IEEE - Aerospace and Electronic Systems - January 2023 - 24

the target has moved out of the beam, no further synthetic
aperture can be formed. We call this stripmap imaging.
To overcome this limit on azimuth resolution we can
manoeuvre the whole spacecraft to continuously illuminate
a point on the ground, as the radar moves past. There is a
limit to this time, however, since the illumination time is
now actually longer than for the simple strip mode described
previously. In this, we can achieve a much larger
synthetic aperture and therefore, a much larger, along track,
resolution. This is called spotlight mode. Needless to say,
there are many variations ofthese modes, beyond the scope
ofthis article.
Most systems focus the image in the slant or radar
delay dimension. Some thought shows that the slant range
must be converted to the ground range dimension for
applications in Earth observation (EO).
Radar interferometry was first demonstrated using
Seasat data by Goldstein [8]. Two SAR images, taken
along the same track are carefully coregistered, and if
there have been no changes in the imaged objects, the
phase difference between each image pixel will be close
to zero. However, even tiny physical changes in the slant
range, equivalent to the fractions of the EM wavelength,
will show very strongly. We can extend this processing [7]
(using images with close but different baselines) to make
height maps, measure soil moisture, and so on.
USES OF SAR
The resulting images look very similar to optical images,
but the major difference is that the coherent source of radiation
means that areas on the ground that are very similar
in average, physical characteristics will have random variations
in amplitude. This is due to the fact that each pixel
in the image is in fact the summation of a number of randomly
placed scatterers within the resolution cell.
Furthermore, electromagnetic waves, depending on
their wavelength, can actually penetrate into, for example,
a forest stand, giving us more information about the structure
of the trees. In the case of longer wavelengths (for
example L band at 23 cm) the wave can actually penetrate
right through to this surface level and the resulting image
shows very distinctly the presence of those stems of the
trees. Figure 3 is an example ofan SAR image.
Another advantage ofSAR is that the transmission and
reception can reflect different polarizations or orientation
of the electromagnetic wave. Where the electric field is
vertical (vertical polarization) with respect to the terrain,
the illumination will interact more strongly with vertical
structures, such as the stems of trees, the stems of maize
plants. An horizontally polarized wave, however, will not
interact with these vertically oriented structures, and the
return will be much less then for the vertical polarization.
Radar polarimetry has become a very powerful tool in
24
fields, such as classification of crops, measurements of the
biomass of forests, measurements of soil moisture [10].
The original Seasat project was launched to make
measurements of the properties of the vast oceans of the
Earth. Ocean waves provide very distinctive returns to
such an imaging radar. In particular, calm water returns
very little signal. These regions show up as dark areas, but
the same applies when oil is present on the surface of the
ocean. Oil changes the surface tension of the water surface,
hindering small wave formation. SAR images are
now used routinely to locate oil spills either accidental or
in violation of international shipping rules [11].
The monitoring of the large rainforests of the Earth,
very important for carbon removal from the atmosphere,
is becoming essential to our fight against global warming.
In addition, monitoring the large monocrops, of the United
States, Russian Federation and Ukraine, are important for
food security on the Earth. In a more modern context, continuous
monitoring applies to applications, such as monitoring
crops for crop insurance applications, monitoring
mines for conformance to rehabilitation standards, growth
of urban sprawl in developing nations.
Returning to oceanic applications, currents and other
hydrological effects can be clearly seen in SAR images
and all of us are very important measure of the transport
of water of different temperatures in the oceans, that influence
the global weather patterns.
The ability of SAR to make images regardless of
weather conditions, day and night, clearly makes the sensor
important to national security concerns, since the
movement of vehicles and other assets can be clearly
seen. Many of the existing SAR systems were funded and
funded by security agencies.
PREHISTORY
The ability to build coherent radars (pulse Doppler) is the
basis of early moving target detection systems that emerged
from the Second WorldWar. Given the difficulty ofcarrying
large airborne antennas and thereby realising narrow beams
for accurate bearing determination, Carl Wiley and colleagues
at Goodyear realized that Doppler could be used to
discriminate targets and clutter illuminated at different angles
within a broad, forward looking, beam. This lead to the first
patent [12], although clearly this idea was widely researched,
independently. Wiley's patent is from the viewpoint of
exploiting the Doppler shift, rather than a synthetic aperture.
An early constraint on the image formation process
described in the " What is SAR? " section was that recording
the range profiles of many along track samples was a challenge
for the analogue world, making synthetic apertures
difficult to process. Photographic film was an early contender
with lenses used to provide the along track focus for
the synthetic aperture. Despite these constraints, numerous
IEEE A&E SYSTEMS MAGAZINE
JANUARY 2023
IEEE - Aerospace and Electronic Systems - January 2023

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