Joint Education Initiative/Joint Education Initiative.iso/dos/user_doc/slar.txt

Text File | 1990-09-11 | 12KB | 237 lines

                    A Primer on Side-Looking Airborne Radar

            Side-looking  airborne radar (SLAR) images are so  different
       from  other  types  of  remotely sensed data,  in  both  mode  of
       acquisition  and  techniques for  interpretation,  that  a  short
       explanation  of important concepts and the use of this technology
       in the earth sciences will aid potential users.

            SLAR  is an electronic image-producing system  that  derives
       its  name  from  the  fact that the  radar  beam  is  transmitted
       perpendicular  to the ground track of the aircraft acquiring  the
       data.   The result is an obliquely illuminated plan (or vertical)
       view of the terrain, a view that enhances subtle surface features
       and   facilitates   geologic  interpretation.    This   enhancing
       characteristic  is  one  of the reasons why SLAR  imagery  is  so
       useful  to  earth resource scientists and  managers  involved  in
       mineral  and  energy  exploration,  earth  hazards  studies,  and
       diverse other geologic, hydrologic, cartographic, and engineering
       applications.

            Another  important property of SLAR is that it is an  active
       system  that provides its own source of illumination in the  form
       of  microwave energy;  thus imagery can be obtained either day or
       night.   Since  SLAR penetrates most clouds,  it can be  used  to
       prepare image-base maps of perpetually cloud-covered areas of the
       world   where  collecting  conventional  aerial  photographs   is
       impractical, such as over the rain forests of Brazil or along the
       Aleutian  Arc of Alaska.   However,  precise topographic  mapping
       using SLAR data is not currently practical because of uncorrected
       geometric distortions inherent in radar imagery.  For example, in
       mountainous  terrain,  positional error may be as much as several
       hundred meters.

            Since the radar signal is transmitted at a small  depression
       angle below the horizontal plane in which the aircraft is flying,
       the  signal strikes the terrain at an rather flat angle,  and the
       surficial  expression  of  the geologic  structure  may  thus  be
       enhanced.   The  topographic expression of some surface features,
       such as subtle faults and folds,  may be more clearly seen on the
       radar image than on conventional aerial photographs or  satellite
       images.    For   example,   a   depression  angle  ranging   from
       approximately  10 to 40 degrees across the imaged swath  is  used
       for  many earth-science applications.   The change in  depression
       angle  across  the  imaged  swath  (a swath  having  a  width  of
       approximately  40  km  is used  by  some  commercially  available
       systems) results in features nearer the flightline having shorter
       radar  shadows than features of equal elevation farther from  the
       flightline.   Since  SLAR is an active system,  when a radar beam
       strikes a sufficiently elevated terrain feature,  a radar  shadow
       is cast by the feature; this is an area of no data return.





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            The  gradual  change  of  shadow  length  across  the  range
       perpendicular  to  the flight path (and parallel to the width  of
       the  image swath) has resulted in the convention  of  designating
       the halves of the radar swath as either near range,  or that half
       of the radar swath nearest the flightline;  or as far range, that
       half  of the swath farthest from the flightline.   Azimuth is the
       term  used to describe the direction of the radar image  parallel
       to the flightline, or the bearing of the flightline itself.

            The  SLAR  products generally used for  analysis  are  image
       strips  and  mosaics.    SLAR  images,  whether  photographic  or
       digital,  are  the  graphic  representations of  the  SLAR  data.
       Usually,  the  strips  are much longer in azimuth than in  range,
       since  it is more efficient to fly long  continuous  flightlines.
       Photographic  copies  of  the strips are  generally  regarded  as
       better  than  photomosaics  for interpretation for  two  reasons:
       they  are  at  least one photographic generation  closer  to  the
       original  than  subsequent products such as  image  mosaics,  and
       strips  are  not  cosmetically  altered  to  produce  a  pleasing
       composite  image as is sometimes done in the mosaicking  process.
       Digital  files retain much more of the recorded dynamic range  of
       the data than do photographic copies.   For example, photographic
       images are generally limited to a dynamic range of about 15 db by
       the performance of the photographic emulsion;  whereas an  8-bit-
       per-pixel  digital image file holds an available dynamic range of
       30-40 db.

            SLAR  mosaics provide a synoptic view of  the  terrain,  but
       both the resolution and information content are slightly degraded
       in mosaic preparation.   In addition,  on mosaics, differences in
       length  of shadow from terrain features,  produced by the changes
       in  depression angle across the range of the image  swath,  occur
       across  mosaic junction lines between  adjoining  strips.   These
       differences  in length of shadow,  as well as possible variations
       in the radar returns from similar features in near and far range,
       can   result   in  misinterpretation  of  terrain   and   surface
       characteristics.

            It should be noted that positive photographic transparencies
       of  both  image  strips  and  mosaics  are  generally  considered
       superior  to photographic paper for interpretation,  because  the
       film  has  a greater density range and  therefore  contains  more
       information.   Again, it should be noted that digital files still
       generally   contain  a  greater  dynamic  range  of  information.
       Because  image  strips generally have an overlap of  60  percent,
       both  near- and far-range mosaics can be prepared by  laying  the
       respective  halves  of adjacent SLAR image strips side  by  side.
       These  two  SLAR  presentations complement each other  since  the
       near-range  data  have less radar shadowing while  the  far-range
       data have more surface enhancement.





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            The   look  direction  of  a  radar  image  refers  to   the
       illumination direction; for example, west looking means the radar
       beam was transmitted to the west.   The choices of mission design
       parameters  such  as  look direction  and  depression  angle  are
       usually  based  on an analysis of the geologic structure  of  the
       area.   The  parameters  are then chosen to assure  optimum  data
       acquisition  for  the goals of the study.   For  example,  linear
       structures such as faults that are parallel or nearly parallel to
       the range,  or look,  direction may not be easily detected  since
       little radar shadow is present and enhancement is minimized.

            In  analyzing radar imagery,  the image is oriented with the
       shadows on the side of the feature toward the  viewer.   "Shadows
       stab stomach" is the old adage used by radar interpreters.   This
       practice  assists  in interpreting hills as hills and valleys  as
       valleys.

            Most  commercially available SLAR systems operate in the  X-
       band  at frequencies of 12-18 GHz and wavelengths of 2.4-3.8  cm.
       Usually these systems transmit and receive horizontally polarized
       signals  (HH).   For some purposes it is advantageous to  acquire
       data with an experimental radar system that operates at more than
       one  frequency  and  utilizes  a  vertically  polarized   (either
       transmitted  or received or both) radar beam;  radar data can  be
       like- (HH  or VV) or cross- (HV or VH) polarized depending on the
       antenna  design.   In addition to X-band,  other  frequencies  in
       current  use are C-band at 8-4 GHz and 3.8-7.6 cm,  and L-band at
       2-1 GHz and 15-30 cm.

            SLAR is a somewhat generic term for two distinctly different
       radar antenna technologies:   real-aperture radar,  also known as
       "brute force";  and synthetic-aperture radar (SAR), also known as
       coherent radar.

            In a real-aperture system, a fan-shaped beam is transmitted,
       reflected by the surface,  received, processed, and recorded as a
       line on the image.   Although the angle of the transmitted  radar
       beam  is constant (for example,  0.45 degrees),  the width of the
       beam  is narrower in the near range than in the  far  range,  and
       thus  the resolution in the flightline direction is better in the
       near range than the far range.  Resolution in the range direction
       (perpendicular  to  the  flightline)  is  constant.    Using  our
       previous  example,  portions of the data in the near range  would
       have  a resolution of approximately 30x75 m in range and  azimuth
       respectively,  while  portions  of  the far range  would  have  a
       resolution of approximately 30x150 m.

            Synthetic-aperture  radar  has constant  range  and  azimuth
       resolutions  through the image.   This constancy is  accomplished
       primarily  by  using more detailed information processing of  the
       returned  signal,  thus simulating a longer  antenna.   A  longer
       antenna produces a narrower radar beam, improving the resolution.
       Presently  available synthetic-aperture systems have  resolutions
       of approximately 10x10 m or better.

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            It  must be noted that resolution and detectability are  not
       the same thing with radar;   objects of less than 1 m size may be
       routinely  detected  because of the strong radar return  of  some
       features.   Bright  radar returns may be caused by such things as
       corner-reflector  geometry,   electrical  resonance  effects,  or
       electronic interference.

            SLAR data are particularly valuable when used in conjunction
       with  traditional  earth-science  data,  as well  as  with  other
       remotely   sensed  data.    Scientists  from  private   industry,
       government,  and  university have effectively used SLAR all  over
       the  world  to detect and map  previously  undiscovered  geologic
       features  that  have contributed to the discovery of new  mineral
       and energy reserves.  More than 25 million square km of SLAR data
       have been gathered for government and private sector interests in
       many countries,  including  Brazil,  Canada,  Columbia,  Ecuador,
       Guinea, Indonesia, Japan, Nigeria, Peru, and the United States.






































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