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 »
SAR Imagery: Is there a way to focus it?
October 15th, 2013 by M. Lorraine Tighe, PhD
Synthetic aperture radar or SAR imagery can be challenging for non-radar geeks to figure out what exactly the SAR image is illustrating. Of course our eyes have little trouble understanding aerial photo images primarily because the cameras used to collected photos operate at similar wavelengths (located in the visible portion of the electromagnetic (EM) spectrum) to our eyes. SAR sensors on the other hand, operate in the microwave portions of the EM spectrum which is very different from how our eyes see.
A SAR or radar image is formed from the mapping of observed radar signals reflected from the terrain and recorded by a radar sensor. For radar systems with digital image processing, like that of Intermap®’s or ASTRIUM’s radar systems, the SAR image consists of a file of digital numbers (DNs) assigned to spatial positions in a grid of pixels (picture elements). Each pixel represents the amount of EM energy recorded by the radar sensor over a known area and is denoted by the DN. See Figure 1.
Figure 1. Illustrates the relationship among a raster radar image, image pixels, and DNs. This illustration is comprised of a radar image (far left view of a residential area under construction), a close-up of a section of the radar image to illustrate the radar image pixels (middle image), and the DN for the corresponding image pixels (far right image).
Hopefully you can interpret this radar image using the content presented in my Radar interpretation blog published on September 3rd . But maybe if you had the advantage of requested a higher resolution SAR image you might notice that it seems as if you put on radar classes because the higher resolution image looks more in focus? Yes? Check out the different resolution images below in Figures 2 and 3 for yourself. In both cases, the left images were collected with 135 MHz (1.25 m pixel resolution) whereas the right images were collected with 270 MHz (0.625 m pixel resolution).
Two SAR images over the same area. Left Image: 1.25 m pixel resoltution; Right Image: 0.625 m pixel resolution
Tags: SAR imagery
Category: Geospatial Information