Radar (Microwave) Remote Sensing
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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.

Color Aerial Photography vs Processed Radar Imagery

Color Photography vs Radar Derived DEM (Digital Elevation Model)

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 visible wavelengths.
Radar captures emitted microwave wavelengths that are bounced back to the antenna.

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
SLAR (Side-Looking Airborne Radar)
- develop by guess who in the 1950's
- airborne, fixed antenna width, sends one pulse at a time and measures what gets scattered back
- resolution determined by wavelength and antenna size (narrow antenna width = higher resolution)
SAR (Synthetic Aperture Radar)
- also developed by those responsible for SLAR, but this configuration is not dependent on the physical antenna size although to achieve higher resolution the receiving antenna components and transmitter components need to be separated.
- "synthesizes" a very broad antenna by sending multiple pulses

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.

Range Resolution - Determined by Pulse Interval and Look Angle.Azimuth Resolution - Determined by Antenna and Wavelength.Azimuth Resolution and Antenna Size - Click to Enlarge

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