M. Lorraine Tighe, PhD
Dr. Tighe has a Ph.D. in Earth Sciences, a graduate degree in Remote Sensing and GIS, and a B.Sc. in Physics and Geology. Dr. Tighe has delivered lectures ranging from a half day workshop to a 4 week training program to over 2000 participants in USA, Canada, Jamaica, Brazil, Ecuador, Honduras, … More »
Radar Image Interpretation
September 3rd, 2013 by M. Lorraine Tighe, PhD
Synthetic aperture radar (SAR) sensors “see” the ground in a different way from optical sensors such as SPOT, aerial cameras, or the human eye; therefore, radar images have certain characteristics that are fundamentally different from those in images collected by optical sensors. The key then to understanding and interpreting radar images lies in the answer to this question: What happens to the electromagnetic energy in a radar pulse as it meets the terrain being imaged, interacts with the terrain, is recorded by the radar sensor, and subsequently is processed to generate a radar image? Answering this question is not easy. However, with the resolution of SAR sensors are getting better and better, details provided in, for example a 50 cm pixel, help our eyes to discern topographic features more readily than ever before.
Pictured here is a 50 cm pixel resolution NEXTMap® orthorectified radar image for a typical suburban development in North America (Colorado, U.S.A.). The high resolution imagery provides great detail about the structure of the residential features which in turn allows your eyes to quickly conclude that this represents a residential area. Taking a closer look at the SAR or radar image will demonstrate why it is important to understand how a SAR signal interacts with the residential features being imaged.
Recall that the gray levels of a radar image are related to the relative strength of the microwave energy backscattered by the terrain being imaged. Tightly knitted homes appear on the radar image as square-shaped, bright image toned features that are evenly spaced. The bright tones are the result of much of the original SAR signal making its way back to signal recorder. The primary and secondary roads, on the other hand, appear dark in image tone, smooth in texture, and exhibit a uniform, linear shape. The roads are dark in image tone because much of the transmitted SAR signal is reflected away from the signal recorded. The larger objects with an overall bright image tone in the center top and center bottom portion of the image are most likely schools, due to their proximity to the homes and because they are large complexes surrounded by sports facilities such as tennis courts, baseball fields, or tracks. We can also see that the intensity of the radar signal is affected by radar look direction where brighter images tones will come from those city blocks oriented parallel and/or perpendicular to the radar illumination (houses along the streets oriented north–south). In contrast, those streets oriented oblique to the radar look direction appear light to medium gray image tone. Can you tell what the radar look direction is? A short lesson on SAR image interpretation to hopefully spark a conversation about what your thoughts are on the skill set required to interpret high resolution SAR images.
Category: Geospatial Information