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Remote sensing for resources development and environmental management
Damen, M. C. J.

Symposium on Remote Sensing for Resources Development end Environmental Management / Enschede / August 1986
Oil drums as resolution targets for quality control
of radar survey data
International Institute for Aerospace Survey and Earth Sciences (ITC), Enschede, Netherlands
ABSTRACT: Spatial resolution in radar imagery is dependent on angular discrimination in along-track (azimuth)
direction. Range discrimination depends on a time delay measurement, using either the duration of the pulse
or a frequency-modulated pulse.
To check effective spatial resolution of Synthetic Aperture Radar (SAR) or Real Aperture Radar (RAR) data,
measurements can be made on the response distribution of a point target. This method, used for digital data,
can assess only the peak width to the background and no effect of side—lobes or geometric distortion is taken
into account. For optically correlated imagery, the method is less applicable.
A pragmatic approach for control of spatial resolution is to measure the smallest distance between two point
targets having approximately the same response strength, which are still visible as two objects on the image.
For this purpose, a number of point targets of materials easily and cheaply available everywhere in the world
(oil drums cut in half lengthwise) were used in a test site during ESA's SAR-580 experiment in Southern
Spain. From their separate identification, a good deduction could be made regarding the effective spatial
resolution in range and azimuth of the imagery.
The SARTHI project - (a side-looking radar and IR
false colour survey over the Iberian Pyrite belt
(Koopmans et al. 1985) - was part of the European
SAR-580 experiment carried out during 1981-84.
During the radar survey flight, an experiment was
carried out with drum reflectors to check the use
fulness of a number of point targets to determine
the spatial resolution of the radar.
The definition of spatial resolution in its simplest
form is: "the minimum distance between two point-
sources (objects) of equal-intensity that a sensor
can record separately". The definition is not always
used in this sense, however, and depending on the
system, other parameters may be taken into consider
Forshaw et al. 1980, recognized four different
types of identification of spatial resolution:
1. Geometric properties of the imaging system.
2. Ability to distinguish between point targets.
3. Ability to measure periodicity of repetitive
4. Ability to measure spectral properties of small
finite objects.
The first type giving the geometric properties of
the imaging system will be briefly mentioned here
and compared with results measured according to the
second type: ability to distinguish point targets.
The third type, ability to measure repetive targets,
deals mainly with the optical approach of defining
spatial resolution in line pairs per milli-meter and
will not be treated here.
For side-looking radar, the spatial resolution on
the basis of geometric properties in range is depen
dent on the half pulse-length and the depression
angle; the latter is variable over the image swath:
Rgr = .. _
^ Z cos ts
where Rgr = resolution in ground range
c = velocity of light
t = pulse duration
3 = depression angle
Here we are dealing with a range discrimination
based on measurement of a time delay (duration that
the pulse is emitted). To improve range resolution,
the pulse may be frequency-modulated.
In azimuth, the spatial resolution depends on the
half-power beam-width and the range towards the
object. It is an angular discrimination, hence the
beam-width increases with range.
The following equation is valid for a real aper
ture radar:
where Ra = resolution in azimuth
X = wavelength
Rs = slant range to target
Da = diameter of aperture in wavelengt
For a focussed synthetic aperture radar system,
the azimuth is one-half of the physical length of
the antenna and is independent of range distance.
This is theoretical, however, and azimuth resolu
tion measured according to the second type is often
"Spatial resolution" should not be confused with
"detectability". Objects smaller than a ground
resolution cell may be detectable if the small ob
ject has a strong radiance or backscatter, forming
sufficient contrast relative to its background.
Well-known examples of such detectable objects
are metal fences, corner reflectors, Lunenburger
lenses, or other radar beacons.
For the European SAR-580, the effective spatial
resolution of the SAR imagery was measured on the
basis of the response distribution of point targets
(Smith, 1985) (type 4 of Forshaw's spatial resolu
tion identification). Algorithms were used for
digital data to select or reject a number of appro
priate point targets and to calculate the average
width of the response function at 3 dB below their
peak value. This method assessed only the peak
width of the point target to the background and no
effect of antenna side-lobes or geometric distor
tion was taken into account. For imagery which is
not in digital form, the method is less appli
The results of these measurements for data corre
lated at the Royal Aircraft Establishment (RAE) at