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Synthetic aperture radar (SAR) is a type of radar sensor. The SAR sensor is mounted on an airplane or satellite and points sideways instead of straight down (nadir). It is an active sensor that sends electromagnetic waves to the earth's surface and receives the reflected signal. The electromagnetic wave received by the sensor is called the measured backscatter. A SAR image is a 2D rendering of the measured backscatter.
A SAR image is commonly delivered as two product types: ground range detected (GRD) and single look complex (SLC). GRD products have been averaged to produce a multilooked image projected to the ground range using an Earth ellipsoid model. A GRD image is stored as a real valued array in which the value in each pixel represents the amplitude of the measured backscatter signal. SLC products are images in the image plane of the data acquisition, known as the slant range plane. An SLC image is stored as a complex valued array in which the single complex value in each pixel represents the amplitude and phase of the measured backscatter signal.
A high digital number (DN) for the amplitude of a SAR image pixel represents strong backscatter, while a low DN represents weak backscatter. The strength of the amplitude of the measured backscatter is used to differentiate between features on the ground. The time delay between the transmitted and received electromagnetic wave determines the location of the feature.
Active sensing
A sensor can be classified as either passive or active. A passive sensor, using optical systems, records electromagnetic waves emitted by the sun and reflected from the ground surface. An active sensor, used by SAR systems, functions as both the source and the receiver. This means the sensor transmits the electromagnetic waves and also records the reflected waves. Unlike an optical sensor, a SAR sensor can operate during the day or night, independent of the sun, since it transmits its own signal.
Microwave wavelength
Through active sensing, a SAR sensor can collect imagery using longer wavelengths compared to an optical sensor. While an optical sensor uses wavelengths from visible (4x10-7 meters) to thermal infrared (15x10-6 meters), a SAR sensor uses microwave wavelengths, ranging from K-band (7.5x10-3 meters) to P-band (1 meter).
The microwave wavelengths allow SAR to be an all-weather imaging system for most bands. The longer wavelengths of C-, S-, L-, and P- bands enable the SAR sensor waves to penetrate clouds, fog, dust, smog, and smoke, making it better suited for monitoring humid tropics and high latitudes. Both precipitation and clouds reduce the strength of a K-band signal, while only precipitation reduces an X-band signal. Rain cores and hydrometeors from severe storms can reduce the strength of K-band, X-band, and C-band signals. A SAR image for these bands will have a weakened backscatter for the pixels where the signal interfaced with these severe storm features. Rain cores occur during heavy rain rates (above 125 mm/h), while hydrometeors are clouds of raindrops in both the liquid and melting phase.
The microwave wavelengths characterizing SAR systems also provide distinct information about the physical properties of the earth's surface, such as roughness, density, and moisture content. Microwave wavelengths typically scatter differently based on the feature that reflects them. The wavelength used strongly impacts the features captured in the SAR image. If the wavelength is longer than the feature of interest, the feature will be undetected by the electromagnetic wave. For example, L-band is ideal for flood mapping in tropical forests where canopies obscure an optical sensor's view of the ground. The canopy leaf will not be detected by the longer L-band wavelengths of 15 to 30 centimeters, allowing the wave to penetrate the canopy cover and image the flooded ground below. In this scenario, X-band data, with wavelengths of 2.4 to 3.75 centimeters, would scatter directly off the canopy and create a SAR image highlighting the canopy instead of the flooded ground.
Microwave wavelengths can also penetrate material such as soil, snow, and ice. The longer the wavelength, the greater the penetration depth. However, the greater the moisture content of the material, the shallower the penetration. This characteristic is used to differentiate frozen and unfrozen soil conditions.
For most microwave wavelengths, smooth, horizontal features such as roads, airport runways, dry lake beds, flattened soil, still water, and sand reflect the electromagnetic waves away from the sensor and exhibit pixels with weak backscatter (low DN). Similarly, for most microwave wavelengths, human-made objects characterized by reflective material and sharp geometries, such as buildings and ships, reflect the electromagnetic waves back to the sensor and exhibit pixels with strong backscatter (high DN).
The following table summarizes the various features and applications that are possible for the individual bands and their associated wavelengths:
Radar applications for each band type
| Application | K-band | X-band | C-band | S-band | L-band | P-band |
|---|---|---|---|---|---|---|
Wavelength (cm) | 0.75 to 2.4 | 2.4 to 3.75 | 3.75 to 7.5 | 7.5 to 15 | 15 to 30 | 30 to 100 |
Precipitation penetration |  | | | | ||
Hydrometeors or rain core penetration | | | | |||
Cloud, fog, dust, smog, or smoke penetration | | | | | | |
Low to moderate crop canopy penetration | | | ||||
High crop canopy penetration | | | ||||
Dry alluvium penetration | | | | | | |
Dry snow or ice penetration | | | | | | |
Wet soil penetration | | | | | | |
Earth surface mapping | | | | | | |
Flood mapping | | | | | | |
Flooded grass mapping | | | | |||
Flooded reed and brush vegetation mapping | | |||||
Flooded canopy mapping | | | ||||
Canopy mapping | | | ||||
Sea ice monitoring | | | | | | |
Oil spill mapping | | | | | | |
Soil moisture monitoring | | | | | | |
Polarization
In addition to sensing longer wavelengths, active sensing also provides the ability to control the polarization of the transmitted electromagnetic waves. By having the SAR sensor define both the transmitted and received polarization, the resulting SAR image can highlight different features on the earth's surface based on the backscatter. The SAR data polarization is denoted by two letters, the first corresponding to the transmitted polarization and the second corresponding to the received polarization.
Dual polarized SAR images feature either VV, VH polarized data or HH, HV polarized data. For VV, VH products, the sensor transmits vertically polarized waves and receives vertically polarized (VV) or horizontally polarized (VH) waves. Similarly, for HH, HV products, the sensor transmits horizontally polarized waves and receives horizontally polarized waves (HH) or vertically polarized waves (HV). If the transmitted and received waves share the same polarization, the data is described as copolarized. If the transmitted and received waves do not share the same polarization, the data is considered cross-polarized. As with the wavelength, the transmitted and received polarization used strongly impacts the features captured in the SAR image and must be taken into consideration.
Spatial resolution
Active sensing enables a SAR sensor to synthetically increase its spatial resolution. A SAR sensor emits electromagnetic waves with a chirp of varying frequency that serves as a marker in the received waves. As a satellite orbits or an aircraft flies along its track, the SAR sensor images a point on the ground surface multiple times. The chirp marker is used to identify the location of the received waves. This feature combined with signal processing techniques enable a SAR sensor with a short antenna to synthetically elongate its antenna, which enhances its spatial resolution. To identify the location of the received wave on the ground, the SAR sensor must be side looking. If a SAR sensor is nadir looking (pointed straight down), it cannot use the travel time to distinguish between features an equal distance from the sensor on opposite sides.
While the characteristics of a SAR sensor provide a unique rendering of ground features, they also introduce unique processing challenges. The most common challenges include removing thermal noise, applying calibration to retrieve a meaningful backscatter value, filtering the speckle noise, and removing radiometric and geometric distortions.