Radio Detection And Ranging
A radar system has three functions:
- Transmit - the radar transmits microwave (radio) signals towards the ground.
- Receive - the transmitted energy returned from the ground (back-scattered) is received by the radar.
- Observe - the radar system records the strength (amplitude) and the timing (phase) of the returns.
This refers to the orientation of the electric vector of an electromagnetic wave. Radar system antennas can be configured to transmit and receive either horizontally (H) or vertically (V) polarized electromagnetic radiation. Images can be collected with multiple channels of information that represent the different polarizations, each yielding different information from the same location.
Radar systems can operate at different wavelengths, typically between 2 to 75 cms. Each wavelength is best suited to specific applications; shorter wavelengths offer high resolution, while longer wavelengths provide deeper ground cover penetration abilities.
Speckle degrades image quality - it is caused by variation the returned energy to the radar sensor from the same resolution cell. Speckle can be minimized by multi-look filtering, or alternately by applying adaptive filters. Applying speckle filters enhances radiometric resolution at the expense of spatial resolution.
Since imaging radars use microwaves, the presence of moisture in a material being imaged determines its dielectric constant. The dielectric constant influences the absorption and reflection of microwave energy. Due to differences in moisture, and thus dielectric constant, identical materials can vary in appearance at different times or locations according to the amount of moisture they contain.
Radar Image Distortions
Two types of distortions are common in Radar images; slant-range scale distortion occurs since the radar is measuring distance to features rather than the true horizontal distance on the ground. Slant range to ground range conversion is required to remove this effect and represent the image as square pixels. Relief displacement refers to geometric distortions typically associated with tall objects/mountains which get displaced toward the sensor due to their geometry relative to the radar. Radar shadow occurs in the down-range direction behind tall objects/mountains, where the radar sensor is obstructed and cannot collect back-scattered energy. Foreshortening in a radar image is the appearance of compression of those features in the scene which are tilted toward the radar.
Online, Instructor-led Training
Receive expert instruction in the comfort of your own facility. PCI’s online training facility offers numerous advantages over conventional training courses. This course was developed and is delivered by Gabriel Gosselin, Ph.D. Mr. Gosselin has over 12 years of experience in remote sensing and GIS. He participated in research projects oriented toward natural resources monitoring using optical and radar data. He also taught undergraduate classes in photo-interpretation, cartography and remote sensing at The University of Montreal. Since 2007 he pursued a specialisation in satellite radar polarimetry for wetland classification and monitoring in boreal and subarctic environments.
In addition to instructor-led online training, PCI Geomatics also allows personal, non-commercial use of the our SAR training materials to help you get acquainted with everything SAR has to offer and how to begin manipulating and interpreting SAR data with Geomatica.
Learn about real-world use cases for SAR imagery by viewing our on-demand SAR webinars. Discover operational examples of SAR data used for ice monitoring, flooding, oils spills, ship detection, agriculture, and also how SAR imagery can be fused with optical images.
Recent SAR webinars include:
- SAR Tools and Capabilities in Geomatica 2014
- Disaster Response Mapping with SAR Imagery
- Putting SAR Imagery to Work in Military Applications
- Crisis Monitoring and Flood Detection using SAR Imagery
Sample Imagery Available for Download
A RADARSAT-2 sample dataset is available for free download. The data includes RADARSAT-2 Quad Polarized modes, offered in 8m Fine Quad-Pol and 25m Standard Quad-Pol mode. Other beam modes of interest include the dual polarized modes, and our high-resolution, single polarized Ultra-Fine 3m mode.
- The Vancouver dataset offers the end-user the chance to evaluate RADARSAT-2 products from all the beam modes and for a variety of different applications that include urban mapping, marine surveillance, agricultural mapping, and infrastructure mapping. This geographic location offers varied terrain from the rugged mountains to the north of Vancouver, to the flat, agricultural lands of the Fraser River Delta.
- The Fine Quad-Pol dataset (Single Look Complex product) coincides with areas that have been the focus of other studies. Data is available for Altona, Canada - Flevoland, Netherlands - Oberfaffenhofen, Germany - San Francisco, USA - and The Straight of Gibraltar.
- An interferometric pair has also been made available in the sample dataset. The Ultra-Fine images making up this pair were acquired 24 days apart. These data are suitable for interferometric processing and have already been used to create a Digital Elevation Model. The scenes have good coherence over most of the image. The area features an arid region with little vegetation on the southern edge of Phoenix, Arizona, USA.
All of the datasets are available via free download on the MDA website here.
Applications with SAR imagery
Although SAR imagery differs considerably to optical data, it has the big advantage of being able to collect data regardless of illumination or adverse weather. Using multiple collections can provide rapid revisit and continued surveillance of key interest areas. Such an approach may be used to detect the presence of overland tracks from vehicles and targets of interest, in order to monitor potential areas of interest to build patterns and monitor activity triggering cross queuing opportunities to collect higher resolution information from UAVs.
Since SAR imagery can be collected in all-weather day/night conditions, it is well suited to collect updated information following emergencies such as flooding, and oil spills. Other applications are possible, however flooding and oil spills are particularly well suited to SAR imagery. Land / water interfaces are clearly observed on SAR imagery due to the sharp contract in surface scattering, whereas Oil/open ocean can be clearly identified under certain sea state conditions the oil having a wave suppression effect - which changes the surface scattering in the SAR imagery.
Optical sensors have traditionally been better suited than Radar to produce crop type / condition maps, mainly due to the sensitivity of the multispectral bands to the chlorophyll present in plants (increased reflectance in near infrared band). Radar sensors provide information on the structure of the plant, and to some extent the moisture content. With newer SAR imaging modes such as multiple polarization collections, more information is available on the structure of the crops and many techniques can be used to calculate signatures based on radar responses. Repeat pass Radar images can be used in conjunction with optical imagery to map crop types and assess conditions.
Seasat, the first earth observation satellite equipped with a Synthetic Aperture Radar instrument, launched in 1978, was designed to study oceanographic phenomena. The RADARSAT program was primarily conceived to provide all weather day/night imagery to map ice conditions and assist ship captains with near real time information to navigate the northern oceans. Maritime applications of SAR are among the most mature areas in Radar remote sensing. Information such as ice conditions (type, shape, thickness), ship positions, ship wakes, wind direction, oil seep and spill detection are very well suited to be observed in SAR imagery.
The petroleum and mineral resources sectors benefit from the collection of earth observation imagery, both for onshore and offshore exploration and mapping. SAR data can be used under ideal sea state conditions to detect the presence of naturally occurring oil seeps, this can help reduce the risks and costs associated with drilling. SAR data can also be used to derive geophysical terrain information due to the sensitivity of the imagery to topography, some of the geological structures of interest to exploration geologists are nicely highlighted, thus allowing discrimination of different geological units.
SAR imagery can used to derive Digital Elevation Models. Two commonly used techniques include Radargrammetry, and Interferommetry. Radargrammetry is a technique that uses same side stereo pairs collected over the same location on two different dates. The known parallax for the sensor across the two collections is used to derive elevation values for matching locations on the two images. Interferommetry another technique that that uses phase differences in the returned SAR signal to derive path length differences to a level of accuracy on the order of the wavelength (centimeter level). Each technique has its benefits and drawbacks - both are very useful to derive elevation in areas where persistent cloud cover is an issue. Sample DEMs derived from SAR data.