Electromagnetic radiation at long wavelengths (0.1 to 30 centimeters) falls into a segment of the spectrum commonly called the microwave region. At still longer wavelengths (centimeters to meters) the radiation is known as radio waves (these can be generated by manmade transmitters or occur naturally [e.g., beamed from energetic stars]). Remote sensing has utilized passive microwaves, emanating from thermally activated bodies. But, in much more common use (since World War II) is another manmade device, radar, an active (transmitter-produced) microwave system that sends out radiation, some of which is reflected back to a receiver. The varying signal, which changes with the positions and shapes of target bodies, and is influenced by their properties, can be used to form kinds of images that superficially resemble those recorded by Landsat-like sensors. This first page introduces certain basic principles, describes the common radar bands in use, and shows a typical radar image.
Radar is an acronym for Radio Detection and Ranging. Radar is an active sensor systems. It generates its own illumination as an outgoing signal that interacts with the target such that some of the signal is returned as backscatter that is picked up by the same antenna that emitted the radar beam. Radar operates in part of the microwave region of the electromagnetic spectrum, specifically in the frequency interval from 40,000 to 300 megahertz (MHz). The latter frequency extends into the higher frequencies of the broadcast-radio region. Commonly used frequencies and their corresponding wavelengths are specified by a band nomenclature, as follows:
- Ka Band: Frequncy 40,000-26,000 MHz; Wavelength (0.8-1.1 cm)
- K Band: 26,500-18,500 MHz; (1.1-1.7 cm)
- X Band: 12,500-8,000 MHz; (2.4-3.8 cm)
- C Band: 8,000-4,000 MHz; (3.8-7.5 cm)
- L Band: 2,000-1,000 MHz; (15.0-30.0 cm)
- P Band: 1,000- 300 MHz; (30.0-100.0 cm)
This chart summarizes the above information on Bands in the Microwave segment of the EM Spectrum:
Unlike other sensors that passively sense radiation from targets illuminated by the Sun or thermal sources, radar generates its own illumination (hence, it is active; another example is the flash camera) by sending bursts or pulses of EM energy that reflect off of a target. A fraction of the reflected energy then returns to the radar’s receiving antenna, which collects it and passes it to an electronic processing system. A radar system is a ranging device that measures distances as a function of round trip travel times (at light speed) of a directed beam of pulses (the signal, whose strength is measured in decibels, dB) spread out over specific distances. In this way radar determines the direction and distance from the instrument (fixed or moving) to an energy-scattering object. We can also derive information about target shapes and certain diagnostic physical properties of materials at and just below the surface, or from within the atmosphere, by analyzing signal modifications.
8-1: In the Introduction, you learned that wavelength times frequency = the speed of light. By choosing the proper units, this equation can be derived: Wavelength (in cm) = 30/frequency (in GHz or 109cycles per second). Given a frequency of 20 GHz, what Wave Band would this radar pulse fall into?
As we shall see in this Section, radar has become increasingly important in various applications of remote sensing. The military continues to be a prime user. Several satellites operating now have radar has their principal sensor. Most of us are most familiar with radar’s use in Meteorology, mainly as a tracker of storms, rainfall, and advancing fronts. We will skip further reference to this last use in this Section; it is defered until Section 14 that deals with Meteorological Remote Sensing.
This ability to mirror ground surfaces for displaying topography is a prime use of radar for a variety of applications. Some radars operate on moving platforms; others are fixed on the ground. The usefulness in determining land surface configurations is strikingly confirmed in the radar image strip (SIR-A system on the Shuttle) shown here, extending 300 km (200 mi) to the northeast (right side) across the folded and dissected South American Andes in Bolivia. The image covers from the high plains (Altiplano) on the west to the lowlands (Amazon Basin) on the east. (Scroll to see this right end.)
Radar systems, as well as passive sensors operating at microwave wavelengths (slightly shorter than radar), are also effective in detecting soil moisture, storms-clouds-rain, and sea states.
- British fleet’s new radar system can detect a supersonic tennis ball 25 km away (gizmag.com)
- Cellcos to FCC: Give Us 2 GHz TV Microwave (dailywireless.org)
- Forget about leprechauns, engineers are catching rainbows (phys.org)
- The High Frequency Active Auroral Research Program (tatoott1009.com)