A radar imaging system can be thought of as an echo measurement system. The radar antenna emits thousands of pulses of microwave radiation and measures the characteristics of associated echoes. The radar determines the range between the antenna and the reflecting object, the amplitude of the return wave, and its phase. That is, the radar can determine if the wave returns at its peak or trough, or somewhere in between. These measurements of range, amplitude, and phase are processed together to form images and many other useful products.
The ability to penetrate clouds is the core advantage of radar imaging. Radars emit pulses of microwave energy, which have long wavelengths in comparison to sunlight, and are unaffected by cloud-cover, dust, and gas in the atmosphere. Radar is the only remote sensing technology that can almost guarantee collection regardless of weather.
Other Useful Radar Properties
- Sunlight Not Needed: Radar does not need sunlight for illumination; it can be configured to collect images at any time of the day or night.
- Flexible Collection: Radars can be designed with flexible collection capabilities. A single system can support high-resolution imaging over small areas, medium-resolution over medium areas, or low-resolution over large areas.
- Multiple Microwave Bands: Radar imaging supports collection in different wavelength bands. Many systems employ X-band radiation with pulse wavelengths of roughly three centimeters, but other wavelengths are possible. For example, P-band radar has a wavelength of about one meter, which is so long that the energy penetrates vegetation and can be used to image through foliage.
- Controlled Polarization: Radars control the orientation, or polarization, in space of the emitted waves. They are designed to image in specific polarizations, or even in multiple polarizations during the same imaging operation. Images of different polarizations record different reflectance patterns, which may reveal surface structure content, such as crop types or drainage patterns.
- Coherent Illumination: In contrast to the random illumination of sunlight, radar energy is emitted in a controlled manner in which the wave and frequency patterns are consistent. This natural coherence means the radar data can be used to generate special products such as 3D global elevation grids and models of very slight changes in ground-surface structure over time.
- High Resolution: The technique called Synthetic Aperture Radar (SAR) permits high-resolution imaging from any distance.
In the early days of radar imaging, the challenge was to achieve useful resolutions in the range and cross-range dimensions of the image. Good range resolution relies principally upon the properties of the transmitted waveform. But the early imaging radars, so-called real-aperture radars, had cross-range resolutions of hundreds of meters, and this degraded as the distance between the sensor and the ground increased. While larger antennas improved resolution, it is not possible to build antennas large enough to provide good cross-range resolution for real-aperture radars.
The SAR technique, invented in the early 1950s, overcomes this problem by using the flight direction of the sensor to simulate, or synthesize, a large antenna. The individual transmit and receive cycles of the SAR imaging operation are completed from different locations as the sensor moves. The locations are treated as array elements of a single long antenna strung out along the flight direction. This SAR “trick” uses the long synthesized aperture to achieve fine cross-range resolution while the smaller physical aperture provides for a wide field of view.
Modern space-based commercial SAR systems, such as Cosmo Skymed, RADARSAT2 and TerraSAR-X, orbiting at approximately seven kilometers per second, and imaging for two-and-a-half seconds in high-resolution mode, have a synthetic aperture of 17.5 kilometers. A physical antenna of that size is inconceivable.
The Value of Radar Imaging
Radar imaging systems can image through almost any weather condition, and they have several other useful remote sensing capabilities. In particular, the precise measurement of phase, which is fundamental to SAR, is simply not available to passive remote sensing systems. Despite the fact that SAR imaging is well outside the human experience, the opportunities it offers are powerful and far-reaching. We look forward to the next generation of scientists, engineers, and innovators to unleash the full potential of a technology that was invented more than 60 years ago.