High-resolution optical imagers provide detailed images of Earth’s surface. In general, these are nadir-viewing instruments with a horizontal spatial resolution in the range 10 m to 100 m and swath widths of order 100 km. High-resolution imagers are, in general, panchromatic (a single waveband) and multi-spectral (multiple waveband) sensors, with spectral bands in the visible and IR range that are simultaneously recorded.
Use of these sensors can be limited by weather conditions, since they are unable to penetrate thick cloud, rain or fog and are typically restricted to fair weather, daytime-only operation.
These instruments transmit at frequencies of around 1 GHz to 10 GHz and measure the backscattered signals to generate microwave images of Earth’s surface at high spatial resolutions (between 10 m and 100 m), with a swath width of 100–500 km. Both synthetic aperture radars and real aperture side-looking imaging radar systems fall into this category.
The images produced have a similar resolution to those from high-resolution optical imagers, but radars have the capability to ‘see’ through clouds, providing data on an all-weather, day/night basis.
SARs also have the ability to penetrate vegetation and to sample surface roughness and surface dielectric properties.