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1.1.2 Scientific Background

1.1.2.1 Radar Imaging

Radar is the commonly used acronym for Radio Detection and Ranging. Radio waves are that part of the electromagnetic spectrum that have wavelengths considerably longer than visible light, as shown in figure1.3 below.

Electromagnetic Spectrum
Figure 1.3 The Electromagnetic Spectrum

Imaging radar is an active illumination system, in contrast to passive optical imaging systems that require the Sun's illumination. An antenna, mounted on an aircraft or spacecraft, transmits a radar signal in a side-looking direction towards the earth's surface. The reflected signal, known as the echo, is backscattered from the surface and received a fraction of a second later at the same antenna, as shown in figure1.4 below. The brightness, or amplitude (A), of this received echo is measured and recorded and the data are then used to construct an image. For coherent Radar systems such as Synthetic Aperture Radar (SAR), the phase of the received echo is also measured and used to construct the image.

image
Figure 1.4 Echoes received back by antenna

Radar uses a single frequency for illumination; therefore there is no color associated with raw Radar imagery (unlike optical imagery which is illuminated by all the various colours from the ambient visible light.) However, Radar provides at least two significant benefits from its not being dependent on natural light: the ability to image through clouds, and the ability to image at night. The wavelength of the microwaves used in Radar are longer than those of visible light, and are less responsive to the boundaries between air and the water droplets within the clouds. The result is that, for Radar, the clouds appear homogeneous with only slight distortions occurring when the waves enter and leave the clouds.

1.1.2.1.1 Polarisation

Irrespective of wavelength, radar signals can be transmitted and/or received in different modes of polarisation. Being electromagnetic waves, microwaves are transverse; that is the vibrations are perpendicular to the direction of wave propagation (unlike sound waves which are longitudinal and vibrate in the same direction as the wave propagation.) For Radar, the waves are typically polarised in a plane, either horizontal or vertical; although it is also possible for the waves to not be restricted to a plane (they could be elliptically or circularly polarised).

1.1.2.1.2 Backscatter

Radar images are composed of many picture elements referred to as pixels. Each pixel in the radar image represents an estimate of the radar backscatter for that area on the ground. Darker areas in the image represent low backscatter, while brighter areas represent high backscatter. Bright features mean that a large fraction of the radar energy was reflected back to the radar, while dark features imply that very little energy was reflected. Backscatter for a target area at a particular wavelength will vary for a variety of conditions, such as the physical size of the scatterers in the target area, the target's electrical properties and the moisture content, with wetter objects appearing bright, and drier targets appearing dark. (The exception to this is a smooth body of water, which will act as a flat surface and reflect incoming pulses away from a target. These bodies will appear dark ). The wavelength and polarisation of the Radar pulses, and the observation angles will also affect backscatter

image
Figure 1.5 Backscatter from various surfaces types

A useful rule-of-thumb in analysing radar images is that the higher or brighter the backscatter on the image, the rougher the surface being imaged. Flat surfaces that reflect little or no radio or microwave energy back towards the radar will always appear dark in radar images. Vegetation is usually moderately rough on the scale of most radar wavelengths and appears as grey or light grey in a radar image. Surfaces inclined towards the radar will have a stronger backscatter than surfaces which slope away from the radar and will tend to appear brighter in a radar image. Some areas not illuminated by the radar, like the back slope of mountains, are in shadow, and will appear dark. When city streets or buildings are lined up in such a way that the incoming radar pulses are able to bounce off the streets and then bounce again off the buildings (called a double-bounce) and directly back towards the radar they appear very bright (white) in radar images. Roads and freeways are flat surfaces and so appear dark. Buildings which do not line up so that the radar pulses are reflected straight back will appear light grey, like very rough surfaces.

1.1.2.2 Real Aperture Radar (RAR)

Before considering the properties of a Synthetic Aperture Radar system, we will consider a Real Aperture System. Aperture means the opening used to collect the reflected energy that is used to form an image. In the case of radar imaging this is the antenna. For RAR systems, only the amplitude (and not the phase) of each echo return is measured and processed.

1.1.2.2.1 Spatial Resolution

The spatial resolution of a Real Aperture Radar system is determined by, among other things, the size of the antenna used. For any given wavelength, the larger the antenna the better the spatial resolution. Other determining factors include the pulse length and the antenna beamwidth. The pulse length of the radar signal is determined by the length of time that the antenna emits its burst of energy.

Consider an image to be a set of values A(x,y), where the x coordinate is in the direction of platform motion and the y coordinate is in the direction of illumination. Then the value of y, or range direction , and its resolution (range resolution) is based on the pulse length, the arrival time of the echo, and the timing precision of the radar. The value of x, which is azimuth direction (also referred to as the along-track direction) , and its resolution (azimuth resolution) depends on the position of the platform that carries the transmitting antenna and the beamwidth of the radar.

1.1.2.2.2 Range Resolution

For a Radar system to image separately two ground features that are close together in the range direction, it is necessary for all parts of the two objects' reflected signals to be received separately by the antenna. Any time overlap between the signals from two objects will cause their images to be blurred together.

1.1.2.2.3 Azimuth Resolution

As mentioned above, the azimuth resolution is affected by the beamwidth. As the antenna beam fans out with increasing distance from the earth to the platform carrying the pulse transmitting source and receiver, the azimuth resolution deteriorates. The beamwidth of the antenna is directly proportional to the wavelength of the transmitted pulses and inversely proportional to the length of the antenna.

So, for any given wavelength, antenna beamwidth can be best controlled by one of two different means:

  • by controlling the physical length of the antenna, or
  • by synthesising an effective length of the antenna
Those systems where beamwidth is controlled by the physical antenna length are referred to as Real Aperture, or Noncoherent Radars and the natural resolution of such an orbiting radar instrument, observing from 1000 km, is typically 10 km on the ground. While these systems enjoy relative simplicity of design and data processing, the resolution difficulties restrict them to short-range, low altitude operation and the use of relatively short wavelengths. These restrictions limit the area of coverage obtainable and the short wavelengths experience more atmospheric dispersion.

1.1.2.3 Synthetic Aperture Radar (SAR)

Synthetic Aperture Radar (SAR) takes advantage of the Doppler history of the radar echoes generated by the forward motion of the spacecraft to synthesise a large antenna, enabling high azimuthal resolution in the resulting image despite a physically small antenna, as shown is figure1.6 . As the radar moves, a pulse is transmitted at each position. The return echoes pass through the receiver and are recorded in an echo store.

image
Figure 1.6 Constructing a synthetic antenna

SAR is a coherent, active, microwave imaging method that improves natural radar resolution by focusing the image through a process known as synthetic aperture processing. This typically requires a complex integrated array of onboard navigational and control systems, with location accuracy provided by both Doppler and inertial navigation equipment. For a C band instrument, (such as ERS-1, ERS-2 or ASAR) 1000 km from its target, the area on the ground covered by a single transmitted EM pulse, known as the radar footprint, is on the order of 5 km long in the along-track (azimuth) direction. In SAR, the satellite must not cover more than half of the azimuth antenna length between the emission of successive pulses, so as not to degrade the range resolution 1.1.2.2.2. . For example, a 10 m antenna should advance only 5 m between pulses, to produce a 5 m long final elementary resolution cell, or pixel. Therefore, each 5 km long footprint is a collection of signals, each one of which is a mixture of a thousand 5 m samples, each of which contributes to a thousand signals. Focusing is the reconstruction of the contribution of each 5 m cell, which results in an improvement of resolution of approximately a thousand times of a Real Aperture Radar. This effectively provides a synthetic aperture of a 20 km antenna.

In essence, return signals from the centre portion of the beamwidth are discriminated by detecting Doppler frequency shifts, which is a change in wave frequency resulting from the relative velocities of a transmitter and a reflector. Within the wide antenna beam, returns from features in the area ahead of the platform will have upshifted, or higher, frequencies resulting from the Doppler effect. Conversely, returns from features behind the platform will have downshifted, or lower, frequencies. Returns from features near the centreline of the beamwidth (the so-called Zero-Doppler line) will experience no frequency shift.

The amplitude and phase of the signals returned from objects are recorded in the echo store throughout the time period in which the objects are within the beam of the moving antenna. By processing the return signals according to their Doppler shifts, a very narrow effective antenna beamwidth can be achieved, even at far ranges, without requiring a physically long antenna or a short operating wavelength.

Because the signals received by a SAR system are recorded over a long time period, the system translates the real antenna over a correspondingly long distance, which becomes the effective length of the antenna. The azimuth resolution 1.1.2.2.3. with this synthetic antenna length is greatly improved, due to the effective narrowing of the beamwidth. The azimuth resolution is also essentially independent of range, because at long range an object is in the beam longer, meaning that returns from it are recorded over a longer distance.

1.1.2.4 Image Interpretation

Determining and describing how various objects reflect radar energy has been mostly derived from empirical observation. It has been found that the primary factors influencing an object's return signal intensity are their geometric and electrical characteristics, as well as their general composition. That is, radar waves interact differently with soil, vegetation, water, ice, or man-made objects.

1.1.2.4.1 Geometric Characteristics

One of the most readily apparent features of radar imagery is its sidelighted character, which arises through variations in the relative sensor to terrain geometry. Shadow areas and radar backscatter 1.1.2.1.2. are affected by different surface properties over a range of incidence angles.

For flat terrain, the local reflection angle is the same as the incidence angle; most of the incident energy is reflected away from the sensor, resulting in a very low return signal. Rough surfaces, on the other hand, will scatter incidence energy in all directions and return a significant portion of the incident energy back to the antenna, as show in the scattering mechanisms figure below.

Scattering Mechanisms
Figure 1.7 Scattering mechanisms

The shape and orientation of objects must be considered, as well as their surface roughness, when evaluating radar returns. For instance a particularly bright response will come from a corner reflector, which will produce a double bounce, as shown above. It is also worth noting that some features, such as corn fields, might appear rough when seen in both the visible and microwave portion of the spectrum, whereas other surfaces, such as roads, may appear rough in the visible range but look smooth in the microwave spectrum. In general, SAR images will manifest many more smooth, or specular, surfaces than those produced with optical sensors.

Since one factor affecting backscatter is the polarisation used, as was discussed previously 1.1.2.1.2. , those SAR systems that can transmit pulses in either horizontal (H) or vertical (V) polarisation and receive in either H or V, such as the ASAR sensor, can better utilise this property for image interpretation.

In addition, SARs measure the phase of the incoming pulse and can therefore measure the phase difference in the return of the HH and VV signals. This difference can be thought of as a difference in the round-trip times of HH and VV signals and is frequently the result of the structural characteristics of the scatterers, or targets. The phase information in the products from these SARs can used to derive an estimate of the correlation coefficient for the HH and VV returns, which can be considered as a measure of how alike the HH and VV scatterers are.

As an aside, the phase information derived by a SAR may also be exploited in Interferometric SAR (InSAR). InSAR requires the phase differences of at least two complex-valued SAR images, acquired from different orbit positions and/or at different times. These phase differences can be compared after proper image registration and produce a new kind of image called an interferogram. This subject is discussed in greater detail in the section entitled "Interferometry 1.1.5.4. ".

1.1.2.4.2 Electrical Characteristics

The electrical characteristics of terrain features interact with their geometric characteristics to determine the intensity of radar returns. One measure of an object's electrical character is the 'complex dielectric constant', which is a parameter that indicates the reflectivity and conductivity of various materials. As reflectivity and conductivity increases, so does the value of this constant.

In the microwave region of the spectrum, most natural materials have a dielectric constant in the range of 3 to 8 when dry, whereas water has a dielectric constant of approximately 80. This means that the presence of moisture in either soil or vegetation will result in significantly greater reflectivity. Other examples of sources of high reflectivity are metal bridges, silos, and railroad tracks.

1.1.2.4.3 Responses of Soil, Vegetation, Mountains, Water and Ice

1.1.2.4.3.1 Soil

Because the dielectric constant for water is at least 10 times that for dry soil, the presence of water in the top few centimetres of bare, unvegetated soil can be detected in radar imagery, becoming particularly apparent at longer wavelengths.

1.1.2.4.3.2 Vegetation

A vegetation canopy will interact with radar waves as a group of volume scatterers, as shown in figure1.7 above. The canopy is composed of a large number of discrete plant components, such as leaves, stems, stalks, limbs and so on. In addition, the canopy is underlain by soil that may result in surface scattering of the energy that penetrates the vegetation canopy. In general, shorter wavelengths, of approximately 2 to 6 cm, are best for sensing crop canopies and tree leaves, because at these wavelengths volume scattering predominates and surface scattering from the underlying soil is minimised. However, longer wavelengths, of approximately 10 to 30 cm are best for sensing tree trunks or limbs.

Another factor influencing the affect of radar backscatter from vegetation is the polarisation of the beam, with like-polarised waves (HH or VV) penetrating vegetation more than cross-polarised (HV or VH) waves. In addition, more energy is returned from crops having their rows aligned in the azimuth direction than from those aligned in the range direction, especially in the case of like-polarised beams.

1.1.2.4.3.3 Mountains

When a spaceborne SAR looks down and to the side toward a steep mountain, many objects on the mountain's facing slope may appear to be located at the same distance from the spacecraft, as though the farther out in range an object is, the higher, or closer, to the spacecraft the ground is raised by the mountain to compensate. Since those many objects are located at nearly the same distance from the SAR, their backscattered signals will return to the spacecraft at about the same time. The SAR will conclude that the object located at that distance backscattered brightly, mapping all those responses into one location while in truth they came from many. This is called foreshortening, or layover in the extreme case where responses from, say, a mountain's peak are positioned before surrounding locations. This is depicted in the figure below entitled "Geometric Distortions".

Geometric Distortion
Figure 1.8 Geometric distortions

The resulting SAR image will show much of the mountain's bright facing slope was mapped onto a few pixels, while the darker backfacing slope was considered to cover many more pixels. Therefore it appears as though the mountains are lying over. Steeper topography or a smaller SAR look angle can worsen foreshortening effects.

1.1.2.4.3.4 Water and Ice

Smooth water surfaces yield no returns to the antenna, but rough water surfaces return radar signals of varying strengths. In addition, waves moving in the range direction, that is moving toward or away from the radar system, are easier to detect that waves moving in the azimuthal direction.

Radar backscatter from sea ice is dependent on the dielectric properties and spatial distribution of the ice. Such factors as ice age, surface roughness, internal geometry, temperature, and snow cover will also all play a role in the affect of radar backscatter.

Each of these above topics are discussed in more detail in the sections within the handbook relating to these applications.

1.1.2.5 General Rules for SAR Image Interpretation

In summary, the general rules for SAR Image Interpretation are as follows:

  • Regions of calm water and other smooth surfaces will appear black, because the incident radar reflects away from the spacecraft.
  • Surface variations near the size of the radar's wavelength cause strong backscattering.
  • A rough surface backscatters more brightly when it is wet.
  • Wind-roughened water can backscatter brightly when the resulting waves are close in size to the incident radar's wavelength.
  • Hills and other large-scale surface variations tend to appear bright on the side that faces the sensor and dim on the side that faces away from the sensor. Mountains show this effect to the extreme, in part due to increased foreshortening.
  • Due to the reflectivity and angular structure of buildings, bridges, and other human-made objects, these targets tend to behave as corner reflectors and show up as bright spots in a SAR image.
  • A particularly strong response, say from a corner reflector, can look like a bright cross in a processed SAR image.

1.1.2.6 References

Ref 1.1
T.M. Lillesand, R.W. Kiefer, "Remote Sensing and Image Interpretation", 2nd ed, Wiley 1987.

Ref 1.2
D. Massonet, K.L.Feigl, "Radar interferometry and its application to changes in the earth surface", Reviews of Geophysics Vol. 36, Number 4, Nov. 1998.

Ref 1.3
R. Bamler, P. Hartl, "Synthetic aperture radar interferometry", 1998.

Ref 1.4
ALASKA Sar Facility, www.asf.alaska.edu