|RADAR stands for "RAdio
Detection And Ranging". By virtue of sending out pulses of microwave
electromagnetic radiation this type of instrument can be classified as
an "active sensor" - it measures the time between pulses and their
reflected components to determine distance. Different pulse intervals,
different wavelengths, different geometry and polarizations can be
combined to roughness characteristics of the earth surface.
Radar wavelengths range between less than 1 millimeter to 1 meter.
All photos can be called images but not all images should be called photos - images that are acquired digitally are usually not referred to as photographs ... there is no such thing as a radar photograph!
Radar uses relative long wavelengths which allows these systems to "see" through clouds, smoke, and some vegetation. Also, being an active system, it can be operated day or night. There are disadvantages, such as the non-unique spectral properties of the returned radar signal. Unlike infrared data that help us to identify different minerals or vegetation types from reflected sunlight, radar only shows the difference in the surface roughness and geometry and moisture content of the ground (the complex dielectric constant). Radar and infrared sensors are complimentary instruments and are often used together to study the same types of earth surfaces.
Unlike instruments that look straight down (i.e. Aerial Cameras and nadir pointing satellite sensors), radar data are collected looking off to the side. Because radar measures the time that it takes for the signals to go from the antenna to the ground and back, this angled perspective is a necessity because this causes there to be a delay between the parts of the returned pulse that are farthest from the antenna and the parts that are closest. This very small delay is measured with very high precision, these measurements are filtered using complex functions for removing noise and then these time measurements are converted to a distance for every location in the radar swath and viola we have a map of the surface topography.
Cameras capture reflected
Plan Position Indicator - the kind of radar used by air traffic controllers is not considered an "imaging radar"
Radar was originally developed in the 1950s, the first airborne system was called SLAR (Side-Looking Airborne Radar) and was used for improving the resolution for military reconnaissance, early airborne radar systems were limited by the physical size of the antenna however. Later, SAR (Synthetic Aperture Radar), was developed and are widely used in many countries for civilian applications.
San Francisco (1950's acquired by an early SLAR system)
Types of Imaging Radar
Radar resolution has two components; the "range" resolution and the "azimuth" resolution. These are determined by, among other factors, the width of the synthesized antenna (which is dictated by the pulse interval) and the wavelength.