The reason I quoted this piece is synthetic aperture radar allows a drone in real time to provide images through clouds, rain or fog, and in daytime or nighttime conditions which basically means a drone fitted with this device can be above clouds in day or night and see almost anything that moves or doesn't move anywhere on earth.
Synthetic aperture radar
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This radar image acquired by the SIR-C/X-SAR radar on board the
Space Shuttle Endeavour shows the
Teide
volcano. The city of Santa Cruz de Tenerife is visible as the purple
and white area on the lower right edge of the island. Lava flows at the
summit crater appear in shades of green and brown, while vegetation
zones appear as areas of purple, green and yellow on the volcano's
flanks.
Synthetic aperture radar (
SAR) is a form of
radar which is used to
create images
of objects, such as landscapes – these images can be either two or
three dimensional representations of the object. SAR uses the motion of
the radar antenna over a targeted region to provide finer spatial
resolution than is possible with conventional beam-scanning radars. SAR
is typically mounted on a moving platform such as an aircraft or
spacecraft, and has its origins in an advanced form of
side-looking airborne radar (SLAR).
The distance the SAR device travels over a target in the time taken for
the radar pulses to return to the antenna creates the large "synthetic"
antenna aperture
(the "size" of the antenna). As a rule of thumb, the larger the
aperture is, the higher the image resolution will be, regardless of
whether the aperture is physical (a large antenna) or 'synthetic' (a
moving antenna) – this allows SAR to create high resolution images with
comparatively small physical antennas.
To create a SAR image, successive pulses of
radio waves are transmitted to "illuminate" a target scene, and the
echo of each pulse is received and recorded. The pulses are transmitted and the echoes received using a single
beam-forming antenna, with
wavelengths
of a meter down to several millimeters. As the SAR device on board the
aircraft or spacecraft moves, the antenna location relative to the
target changes with time.
Signal processing
of the successive recorded radar echoes allows the combining of the
recordings from these multiple antenna positions – this process forms
the 'synthetic antenna aperture', and allows the creation of higher
resolution images than would otherwise be possible with a given physical
antenna.
[1]
Current (2010) airborne systems provide resolutions of about 10 cm,
ultra-wideband systems provide resolutions of a few millimeters, and experimental
terahertz SAR has provided sub-millimeter resolution in the laboratory.
[citation needed]
SAR images have wide applications in remote sensing and mapping of
the surfaces of both the Earth and other planets. SAR can also be
implemented as
inverse SAR by observing a moving target over a substantial time with a stationary antenna.
Functional principle
![[icon]](https://upload.wikimedia.org/wikipedia/commons/thumb/1/1c/Wiki_letter_w_cropped.svg/20px-Wiki_letter_w_cropped.svg.png) |
This section requires expansion with: a short description of the general functional principle of SAR, with illustrative images. (June 2014) |
Algorithm
The SAR algorithm, as given here, applies to phased arrays generally.
A three-dimensional array (a volume) of scene elements is defined
which will represent the volume of space within which targets exist.
Each element of the array is a cubical
voxel
representing the probability (a "density") of a reflective surface
being at that location in space. (Note that two-dimensional SARs are
also possible—showing only a top-down view of the target area).
Initially, the SAR algorithm gives each voxel a density of zero.
Then, for each captured waveform, the entire volume is iterated. For a
given waveform and voxel, the distance from the position represented by
that voxel to the antenna(e) used to capture that waveform is
calculated. That distance represents a time delay into the waveform. The
sample value at that position in the waveform is then added to the
voxel's density value. This represents a possible echo from a target at
that position. Note that there are several optional approaches here,
depending on the precision of the waveform timing, among other things.
For example, if phase cannot be accurately known, then only the envelope
magnitude (with the help of a
Hilbert transform)
of the waveform sample might be added to the voxel. If polarization and
phase are known in the waveform, and are accurate enough, then these
values might be added to a more complex voxel that holds such
measurements separately.
After all waveforms have been iterated over all voxels, the basic SAR processing is complete.
What remains, in the simplest approach, is to decide what voxel
density value represents a solid object. Voxels whose density is below
that threshold are ignored. Note that the threshold level chosen must at
least be higher than the peak energy of any single wave—otherwise that
wave peak would appear as a sphere (or ellipse, in the case of
multistatic operation) of false "density" across the entire volume. Thus
to detect a point on a target, there must be at least two different
antenna echoes from that point. Consequently, there is a need for large
numbers of antenna positions to properly characterize a target.
The voxels that passed the threshold criteria are visualized in 2D or
3D. Optionally, added visual quality can sometimes be had by use of a
surface detection algorithm like
marching cubes.
More complex operation
The basic design of a synthetic aperture radar system can be enhanced
to collect more information. Most of these methods use the same basic
principle of combining many pulses to form a synthetic aperture, but may
involve additional antennas or significant additional processing.
Multistatic operation
SAR requires that echo captures be taken at multiple antenna
positions. The more captures taken (at different antenna locations) the
more reliable the target characterization.
Multiple captures can be obtained by moving a single antenna to
different locations, by placing multiple stationary antennas at
different locations, or combinations thereof.
The advantage of a single moving antenna is that it can be easily
placed in any number of positions to provide any number of monostatic
waveforms. For example, an antenna mounted on an airplane takes many
captures per second as the plane travels.
The principal advantages of multiple static antennas are that a
moving target can be characterized (assuming the capture electronics are
fast enough), that no vehicle or motion machinery is necessary, and
that antenna positions need not be derived from other, sometimes
unreliable, information. (One problem with SAR aboard an airplane is
knowing precise antenna positions as the plane travels).
For multiple static antennas, all combinations of monostatic and
multistatic radar
waveform captures are possible. Note, however, that it is not
advantageous to capture a waveform for each of both transmission
directions for a given pair of antennas, because those waveforms will be
identical. When multiple static antennas are used, the total number of
unique echo waveforms that can be captured is
where
N is the number of unique antenna positions.
Polarimetry
Main article:
Polarimetry
Radar waves have a
polarization. Different materials reflect radar waves with different intensities, but
anisotropic
materials such as grass often reflect different polarizations with
different intensities. Some materials will also convert one polarization
into another. By emitting a mixture of polarizations and using
receiving antennas with a specific polarization, several images can be
collected from the same series of pulses. Frequently three such RX-TX
polarizations (HH-pol, VV-pol, VH-pol) are used as the three color
channels in a synthesized image. This is what has been done in the
picture at right. Interpretation of the resulting colors requires
significant testing of known materials.
New developments in polarimetry include using the changes in the
random polarization returns of some surfaces (such as grass or sand) and
between two images of the same location at different times to determine
where changes not visible to optical systems occurred. Examples include
subterranean tunneling or paths of vehicles driving through the area
being imaged. Enhanced SAR sea oil slick observation has been developed
by appropriate physical modelling and use of fully polarimetric and
dual-polarimetric measurements.
Interferometry
Rather than discarding the phase data, information can be extracted
from it. If two observations of the same terrain from very similar
positions are available,
aperture synthesis
can be performed to provide the resolution performance which would be
given by a radar system with dimensions equal to the separation of the
two measurements. This technique is called
Interferometric SAR or InSAR.
If the two samples are obtained simultaneously (perhaps by placing
two antennas on the same aircraft, some distance apart), then any phase
difference will contain information about the angle from which the radar
echo returned. Combining this with the distance information, one can
determine the position in three dimensions of the image pixel. In other
words, one can extract terrain altitude as well as radar reflectivity,
producing a
digital elevation model (DEM) with a single airplane pass. One aircraft application at the
Canada Centre for Remote Sensing
produced digital elevation maps with a resolution of 5 m and altitude
errors also on the order of 5 m. Interferometry was used to map many
regions of the Earth's surface with unprecedented accuracy using data
from the
Shuttle Radar Topography Mission.
If the two samples are separated in time, perhaps from two flights
over the same terrain, then there are two possible sources of phase
shift. The first is terrain altitude, as discussed above. The second is
terrain motion: if the terrain has shifted between observations, it will
return a different phase. The amount of shift required to cause a
significant phase difference is on the order of the wavelength used.
This means that if the terrain shifts by centimeters, it can be seen in
the resulting image (a
digital elevation map must be available to separate the two kinds of phase difference; a third pass may be necessary to produce one).
This second method offers a powerful tool in
geology and
geography.
Glacier flow can be mapped with two passes. Maps showing the land deformation after a minor
earthquake or after a
volcanic eruption (showing the shrinkage of the whole volcano by several centimeters) have been published (where?).
Differential interferometry
Differential interferometry (D-InSAR) requires taking at least two
images with addition of a DEM. The DEM can be either produced by GPS
measurements or could be generated by interferometry as long as the time
between acquisition of the image pairs is short, which guarantees
minimal distortion of the image of the target surface. In principle, 3
images of the ground area with similar image acquisition geometry is
often adequate for D-InSar. The principle for detecting ground movement
is quite simple. One interferogram is created from the first two images;
this is also called the reference interferogram or topographical
interferogram. A second interferogram is created that captures
topography + distortion. Subtracting the latter from the reference
interferogram can reveal differential fringes, indicating movement. The
described 3 image D-InSAR generation technique is called 3-pass or
double-difference method.
Differential fringes which remain as fringes in the differential
interferogram are a result of SAR range changes of any displaced point
on the ground from one interferogram to the next. In the differential
interferogram, each fringe is directly proportional to the SAR
wavelength, which is about 5.6 cm for ERS and RADARSAT single phase
cycle. Surface displacement away from the satellite look direction
causes an increase in path (translating to phase) difference. Since the
signal travels from the SAR antenna to the target and back again, the
measured displacement is twice the unit of wavelength. This means in
differential interferometry one fringe cycle −
π to +
π
or one wavelength corresponds to a displacement relative to SAR antenna
of only half wavelength (2.8 cm). There are various publications on
measuring subsidence movement, slope stability analysis, landslide,
glacier movement, etc. tooling D-InSAR. Further advancement to this
technique whereby differential interferometry from satellite SAR
ascending pass and descending pass can be used to estimate 3-D ground
movement. Research in this area has shown accurate measurements of 3-D
ground movement with accuracies comparable to GPS based measurements can
be achieved.
Ultra-wideband SAR
Conventional radar systems emit bursts of radio energy with a fairly
narrow range of frequencies. A narrow-band channel, by definition, does
not allow rapid changes in modulation. Since it is the change in a
received signal that reveals the time of arrival of the signal
(obviously an unchanging signal would reveal nothing about "when" it
reflected from the target), a signal with only a slow change in
modulation cannot reveal the distance to the target as well as can a
signal with a quick change in modulation.
Ultra-wideband
(UWB) refers to any radio transmission that uses a very large bandwidth
– which is the same as saying it uses very rapid changes in modulation.
Although there is no set bandwidth value that qualifies a signal as
"UWB", systems using bandwidths greater than a sizable portion of the
center frequency (typically about ten percent, or so) are most often
called "UWB" systems. A typical UWB system might use a bandwidth of
one-third to one-half of its center frequency. For example, some systems
use a bandwidth of about 1 GHz centered around 3 GHz.
There are as many ways to increase the bandwidth of a signal as there
are forms of modulation – it is simply a matter of increasing the rate
of that modulation. However, the two most common methods used in UWB
radar, including SAR, are very short pulses and high-bandwidth chirping.
A general description of chirping appears elsewhere in this article.
The bandwidth of a chirped system can be as narrow or as wide as the
designers desire. Pulse-based UWB systems, being the more common method
associated with the term "UWB radar", are described here.
A pulse-based radar system transmits very short pulses of
electromagnetic energy, typically only a few waves or less. A very short
pulse is, of course, a very rapidly changing signal, and thus occupies a
very wide bandwidth. This allows far more accurate measurement of
distance, and thus resolution.
The main disadvantage of pulse-based UWB SAR is that the transmitting
and receiving front-end electronics are difficult to design for
high-power applications. Specifically, the transmit duty cycle is so
exceptionally low and pulse time so exceptionally short, that the
electronics must be capable of extremely high instantaneous power to
rival the average power of conventional radars. (Although it is true
that UWB provides a notable gain in
channel capacity over a narrow band signal because of the relationship of bandwidth in the
Shannon–Hartley theorem and because the low receive duty cycle receives less noise, increasing the
signal-to-noise ratio,
there is still a notable disparity in link budget because conventional
radar might be several orders of magnitude more powerful than a typical
pulse-based radar.) So pulse-based UWB SAR is typically used in
applications requiring average power levels in the microwatt or
milliwatt range, and thus is used for scanning smaller, nearer target
areas (several tens of meters), or in cases where lengthy integration
(over a span of minutes) of the received signal is possible. Note,
however, that this limitation is solved in chirped UWB radar systems.
The principal advantages of UWB radar are better resolution (a few millimeters using
commercial off-the-shelf electronics) and more spectral information of target reflectivity.
Doppler-beam sharpening
Doppler Beam Sharpening commonly refers to the method of processing
unfocused real-beam phase history to achieve better resolution than
could be achieved by processing the real beam without it. Because the
real aperture of the radar antenna is so small (compared to the
wavelength in use), the radar energy spreads over a wide area (usually
many degrees wide in a direction orthogonal (at right angles) to the
direction of the platform (aircraft)). Doppler-beam sharpening takes
advantage of the motion of the platform in that targets ahead of the
platform return a Doppler upshifted signal (slightly higher in
frequency) and targets behind the platform return a Doppler downshifted
signal (slightly lower in frequency).
The amount of shift varies with the angle forward or backward from
the ortho-normal direction. By knowing the speed of the platform, target
signal return is placed in a specific angle "bin" that changes over
time. Signals are integrated over time and thus the radar "beam" is
synthetically reduced to a much smaller aperture – or more accurately
(and based on the ability to distinguish smaller Doppler shifts) the
system can have hundreds of very "tight" beams concurrently. This
technique dramatically improves angular resolution; however, it is far
more difficult to take advantage of this technique for range resolution.
(See
pulse-doppler radar).
Chirped (pulse-compressed) radars
Further information:
Chirp
A common technique for many radar systems (usually also found in SAR systems) is to "
chirp"
the signal. In a "chirped" radar, the pulse is allowed to be much
longer. A longer pulse allows more energy to be emitted, and hence
received, but usually hinders range resolution. But in a chirped radar,
this longer pulse also has a frequency shift during the pulse (hence the
chirp or frequency shift). When the "chirped" signal is returned, it
must be correlated with the sent pulse. Classically, in analog systems,
it is passed to a dispersive delay line (often a
SAW
device) that has the property of varying velocity of propagation based
on frequency. This technique "compresses" the pulse in time – thus
having the effect of a much shorter pulse (improved range resolution)
while having the benefit of longer pulse length (much more signal
returned). Newer systems use digital pulse correlation to find the pulse
return in the signal.
Typical operation
NASA's AirSAR instrument is attached to the side of a
DC-8
In a typical SAR application, a single radar antenna is attached to
an aircraft or spacecraft so as to radiate a beam whose wave-propagation
direction has a substantial component perpendicular to the flight-path
direction. The beam is allowed to be broad in the vertical direction so
it will illuminate the terrain from nearly beneath the aircraft out
toward the horizon.
Resolution in the range dimension of the image is accomplished by
creating pulses which define very short time intervals, either by
emitting short pulses consisting of a carrier frequency and the
necessary sidebands, all within a certain bandwidth, or by using longer "
chirp pulses"
in which frequency varies (often linearly) with time within that
bandwidth. The differing times at which echoes return allow points at
different distances to be distinguished.
The total signal is that from a beamwidth-sized patch of the ground.
To produce a beam that is narrow in the cross-range direction
[clarification needed],
diffraction
effects require that the antenna be wide in that dimension. Therefore,
the distinguishing, from each other, of co-range points simply by
strengths of returns that persist for as long as they are within the
beam width is difficult with aircraft-carryable antennas, because their
beams can have linear widths only about two orders of magnitude
(hundreds of times) smaller than the range. (Spacecraft-carryable ones
can do 10 or more times better.) However, if both the amplitude and the
phase of returns are recorded, then the portion of that multi-target
return that was scattered radially from any smaller scene element can be
extracted by phase-vector correlation of the total return with the form
of the return expected from each such element. Careful design and
operation can accomplish resolution of items smaller than a millionth of
the range, for example, 30 cm at 300 km, or about one foot at nearly
200 miles (320 km).
The process can be thought of as combining the series of spatially
distributed observations as if all had been made simultaneously with an
antenna as long as the beamwidth and focused on that particular point.
The "synthetic aperture" simulated at maximum system range by this
process not only is longer than the real antenna, but, in practical
applications, it is much longer than the radar aircraft, and
tremendously longer than the radar spacecraft.
Image resolution of SAR in its range coordinate (expressed in image
pixels per distance unit) is mainly proportional to the radio bandwidth
of whatever type of pulse is used. In the cross-range coordinate, the
similar resolution is mainly proportional to the bandwidth of the
Doppler shift of the signal returns within the beamwidth. Since Doppler
frequency depends on the angle of the scattering point's direction from
the broadside direction, the Doppler bandwidth available within the
beamwidth is the same at all ranges. Hence the theoretical spatial
resolution limits in both image dimensions remain constant with
variation of range. However, in practice, both the errors that
accumulate with data-collection time and the particular techniques used
in post-processing further limit cross-range resolution at long ranges.
The conversion of return delay time to geometric range can be very
accurate because of the natural constancy of the speed and direction of
propagation of electromagnetic waves. However, for an aircraft flying
through the never-uniform and never-quiescent atmosphere, the relating
of pulse transmission and reception times to successive geometric
positions of the antenna must be accompanied by constant adjusting of
the return phases to account for sensed irregularities in the flight
path. SAR's in spacecraft avoid that atmosphere problem, but still must
make corrections for known antenna movements due to rotations of the
spacecraft, even those that are reactions to movements of onboard
machinery. Locating a SAR in a manned space vehicle may require that the
humans carefully remain motionless relative to the vehicle during data
collection periods.
Although some references to SARs have characterized them as "radar
telescopes", their actual optical analogy is the microscope, the detail
in their images being smaller than the length of the synthetic aperture.
In radar-engineering terms, while the target area is in the "
far field" of the illuminating antenna, it is in the "near field" of the simulated one.
Returns from scatterers within the range extent of any image are
spread over a matching time interval. The inter-pulse period must be
long enough to allow farthest-range returns from any pulse to finish
arriving before the nearest-range ones from the next pulse begin to
appear, so that those do not overlap each other in time. On the other
hand, the interpulse rate must be fast enough to provide sufficient
samples for the desired across-range (or across-beam) resolution. When
the radar is to be carried by a high-speed vehicle and is to image a
large area at fine resolution, those conditions may clash, leading to
what has been called SAR's ambiguity problem. The same considerations
apply to "conventional" radars also, but this problem occurs
significantly only when resolution is so fine as to be available only
through SAR processes. Since the basis of the problem is the
information-carrying capacity of the single signal-input channel
provided by one antenna, the only solution is to use additional channels
fed by additional antennas. The system then becomes a hybrid of a SAR
and a phased array, sometimes being called a Vernier array.
Combining the series of observations requires significant computational resources, usually using
Fourier transform
techniques. The high digital computing speed now available allows such
processing to be done in near-real time on board a SAR aircraft. (There
is necessarily a minimum time delay until all parts of the signal have
been received.) The result is a map of radar reflectivity, including
both amplitude and phase. The amplitude information, when shown in a
map-like display, gives information about ground cover in much the same
way that a black-and-white photo does. Variations in processing may also
be done in either vehicle-borne stations or ground stations for various
purposes, so as to accentuate certain image features for detailed
target-area analysis.
Although the phase information in an image is generally not made
available to a human observer of an image display device, it can be
preserved numerically, and sometimes allows certain additional features
of targets to be recognized. Unfortunately, the phase differences
between adjacent image picture elements ("pixels") also produce random
interference effects called "coherence
speckle",
which is a sort of graininess with dimensions on the order of the
resolution, causing the concept of resolution to take on a subtly
different meaning. This effect is the same as is apparent both visually
and photographically in laser-illuminated optical scenes. The scale of
that random speckle structure is governed by the size of the synthetic
aperture in wavelengths, and cannot be finer than the system's
resolution. Speckle structure can be subdued at the expense of
resolution.
Before rapid digital computers were available, the data processing was done using an optical
holography
technique. The analog radar data were recorded as a holographic
interference pattern on photographic film at a scale permitting the film
to preserve the signal bandwidths (for example, 1:1,000,000 for a radar
using a 0.6-meter wavelength). Then light using, for example,
0.6-micrometer waves (as from a
helium–neon laser)
passing through the hologram could project a terrain image at a scale
recordable on another film at reasonable processor focal distances of
around a meter. This worked because both SAR and phased arrays are
fundamentally similar to optical holography, but using microwaves
instead of light waves. The "optical data-processors" developed for this
radar purpose
[2][3][4] were the first effective analog
optical computer
systems, and were, in fact, devised before the holographic technique
was fully adapted to optical imaging. Because of the different sources
of range and across-range signal structures in the radar signals,
optical data-processors for SAR included not only both spherical and
cylindrical lenses, but sometimes conical ones.
Image appearance
Main article:
Radar imaging
The following considerations apply also to real-aperture
terrain-imaging radars, but are more consequential when resolution in
range is matched to a cross-beam resolution that is available only from a
SAR.
The two dimensions of a radar image are range and cross-range. Radar
images of limited patches of terrain can resemble oblique photographs,
but not ones taken from the location of the radar. This is because the
range coordinate in a radar image is perpendicular to the vertical-angle
coordinate of an oblique photo. The apparent
entrance-pupil position (or
camera center)
for viewing such an image is therefore not as if at the radar, but as
if at a point from which the viewer's line of sight is perpendicular to
the slant-range direction connecting radar and target, with slant-range
increasing from top to bottom of the image.
Because slant ranges to level terrain vary in vertical angle, each
elevation of such terrain appears as a curved surface, specifically a
hyperbolic cosine
one. Verticals at various ranges are perpendiculars to those curves.
The viewer's apparent looking directions are parallel to the curve's
"hypcos" axis. Items directly beneath the radar appear as if optically
viewed horizontally (i.e., from the side) and those at far ranges as if
optically viewed from directly above. These curvatures are not evident
unless large extents of near-range terrain, including steep slant
ranges, are being viewed.
When viewed as specified above, fine-resolution radar images of small
areas can appear most nearly like familiar optical ones, for two
reasons. The first reason is easily understood by imagining a flagpole
in the scene. The slant-range to its upper end is less than that to its
base. Therefore, the pole can appear correctly top-end up only when
viewed in the above orientation. Secondly, the radar illumination then
being downward, shadows are seen in their most-familiar
"overhead-lighting" direction.
Note that the image of the pole's top will overlay that of some
terrain point which is on the same slant range arc but at a shorter
horizontal range ("ground-range"). Images of scene surfaces which faced
both the illumination and the apparent eyepoint will have geometries
that resemble those of an optical scene viewed from that eyepoint.
However, slopes facing the radar will be foreshortened and ones facing
away from it will be lengthened from their horizontal (map) dimensions.
The former will therefore be brightened and the latter dimmed.
Returns from slopes steeper than perpendicular to slant range will be
overlaid on those of lower-elevation terrain at a nearer ground-range,
both being visible but intermingled. This is especially the case for
vertical surfaces like the walls of buildings. Another viewing
inconvenience that arises when a surface is steeper than perpendicular
to the slant range is that it is then illuminated on one face but
"viewed" from the reverse face. Then one "sees", for example, the
radar-facing wall of a building as if from the inside, while the
building's interior and the rear wall (that nearest to, hence expected
to be optically visible to, the viewer) have vanished, since they lack
illumination, being in the shadow of the front wall and the roof. Some
return from the roof may overlay that from the front wall, and both of
those may overlay return from terrain in front of the building. The
visible building shadow will include those of all illuminated items.
Long shadows may exhibit blurred edges due to the illuminating antenna's
movement during the "time exposure" needed to create the image.
Surfaces that we usually consider rough will, if that roughness
consists of relief less than the radar wavelength, behave as smooth
mirrors, showing, beyond such a surface, additional images of items in
front of it. Those mirror images will appear within the shadow of the
mirroring surface, sometimes filling the entire shadow, thus preventing
recognition of the shadow.
An important fact that applies to SARs but not to real-aperture
radars is that the direction of overlay of any scene point is not
directly toward the radar, but toward that point of the SAR's current
path direction that is nearest to the target point. If the SAR is
"squinting" forward or aft away from the exactly broadside direction,
then the illumination direction, and hence the shadow direction, will
not be opposite to the overlay direction, but slanted to right or left
from it. An image will appear with the correct projection geometry when
viewed so that the overlay direction is vertical, the SAR's flight-path
is above the image, and range increases somewhat downward.
Objects in motion within a SAR scene alter the Doppler frequencies of
the returns. Such objects therefore appear in the image at locations
offset in the across-range direction by amounts proportional to the
range-direction component of their velocity. Road vehicles may be
depicted off the roadway and therefore not recognized as road traffic
items. Trains appearing away from their tracks are more easily properly
recognized by their length parallel to known trackage as well as by the
absence of an equal length of railbed signature and of some adjacent
terrain, both having been shadowed by the train. While images of moving
vessels can be offset from the line of the earlier parts of their wakes,
the more recent parts of the wake, which still partake of some of the
vessel's motion, appear as curves connecting the vessel image to the
relatively quiescent far-aft wake. In such identifiable cases, speed and
direction of the moving items can be determined from the amounts of
their offsets. The along-track component of a target's motion causes
some defocus. Random motions such as that of wind-driven tree foliage,
vehicles driven over rough terrain, or humans or other animals walking
or running generally render those items not focusable, resulting in
blurring or even effective invisibility.
These considerations, along with the speckle structure due to
coherence, take some getting used to in order to correctly interpret SAR
images. To assist in that, large collections of significant target
signatures have been accumulated by performing many test flights over
known terrains and cultural objects.
History
Carl A. Wiley,
[5] a mathematician at
Goodyear Aircraft Company in
Litchfield Park, Arizona, invented synthetic aperture radar in June 1951 while working on a correlation guidance system for the
Atlas ICBM program.
[6] In early 1952, Wiley, together with Fred Heisley and Bill Welty, constructed a concept validation system known as DOUSER ("
Doppler
Unbeamed Search Radar"). During the 1950s and 1960s, Goodyear Aircraft
(later Goodyear Aerospace) introduced numerous advancements in SAR
technology, many with the help from
Don Beckerleg.
[7]
Independently of Wiley's work, experimental trials in early 1952 by Sherwin and others at the
University of Illinois'
Control Systems Laboratory showed results that they pointed out "could
provide the basis for radar systems with greatly improved angular
resolution" and might even lead to systems capable of focusing at all
ranges simultaneously.
[8]
In both of those programs, processing of the radar returns was done
by electrical-circuit filtering methods. In essence, signal strength in
isolated discrete bands of Doppler frequency defined image intensities
that were displayed at matching angular positions within proper range
locations. When only the central (zero-Doppler band) portion of the
return signals was used, the effect was as if only that central part of
the beam existed. That led to the term Doppler Beam Sharpening.
Displaying returns from several adjacent non-zero Doppler frequency
bands accomplished further "beam-subdividing" (sometimes called
"unfocused radar", though it could have been considered "semi-focused").
Wiley's patent, applied for in 1954, still proposed similar processing.
The bulkiness of the circuitry then available limited the extent to
which those schemes might further improve resolution.
The principle was included in a memorandum
[9]
authored by Walter Hausz of General Electric that was part of the
then-secret report of a 1952 Dept. of Defense summer study conference
called TEOTA ("The Eyes of the Army"),
[10]
which sought to identify new techniques useful for military
reconnaissance and technical gathering of intelligence. A follow-on
summer program in 1953 at the
University of Michigan,
called Project Wolverine, identified several of the TEOTA subjects,
including Doppler-assisted sub-beamwidth resolution, as research efforts
to be sponsored by the Department of Defense (DoD) at various academic
and industrial research laboratories. In that same year, the
Illinois group produced a "strip-map" image exhibiting a considerable amount of sub-beamwidth resolution.
A more advanced focused-radar project was among several remote
sensing schemes assigned in 1953 to Project Michigan, a
tri-service-sponsored (Army, Navy, Air Force) program at the University
of Michigan's
Willow Run Research Center (WRRC), that program being administered by the
Army Signal Corps.
Initially called the side-looking radar project, it was carried out by a
group first known as the Radar Laboratory and later as the Radar and
Optics Laboratory. It proposed to take into account, not just the
short-term existence of several particular Doppler shifts, but the
entire history of the steadily varying shifts from each target as the
latter crossed the beam. An early analysis by Dr. Louis J. Cutrona,
Weston E. Vivian, and
Emmett N. Leith
of that group showed that such a fully focused system should yield, at
all ranges, a resolution equal to the width (or, by some criteria, the
half-width) of the real antenna carried on the radar aircraft and
continually pointed broadside to the aircraft's path.
[11]
The required data processing amounted to calculating
cross-correlations of the received signals with samples of the forms of
signals to be expected from unit-amplitude sources at the various
ranges. At that time, even large digital computers had capabilities
somewhat near the levels of today's four-function handheld calculators,
hence were nowhere near able to do such a huge amount of computation.
Instead, the device for doing the correlation computations was to be an
optical correlator.
It was proposed that signals received by the traveling antenna and
coherently detected be displayed as a single range-trace line across the
diameter of the face of a
cathode-ray tube,
the line's successive forms being recorded as images projected onto a
film traveling perpendicular to the length of that line. The information
on the developed film was to be subsequently processed in the
laboratory on equipment still to be devised as a principal task of the
project. In the initial processor proposal, an arrangement of lenses was
expected to multiply the recorded signals point-by-point with the known
signal forms by passing light successively through both the signal film
and another film containing the known signal pattern. The subsequent
summation, or integration, step of the correlation was to be done by
converging appropriate sets of multiplication products by the focusing
action of one or more spherical and cylindrical lenses. The processor
was to be, in effect, an optical
analog computer performing large-scale
scalar arithmetic
calculations in many channels (with many light "rays") at once.
Ultimately, two such devices would be needed, their outputs to be
combined as quadrature components of the complete solution.
Fortunately (as it turned out), a desire to keep the equipment small had led to recording the reference pattern on
35 mm film.
Trials promptly showed that the patterns on the film were so fine as to
show pronounced diffraction effects that prevented sharp final
focusing.
[3]
That led Leith, a physicist who was devising the correlator, to
recognize that those effects in themselves could, by natural processes,
perform a significant part of the needed processing, since along-track
strips of the recording operated like diametrical slices of a series of
circular optical zone plates. Any such plate performs somewhat like a
lens, each plate having a specific focal length for any given
wavelength. The recording that had been considered as scalar became
recognized as pairs of opposite-sign vector ones of many spatial
frequencies plus a zero-frequency "bias" quantity. The needed
correlation summation changed from a pair of scalar ones to a single
vector one.
Each zone plate strip has two equal but oppositely signed focal
lengths, one real, where a beam through it converges to a focus, and one
virtual, where another beam appears to have diverged from, beyond the
other face of the zone plate. The zero-frequency (
DC bias)
component has no focal point, but overlays both the converging and
diverging beams. The key to obtaining, from the converging wave
component, focused images that are not overlaid with unwanted haze from
the other two is to block the latter, allowing only the wanted beam to
pass through a properly positioned frequency-band selecting aperture.
Each radar range yields a zone plate strip with a focal length
proportional to that range. This fact became a principal complication in
the design of
optical processors.
Consequently, technical journals of the time contain a large volume of
material devoted to ways for coping with the variation of focus with
range.
For that major change in approach, the light used had to be both
monochromatic and coherent, properties that were already a requirement
on the radar radiation.
Lasers also then being in the future, the best then-available approximation to a coherent light source was the output of a
mercury vapor lamp,
passed through a color filter that was matched to the lamp spectrum's
green band, and then concentrated as well as possible onto a very small
beam-limiting aperture. While the resulting amount of light was so weak
that very long exposure times had to be used, a workable optical
correlator was assembled in time to be used when appropriate data became
available.
Although creating that radar was a more straightforward task based on
already-known techniques, that work did demand the achievement of
signal linearity and frequency stability that were at the extreme state
of the art. An adequate instrument was designed and built by the Radar
Laboratory and was installed in a C-46 (
Curtiss Commando)
aircraft. Because the aircraft was bailed to WRRC by the U. S. Army and
was flown and maintained by WRRC's own pilots and ground personnel, it
was available for many flights at times matching the Radar Laboratory's
needs, a feature important for allowing frequent re-testing and
"debugging" of the continually developing complex equipment. By
contrast, the Illinois group had used a C-46 belonging to the Air Force
and flown by AF pilots only by pre-arrangement, resulting, in the eyes
of those researchers, in limitation to a less-than-desirable frequency
of flight tests of their equipment, hence a low bandwidth of feedback
from tests. (Later work with newer Convair aircraft continued the
Michigan group's local control of flight schedules.)
Michigan's chosen 5-foot (1.5 m)-wide World War II-surplus antenna
was theoretically capable of 5-foot (1.5 m) resolution, but data from
only 10% of the beamwidth was used at first, the goal at that time being
to demonstrate 50-foot (15 m) resolution. It was understood that finer
resolution would require the added development of means for sensing
departures of the aircraft from an ideal heading and flight path, and
for using that information for making needed corrections to the antenna
pointing and to the received signals before processing. After numerous
trials in which even small atmospheric turbulence kept the aircraft from
flying straight and level enough for good 50-foot (15 m) data, one
pre-dawn flight in August 1957
[12]
yielded a map-like image of the Willow Run Airport area which did
demonstrate 50-foot (15 m) resolution in some parts of the image,
whereas the illuminated beam width there was 900 feet (270 m). Although
the program had been considered for termination by DoD due to what had
seemed to be a lack of results, that first success ensured further
funding to continue development leading to solutions to those recognized
needs.
First successful focussed airborne synthetic aperture radar image,
Willow Run Airport and vicinity, August 1957. Image courtesy University
of Michigan.
The SAR principle was first acknowledged publicly via an April 1960
press release about the U. S. Army experimental AN/UPD-1 system, which
consisted of an airborne element made by
Texas Instruments and installed in a
Beech L-23D
aircraft and a mobile ground data-processing station made by WRRC and
installed in a military van. At the time, the nature of the data
processor was not revealed. A technical article in the journal of the
IRE (
Institute of Radio Engineers) Professional Group on Military Electronics in February 1961
[13]
described the SAR principle and both the C-46 and AN/UPD-1 versions,
but did not tell how the data were processed, nor that the UPD-1's
maximum resolution capability was about 50 feet (15 m). However, the
June 1960 issue of the IRE Professional Group on Information Theory had
contained a long article
[14]
on "Optical Data Processing and Filtering Systems" by members of the
Michigan group. Although it did not refer to the use of those techniques
for radar, readers of both journals could quite easily understand the
existence of a connection between articles sharing some authors.
An operational system to be carried in a reconnaissance version of the
F-4
"Phantom" aircraft was quickly devised and was used briefly in Vietnam,
where it failed to favorably impress its users, due to the combination
of its low resolution (similar to the UPD-1's), the speckly nature of
its coherent-wave images (similar to the speckliness of laser images),
and the poorly understood dissimilarity of its range/cross-range images
from the angle/angle optical ones familiar to military photo
interpreters. The lessons it provided were well learned by subsequent
researchers, operational system designers, image-interpreter trainers,
and the
DoD sponsors of further development and acquisition.
In subsequent work the technique's latent capability was eventually
achieved. That work, depending on advanced radar circuit designs and
precision sensing of departures from ideal straight flight, along with
more sophisticated optical processors using laser light sources and
specially designed very large lenses made from remarkably clear glass,
allowed the
Michigan
group to advance system resolution, at about 5-year intervals, first to
15 feet (4.6 m), then 5 feet (1.5 m), and, by the mid-1970s, to 1 foot
(the latter only over very short range intervals while processing was
still being done optically). The latter levels and the associated very
wide dynamic range proved suitable for identifying many objects of
military concern as well as soil, water, vegetation, and ice features
being studied by a variety of environmental researchers having security
clearances allowing them access to what was then classified imagery.
Similarly improved operational systems soon followed each of those
finer-resolution steps.
Comparison of earliest SAR image with a later improved-resolution one.
Additionally, the data-processing light source had been changed from a
mercury lamp to a laser. Image data courtesy of University of Michigan
and Natural Resources Canada.
Even the 5-foot (1.5 m) resolution stage had over-taxed the ability
of cathode-ray tubes (limited to about 2000 distinguishable items across
the screen diameter) to deliver fine enough details to signal films
while still covering wide range swaths, and taxed the optical processing
systems in similar ways. However, at about the same time, digital
computers finally became capable of doing the processing without similar
limitation, and the consequent presentation of the images on cathode
ray tube monitors instead of film allowed for better control over tonal
reproduction and for more convenient image mensuration.
Achievement of the finest resolutions at long ranges was aided by
adding the capability to swing a larger airborne antenna so as to more
strongly illuminate a limited target area continually while collecting
data over several degrees of aspect, removing the previous limitation of
resolution to the antenna width. This was referred to as the spotlight
mode, which no longer produced continuous-swath images but, instead,
images of isolated patches of terrain.
It was understood very early in SAR development that the extremely
smooth orbital path of an out-of-the-atmosphere platform made it ideally
suited to SAR operation. Early experience with artificial earth
satellites had also demonstrated that the Doppler frequency shifts of
signals traveling through the ionosphere and atmosphere were stable
enough to permit very fine resolution to be achievable even at ranges of
hundreds of kilometers.
[15] While further experimental verification of those facts by a project now referred to as the Quill satellite
[16]
(declassified in 2012) occurred within the second decade after the
initial work began, several of the capabilities for creating useful
classified systems did not exist for another two decades.
That seemingly slow rate of advances was often paced by the progress of other inventions, such as the laser, the
digital computer,
circuit miniaturization, and compact data storage. Once the laser
appeared, optical data processing became a fast process because it
provided many parallel analog channels, but devising optical chains
suited to matching signal focal lengths to ranges proceeded by many
stages and turned out to call for some novel optical components. Since
the process depended on diffraction of light waves, it required
anti-vibration mountings,
clean rooms,
and highly trained operators. Even at its best, its use of CRTs and
film for data storage placed limits on the range depth of images.
At several stages, attaining the frequently over-optimistic
expectations for digital computation equipment proved to take far longer
than anticipated. For example, the
SEASAT
system was ready to orbit before its digital processor became
available, so a quickly assembled optical recording and processing
scheme had to be used to obtain timely confirmation of system operation.
In 1978, the first digital SAR processor was developed by the Canadian
aerospace company
MacDonald Dettwiler (MDA).
[17]
When its digital processor was finally completed and used, the digital
equipment of that time took many hours to create one swath of image from
each run of a few seconds of data.
[18]
Still, while that was a step down in speed, it was a step up in image
quality. Modern methods now provide both high speed and high quality.
Although the above specifies the system development contributions of
only a few organizations, many other groups had also become players as
the value of SAR became more and more apparent. Especially crucial to
the organization and funding of the initial long development process was
the technical expertise and foresight of a number of both civilian and
uniformed project managers in equipment procurement agencies in the
federal government, particularly, of course, ones in the armed forces
and in the intelligence agencies, and also in some civilian space
agencies.
Since a number of publications and Internet sites refer to a young
MIT physics graduate named Robert Rines as having invented
fine-resolution radar in the 1940s, persons who have been exposed to
those may wonder why that has not been mentioned here. Actually, none of
his several radar-image-related patents
[19]
actually had that goal. Instead, they presumed that fine-resolution
images of radar object fields could be accomplished by already-known
"dielectric lenses", the inventive parts of those patents being ways to
convert those microwave-formed images to visible ones. However, that
presumption incorrectly implied that such lenses and their images could
be of sizes comparable to their optical-wave counterparts, whereas the
tremendously larger wavelengths of microwaves would actually require the
lenses to have apertures thousands of feet (or meters) wide, like the
ones simulated by SARs, and the images would be comparably large.
Apparently not only did that inventor fail to recognize that fact, but
so also did the patent examiners who approved his several applications,
and so also have those who have propagated the erroneous tale so widely.
Persons seeking to understand SAR should not be misled by references to
those patents.
Relationship to phased arrays
A technique closely related to SAR uses an array (referred to as a "
phased array")
of real antenna elements spatially distributed over either one or two
dimensions perpendicular to the radar-range dimension. These physical
arrays are truly synthetic ones, indeed being created by synthesis of a
collection of subsidiary physical antennas. Their operation need not
involve motion relative to targets. All elements of these arrays receive
simultaneously in real time, and the signals passing through them can
be individually subjected to controlled shifts of the phases of those
signals. One result can be to respond most strongly to radiation
received from a specific small scene area, focusing on that area to
determine its contribution to the total signal received. The coherently
detected set of signals received over the entire array aperture can be
replicated in several data-processing channels and processed differently
in each. The set of responses thus traced to different small scene
areas can be displayed together as an image of the scene.
In comparison, a SAR's (commonly) single physical antenna element
gathers signals at different positions at different times. When the
radar is carried by an aircraft or an orbiting vehicle, those positions
are functions of a single variable, distance along the vehicle's path,
which is a single mathematical dimension (not necessarily the same as a
linear geometric dimension). The signals are stored, thus becoming
functions, no longer of time, but of recording locations along that
dimension. When the stored signals are read out later and combined with
specific phase shifts, the result is the same as if the recorded data
had been gathered by an equally long and shaped phased array. What is
thus synthesized is a set of signals equivalent to what could have been
received simultaneously by such an actual large-aperture (in one
dimension) phased array. The SAR simulates (rather than synthesizes)
that long one-dimensional phased array. Although the term in the title
of this article has thus been incorrectly derived, it is now firmly
established by half a century of usage.
While operation of a phased array is readily understood as a
completely geometric technique, the fact that a synthetic aperture
system gathers its data as it (or its target) moves at some speed means
that phases which varied with the distance traveled originally varied
with time, hence constituted temporal frequencies. Temporal frequencies
being the variables commonly used by radar engineers, their analyses of
SAR systems are usually (and very productively) couched in such terms.
In particular, the variation of phase during flight over the length of
the synthetic aperture is seen as a sequence of
Doppler
shifts of the received frequency from that of the transmitted
frequency. It is significant, though, to realize that, once the received
data have been recorded and thus have become timeless, the SAR
data-processing situation is also understandable as a special type of
phased array, treatable as a completely geometric process.
The core of both the SAR and the phased array techniques is that the
distances that radar waves travel to and back from each scene element
consist of some integer number of wavelengths plus some fraction of a
"final" wavelength. Those fractions cause differences between the phases
of the re-radiation received at various SAR or array positions.
Coherent detection is needed to capture the signal phase information in
addition to the signal amplitude information. That type of detection
requires finding the differences between the phases of the received
signals and the simultaneous phase of a well-preserved sample of the
transmitted illumination.
Every wave scattered from any point in the scene has a circular
curvature about that point as a center. Signals from scene points at
different ranges therefore arrive at a planar array with different
curvatures, resulting in signal phase changes which follow different
quadratic variations across a planar phased array. Additional linear
variations result from points located in different directions from the
center of the array. Fortunately, any one combination of these
variations is unique to one scene point, and is calculable. For a SAR,
the two-way travel doubles that phase change.
In reading the following two paragraphs, be particularly careful to
distinguish between array elements and scene elements. Also remember
that each of the latter has, of course, a matching image element.
Comparison of the array-signal phase variation across the array with
the total calculated phase variation pattern can reveal the relative
portion of the total received signal that came from the only scene point
that could be responsible for that pattern. One way to do the
comparison is by a correlation computation, multiplying, for each scene
element, the received and the calculated field-intensity values array
element by array element and then summing the products for each scene
element. Alternatively, one could, for each scene element, subtract each
array element's calculated phase shift from the actual received phase
and then vectorially sum the resulting field-intensity differences over
the array. Wherever in the scene the two phases substantially cancel
everywhere in the array, the difference vectors being added are in
phase, yielding, for that scene point, a maximum value for the sum.
The equivalence of these two methods can be seen by recognizing that
multiplication of sinusoids can be done by summing phases which are
complex-number exponents of e, the base of natural logarithms.
However it is done, the image-deriving process amounts to
"backtracking" the process by which nature previously spread the scene
information over the array. In each direction, the process may be viewed
as a
Fourier transform,
which is a type of correlation process. The image-extraction process we
use can then be seen as another Fourier transform which is a reversal
of the original natural one.
It is important to realize that only those sub-wavelength differences
of successive ranges from the transmitting antenna to each target point
and back, which govern signal phase, are used to refine the resolution
in any geometric dimension. The central direction and the angular width
of the illuminating beam do not contribute directly to creating that
fine resolution. Instead, they serve only to select the solid-angle
region from which usable range data are received. While some
distinguishing of the ranges of different scene items can be made from
the forms of their sub-wavelength range variations at short ranges, the
very large depth of focus that occurs at long ranges usually requires
that over-all range differences (larger than a wavelength) be used to
define range resolutions comparable to the achievable cross-range
resolution.
Data collection
A model of a German
SAR-Lupe reconnaissance satellite inside a Cosmos-3M rocket.
Highly accurate data can be collected by aircraft overflying the
terrain in question. In the 1980s, as a prototype for instruments to be
flown on the NASA Space Shuttles, NASA operated a synthetic aperture
radar on a NASA
Convair 990. In 1986, this plane caught fire on takeoff. In 1988, NASA rebuilt a C, L, and P-band SAR to fly on the NASA
DC-8 aircraft. Called
AIRSAR, it flew missions at sites around the world until 2004. Another such aircraft, the
Convair 580,
was flown by the Canada Center for Remote Sensing until about 1996 when
it was handed over to Environment Canada due to budgetary reasons. Most
land-surveying applications are now carried out by
satellite observation. Satellites such as
ERS-1/2,
JERS-1,
Envisat ASAR, and
RADARSAT-1
were launched explicitly to carry out this sort of observation. Their
capabilities differ, particularly in their support for interferometry,
but all have collected tremendous amounts of valuable data. The
Space Shuttle also carried synthetic aperture radar equipment during the
SIR-A and
SIR-B missions during the 1980s, the
Shuttle Radar Laboratory (SRL) missions in 1994 and the
Shuttle Radar Topography Mission in 2000.
The
Venera 15 and
Venera 16 followed later by the
Magellan space probe mapped the surface of Venus over several years using synthetic aperture radar.
Synthetic aperture radar was first used by NASA on JPL's
Seasat oceanographic satellite in 1978 (this mission also carried an
altimeter and a
scatterometer); it was later developed more extensively on the
Spaceborne Imaging Radar (SIR) missions on the space shuttle in 1981, 1984 and 1994. The
Cassini mission to
Saturn is currently using SAR to map the surface of the planet's major moon
Titan, whose surface is partly hidden from direct optical inspection by atmospheric haze. The
SHARAD sounding radar on the
Mars Reconnaissance Orbiter and
MARSIS instrument on
Mars Express
have observed bedrock beneath the surface of the Mars polar ice and
also indicated the likelihood of substantial water ice in the Martian
middle latitudes. The
Lunar Reconnaissance Orbiter, launched in 2009, carries a SAR instrument called
Mini-RF, which was designed largely to look for
water ice deposits on the poles of the Moon.
The
Mineseeker Project is designing a system for determining whether regions contain
landmines based on a
blimp
carrying ultra-wideband synthetic aperture radar. Initial trials show
promise; the radar is able to detect even buried plastic mines.
SAR has been used in
radio astronomy
for many years to simulate a large radio telescope by combining
observations taken from multiple locations using a mobile antenna.
The
National Reconnaissance Office maintains a fleet of (now declassified) synthetic aperture radar satellites commonly designated as
Lacrosse or Onyx.
In February 2009, the
Sentinel R1 surveillance aircraft entered service in the RAF, equipped with the SAR-based Airborne Stand-Off Radar (
ASTOR) system.
The German Armed Forces' (
Bundeswehr) military
SAR-Lupe reconnaissance satellite system has been fully operational since 22 July 2008.
Data distribution
The
Alaska Satellite Facility
provides production, archiving and distribution to the scientific
community of SAR data products and tools from active and past missions,
including the June 2013 release of newly processed, 35-year-old Seasat
SAR imagery.
CSTARS downlinks and processes SAR data (as well as other data) from a variety of satellites and supports the
University of Miami Rosenstiel School of Marine and Atmospheric Science.
CSTARS also supports disaster relief operations, oceanographic and
meteorological research, and port and maritime security research
projects.
See also
References
Synthetic aperture radar (SAR) is a form of radar which is used to create images of objects, such as landscapes – these images can be either two or three dimensional representations of the object. SAR uses the motion of the radar antenna over a targeted region to provide finer spatial resolution than is possible with conventional beam-scanning radars. SAR is typically mounted on a moving platform such as an aircraft or spacecraft, and has its origins in an advanced form of side-looking airborne radar (SLAR). The distance the SAR device travels over a target in the time taken for the radar pulses to return to the antenna creates the large "synthetic" antenna aperture (the "size" of the antenna). As a rule of thumb, the larger the aperture is, the higher the image resolution will be, regardless of whether the aperture is physical (a large antenna) or 'synthetic' (a moving antenna) – this allows SAR to create high resolution images with comparatively small physical antennas.This radar image acquired by the SIR-C/X-SAR radar on board the Space Shuttle Endeavour shows the Teide volcano. The city of Santa Cruz de Tenerife is visible as the purple and white area on the lower right edge of the island. Lava flows at the summit crater appear in shades of green and brown, while vegetation zones appear as areas of purple, green and yellow on the volcano's flanks.
The surface of Venus, as imaged by the Magellan probe using SAR
Radar waves have a polarization. Different materials reflect radar waves with different intensities, but anisotropic materials such as grass often reflect different polarizations with different intensities. Some materials will also convert one polarization into another. By emitting a mixture of polarizations and using receiving antennas with a specific polarization, several images can be collected from the same series of pulses. Frequently three such RX-TX polarizations (HH-pol, VV-pol, VH-pol) are used as the three color channels in a synthesized image. This is what has been done in the picture at right. Interpretation of the resulting colors requires significant testing of known materials.SAR image of Death Valley colored using polarimetry
The SAR principle was first acknowledged publicly via an April 1960 press release about the U. S. Army experimental AN/UPD-1 system, which consisted of an airborne element made by Texas Instruments and installed in a Beech L-23D aircraft and a mobile ground data-processing station made by WRRC and installed in a military van. At the time, the nature of the data processor was not revealed. A technical article in the journal of the IRE (Institute of Radio Engineers) Professional Group on Military Electronics in February 1961[13] described the SAR principle and both the C-46 and AN/UPD-1 versions, but did not tell how the data were processed, nor that the UPD-1's maximum resolution capability was about 50 feet (15 m). However, the June 1960 issue of the IRE Professional Group on Information Theory had contained a long article[14] on "Optical Data Processing and Filtering Systems" by members of the Michigan group. Although it did not refer to the use of those techniques for radar, readers of both journals could quite easily understand the existence of a connection between articles sharing some authors.First successful focussed airborne synthetic aperture radar image, Willow Run Airport and vicinity, August 1957. Image courtesy University of Michigan.
Even the 5-foot (1.5 m) resolution stage had over-taxed the ability of cathode-ray tubes (limited to about 2000 distinguishable items across the screen diameter) to deliver fine enough details to signal films while still covering wide range swaths, and taxed the optical processing systems in similar ways. However, at about the same time, digital computers finally became capable of doing the processing without similar limitation, and the consequent presentation of the images on cathode ray tube monitors instead of film allowed for better control over tonal reproduction and for more convenient image mensuration.Comparison of earliest SAR image with a later improved-resolution one. Additionally, the data-processing light source had been changed from a mercury lamp to a laser. Image data courtesy of University of Michigan and Natural Resources Canada.
Highly accurate data can be collected by aircraft overflying the terrain in question. In the 1980s, as a prototype for instruments to be flown on the NASA Space Shuttles, NASA operated a synthetic aperture radar on a NASA Convair 990. In 1986, this plane caught fire on takeoff. In 1988, NASA rebuilt a C, L, and P-band SAR to fly on the NASA DC-8 aircraft. Called AIRSAR, it flew missions at sites around the world until 2004. Another such aircraft, the Convair 580, was flown by the Canada Center for Remote Sensing until about 1996 when it was handed over to Environment Canada due to budgetary reasons. Most land-surveying applications are now carried out by satellite observation. Satellites such as ERS-1/2, JERS-1, Envisat ASAR, and RADARSAT-1 were launched explicitly to carry out this sort of observation. Their capabilities differ, particularly in their support for interferometry, but all have collected tremendous amounts of valuable data. The Space Shuttle also carried synthetic aperture radar equipment during the SIR-A and SIR-B missions during the 1980s, the Shuttle Radar Laboratory (SRL) missions in 1994 and the Shuttle Radar Topography Mission in 2000.A model of a German SAR-Lupe reconnaissance satellite inside a Cosmos-3M rocket.
Synthetic aperture radar was first used by NASA on JPL's Seasat oceanographic satellite in 1978 (this mission also carried an altimeter and a scatterometer); it was later developed more extensively on the Spaceborne Imaging Radar (SIR) missions on the space shuttle in 1981, 1984 and 1994. The Cassini mission to Saturn is currently using SAR to map the surface of the planet's major moon Titan, whose surface is partly hidden from direct optical inspection by atmospheric haze. The SHARAD sounding radar on the Mars Reconnaissance Orbiter and MARSIS instrument on Mars Express have observed bedrock beneath the surface of the Mars polar ice and also indicated the likelihood of substantial water ice in the Martian middle latitudes. The Lunar Reconnaissance Orbiter, launched in 2009, carries a SAR instrument called Mini-RF, which was designed largely to look for water ice deposits on the poles of the Moon.Titan - Evolving feature in Ligeia Mare (SAR; 21 August 2014).
The Mineseeker Project is designing a system for determining whether regions contain landmines based on a blimp carrying ultra-wideband synthetic aperture radar. Initial trials show promise; the radar is able to detect even buried plastic mines.Titan - Ligeia Mare - SAR and clearer despeckled views.
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