo arose. The side turning towards us and the side turning away shift the wavelength of the echo in opposite directions.

Radar needs no sunlight. Its signal also passes through clouds, whether they are made of sulphuric acid or of water. So radar can image the surface in the dark and under full cloud cover. Both the 305 m Arecibo radio telescope and the Magellan probe observed at a wavelength of 12.6 cm (2380 MHz).

From the 1980s the Arecibo radio telescope could image Venus at 1–2 km resolution. These observations continued until 2020, when the ageing telescope collapsed. Today the site is a museum.

From orbit, a radar would need a huge antenna, several kilometres across, to give a sharp image. Instead, the radar is built to “simulate” such an antenna. As the probe orbits the planet, it observes the same area for a long time. The distance it covers along its orbit becomes the “virtual” size of the antenna. This is called synthetic aperture radar (SAR). A SAR sends its signal sideways, so radar shadows also appear in its images.

From 1990 the SAR of the Magellan probe imaged the surface until it had mapped almost the whole planet at about 100 m resolution. Its radar altimeter also produced a topographic map at about 20 km resolution. To this day (2023), these are the sharpest images of Venus.

Magellan’s main results were published in 1992. Detailed geological mapping of the planet began on this basis. The surface was divided into 62 map sheets at a scale of 1:5,000,000 (Tanaka et al. 1993). By 2019, 34 of them had been completed. In 2012 Herrick and colleagues produced a digital terrain model (DTM) with better resolution than before. They used areas imaged in stereo, from both the right and the left side.