Digital Rectification for Slar Imagery
Wen wogen, wang Xuquin, Liu Yuxian
Research Institute of Surveying and Mapping
Beijing , China
Abstract
A SLAR imagery is a slant-range imagery with scanning dynamically. There are the deformation differing from air-photograph. This paper discusses how to pixel to pixel for SLAR imagery with a DTM. The experimental results are Mx = 12.8 meter and My= 11.0 meter using SLAR imagery revolution 0f 3-meter.
Introduction
Imaging radar transmits itself the pulses of microwave energy to shine the earth. It operates entirely independent of sunlight. So it has strongly penetrating power of cloud, rain and fog as well as imaging capability of all weather and day and night.
A side-looking airborne radar (SLAR) of synthetic aperture has a high resolution, sharp imagery and big covered area. The SLAR imagery, therefore, has wide application potentials . The experts, in early 1970s , had successfully experimented on mapping of the Amazon valley using SLAR imagery . Later on many countries had extended such an application. At that time, the mapping scales almost were 1:250,000 to 1:100,000. Digital rectification we do now is a primal part for the scale of 1:50,000 mapping revision with SLAR imagery.
Geometric Analysis of Slar Imagery
A SLAR imagery is created by which the distance (R) and its change rate (Dr) are determined measuring the delay time of pulses and Doppler shift. Owing to the result of radar ranging, there is an imagery point for the different ground points. It is called as radar layover. At the same time, the geometric deformation of SLAR imagery will be generated by a variety of errors of radar system, flying behavious of aircraft and environments. Most of the operating errors of the radar system will be compensated. The environmental influences are, however, still existence.
For cartography, first of all the ortho-position of SLAR imagery must be solved. Suppose the terrain surface is a flat one, due to distant projection, the imaging scale of the slant-range imagery is not uniform, the further off an antenna a target, the bigger a scale of imagery, and the nearer
an antenna, the smaller a scale. This is called as the scale compression. The larger the depression angle of antenna, the more serious of compression (M ‘ = M/cos (phi). This is a non-linear in-fluence. So transforming the slant-range imagery to distant imagery have the error of the scale.
The terrain relief, however, is quite general case and the displacement of imagery is always come into existence. The displacement of imagery generated by terrain relief are taken place always in the side looking plane and parallel one another. Suppose a hill called as ABC shown as figure 1. There are the foreslop surface AB and the foreslop angle (alpha), the backslop surface BC and the backslop angle (bet) and an elevation h above the average sea level. If (alpha) >) theta, u)
Fig. 1-a
Fig. 1-b
(Fig. 1-a), the imagery of the slop surface AB and BC will generate the layover each other. If (beta) > (theta.d) (Fig.1-b), the radar beam does not illuminate the backslop surface. So the shadow BsDs is generated by the backslop surface BC and the flat ground CD. if (beta) > = (theta d) the backslop surface BC is fully illuminated with the radar beam and then non-shadow will be take place. Generally speaking, the displacement and shadow of the SLAR imagery is a function of the terrain relief except the radar line. So it is the best to perform fine geometric rectification with a DTM.
