© 2012 Earthwide Communications LLC, www.eijournal.com

By William V. Parker, principal, Global Engagement Solutions, and senior associate, PACE Government Services (http://pacegs-dc.com), Washington, D.C.

he last two decades have wit-nessed unprecedented growth in the satellite-based Earth observa-tion industry. Although the market is still strongly biased toward

electro-optically derived imagery, a rising tide of acceptance and usage of satellite-derived synthetic aperture radar (SAR) data has occurred during the last few years. This trend is the result of the increasing availability of commercial SAR satellite data; development of so-phisticated processing and analysis tools; and industry-driven training initiatives to familiarize image analysts with SAR imagery, including its interpretation and utility.

Electro-Optical/SAR ComparisonsIntuitively the colored imagery derived

from electro-optical systems provides the human eye with familiar representations of Earth’s surface that are instinctively easy to interpret. Additionally, electro- optical imagery has been known to the

user community since the 1970s, so there’s a lot of expertise available.

Regardless, the user community is recognizing there’s much more than meets the eye in black-and-white SAR data and imagery. The most obvi-ous SAR advantage is the weather and daylight independence of radar systems, which ensure a guaranteed acquisition of the area of interest. This also enables consistent monitoring independent of lighting, weather or cloud-cover conditions.

This, however, is just one side of the coin. The real advantages of SAR unfold when the data are processed and analyzed appropriately to meet the mission. Many unique effects of SAR satellite data, such as radar shadow, can be exploited to extract information from the derived imagery that isn’t detectable through visual interpretation alone.

For example, SAR imagery can be used to detect and even quantify the motion of objects on both land and sea. Thanks to the measurability of a SAR signal’s intensity and phase, imagery analysts can determine elevation in-formation and even subtle changes to surface conditions.

Discover the Benefits of

THE TOP 10 CONSIDERATIONS for Buying and Using

Synthetic Aperture Radar Imagery

RADAR IMAGINGSurface movement monitoring (left) and radar interferometry (right) are two of the many applications made possible by SAR imagery. For surface movement monitoring, images acquired with identical acquisition parameters at different points in time are combined, allowing users to map changes in topography in the order of millimeters. Radar interferometry is a technique that enables high-resolution digital elevation mapping of Earth’s surface.

T

Table 1: Commercial Radar Remote Sensing Satellites

Satellite Mission Launch Date Band Resolution Swath Width Repeat Rate Comment

TerraSAR-X / TanDEM-X

2007 / 2010 X Band 1-18 meters 5 - 150 km 11 days A German mission carried out under a public-private-partnership with the German Aerospace Center and EADS Astrium. Both satellites used as radar interferometer for WorldDEM™.

COSMO-SkyMed 2007 / 2008 X Band 1-100 meters 10 - 200 km 16 days Italian constellation of four satellites

RADARSAT-1 / RADARSAT-2

1995 / 2007 C Band 3-100 meters 20 - 500 km 24 days Canadian commercial mission

PAZ 2013 X Band 1-18 meters 5 - 150 km 11 days Spanish dual-use mission, constellation with TerraSAR-X and TanDEM-X envisioned

Astrium/Infoterra DLR

SAR BasicsElectro-optical systems are passive, which means they

require the illumination of the sun for imaging. Radar, however, is an active remote sensing system, which means it provides its own energy source to illuminate the imaging area. A radar imaging system has three main functions: It transmits the microwave signal toward the scene, receives a portion of that transmitted energy as backscatter from the scene, and then observes the strength and time delay of the returned signal.

The energy of the radar pulse is scattered in all direc-tions at the Earth’s surface, with some reflected back to the antenna. The surface’s roughness—i.e., the irregularity of the terrain vertically and horizontally—determines the return signal’s amplitude. Surfaces can be classified as smooth, slightly rough, moderately rough or very rough.

Generally, bright areas in a SAR image are strong reflec-tors, such as buildings in urban areas, while dark parts of the image represent surfaces that reflect little or no energy, such as water surfaces or oil film on an ocean. Depending on the wavelength of the radar signal, SAR can penetrate forest canopy and Earth surfaces, detecting dielectric features such as metal objects, water, freeze/thaw, salt content, iron oxides and clay in soils. To fully exploit the advantages of SAR data and imagery, key mission-specific collection characteristics and parameters must be understood. The following sections clarify these parameters and characteristics.

1. WavelengthAs detailed in table 2, radar remote

sensing uses the microwave portion of the electromagnetic spectrum, from a frequency of 0.3 GHz to 300 GHz. Most radar satellites operate at wavelengths between 0.5 cm and 75 cm.

Shorter wavelengths—e.g., X-band imagery at 3 cm—are reflected from the top of the canopy, while longer wavelengths—e.g., L-band imagery at 24 cm—normally go down to the ground and are reflected from there. Using this characteristic of differ-ent wavelengths makes it possible to discern information about the canopy structure of a forested area from a

multiwavelength image and thus estimate above-ground biomass.

Furthermore, the choice of wavelength needs to be matched to the size of the surface feature that should be dis-tinguishable. Small features are best recognized with X-band imagery—i.e., short wavelengths—while large features, such as geology, are better marked in L-band imagery.

2. PolarizationTransmitted and received radar signals propagate in a certain

plane—the polarization. The propagation planes are usually horizontal (H) and vertical (V). Vertically polarized waves will interact with the vertical stalks of plant canopy, while horizon-tally polarized waves will penetrate through plant canopy. Thus, the combination of the image channels into a red-green-blue (RGB) image results in a false-color image, which can differenti-ate ground cover such as vegetation classes.

The different combination options for the polarization will provide different image characteristics:

Single polarization. The radar system operates with the same polarization for transmitting and receiving the signal.

Cross polarization. A different polarization is used to trans-mit and receive the signal.

Dual polarization. The radar system operates with one polarization to transmit the signal and both polarizations simultaneously to receive the signal.

Quad polarization. H and V polarizations are used for alter-nate pulses to transmit the signal and with both simulta-neously to receive the signal.

Multipolarized images are provided in the form of mul-tiple layers, each corresponding to a different polarization chan-nel. Each polarization channel is identified by two letters. The first letter denotes the transmit polarization, and the second re-fers to the receive polarization. Multipolarized SAR imagery allows users to measure the terrain’s polarization properties and not simply the backscatter at a single polarization, thus providing improved classifica-tion information.

Canopy penetration varies with different wavelengths.

Table 2: Frequency and Wavelength of Commonly Used Radar Remote Sensing Bands

Band Frequency Wavelength Key Characteristics

X Band 12.5-8 GHz 2.4-3.75 cm Widely used for military reconnaissance, mapping and surveillance (TerraSAR-X, TanDEM-X, COSMO-SkyMed)

C Band 8-4 GHz 3.75-7.5 cm Penetration capability of vegetation or solids is limited and restricted to the top layers. Useful for sea-ice surveillance (RADARSAT, ERS-1).

S Band 4-2 GHz 7.5-15 cm Used for medium-range meteorological applications—e.g., rainfall measurement, airport surveillance

L Band 2-1 GHz 15-30 cm Penetrates vegetation to support observation applications over vegetated sur-faces and for monitoring ice sheet and glacier dynamics (ALOS PALSAR)

P Band 1-0.3 GHz 30-100 cm To date only used for research and experimental applications. Significant penetration capabilities regarding vegetation canopy (key element for estimating vegetation biomass), sea ice, soil, glaciers.

3. ModeAs detailed in table 3, the acquisition mode is directly

linked with the resolution of the resulting image and the size of the scene area covered.

The highest resolution is achieved with a SpotLight im-age. For this the radar beam continuously illuminates one terrain patch while the satellite is moving along its flight path. This sophisticated imaging mode makes it possible to acquire data with up to 1-meter resolution but restricts the scene size.

An image in StripMap mode is acquired by illuminating the ground swath with a continuous sequence of pulses while the antenna beam is pointed to a fixed angle in elevation and azimuth. This results in an image strip with constant image quality in azimuth. StripMap is the most commonly used ac-quisition mode, as it provides a good trade off between the size of the area covered and the resolution.

The ScanSAR mode overcomes the constraints of the nar-row swath of the StripMap and is intended for use in applica-tions requiring large area coverage such as monitoring applica-tions. In this mode, electronic antenna elevation steering is used to acquire adjacent, slightly overlapping coverages with different incidence angles that are processed into one scene. For example, in the case of TerraSAR-X and RADARSAT-2, up to four single beams covering adjoining swaths are combined. Due to the switching between the beams, only bursts of SAR echoes are received, resulting in a reduced bandwidth and hence reduced azimuth resolution.

4. Incidence AngleThe incidence angle refers to the angle between the

“straight to ground” and the radar illumination. The interac-tion of microwaves with the ground is complex, and differ-ent reflections occur in different angular regions. Returns are

normally strong at low incidence angles and decrease with increasing incidence angle.

Because of SAR’s side-looking perspective, tall objects and relief structures are subject to displacements. There are three main radar effects, which must be taken into account when using SAR data:

• Radar shadows are areas on the ground that aren’t illumi-nated by the radar signal, thus no return signal is received, and these areas appear dark in the imagery. As the incidence angle of an image increases from near-range to far-range, shadowing becomes more prominent toward far-range. Shadowing in a radar image is an important key for terrain relief interpretation, as the height of an object can be derived from measuring the radar shadow. Consequently, this apparently negative radar effect provides valuable information about a scene.

• Foreshortening describes the compression appearance of features that are tilted toward the radar. For a given slope, fore-shortening effects are reduced with increasing incident angles. With this reduction of incidence angle, however, the shadow effect increases. Thus, the selection of the incidence angle is always a trade-off between the acquisition’s occurrence of fore-shortening and radar shadow.

• Layover occurs when the reflected signal from a feature’s upper portion is received before the return from the feature’s lower portion. In this case, the top of the feature will be dis-placed relative to its base. This effect is more prevalent for view-ing geometries with smaller incident angles.

These effects can be compensated for only through a trade-off among them—if you don‘t have one, you will have the others. They can, however, support image analysis.

Table 3: Basic Imaging Modes of Commercial Radar Remote Sensing Systems

SpotLight StripMap ScanSAR

TerraSAR-X Resolution: 1 meter 3 meter 18 meter

Scene size: 10 km x 5 km 30 km x 50 km** extendable up to 1,650 km

100 km x 150 km** extendable up to 1,650 km

COSMO-SkyMed

Resolution: 1 meter 3 meters (HImage)15 meters (Ping Pong)

30 meters (ScanSAR Wide)100 meters (ScanSAR Huge)

Scene size: 10 km x 10 km 40 km x 40 km (HImage)30 km x 30 km (Ping Pong)

100 km x 100 km (ScanSAR Wide)200 km x 200 km (ScanSAR Huge)

RADARSAT-2 Resolution: ~ 1.6 meters (SpotLight)3 meters (Wide Ultra Fine)

8 meters (Wide Multi-Look Fine)8 meters (Wide Fine)

50 meters (ScanSAR Narrow)100 meters (ScanSAR Wide)

Scene size: 18 km x 8 km (SpotLight)50 km x 50 km (Wide Ultra Fine)

90 km x 50 km (Wide Multi-Look Fine)150 km x 170 km (Wide Fine)

300 km x 300 km (ScanSAR Narrow)500 km x 500 km (ScanSAR Wide)

Acquisition mode is directly linked with the resolution of the resulting image and the size of the scene area covered.

SpotLight Mode

StripMap Mode

ScanSAR Mode

514 km

20°

20°

20°

45°

45°

55°

A slope away from the radar illumination with an angle that’s steeper than the sensor depression angle provokes radar shadows.

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5. Repeat FrequencyApplications such as interferometry, surface movement

monitoring or change detection require the acquisition of data stacks that have identical acquisition parameters (orbit, inci-dence angle and polarization). Analyzing changes in the ampli-tude and the phase of the return signal at the various acquisi-tion dates provides information to derive small changes on the imaged surface. Not all changes, such as vegetation changes, are relevant for analysis, but the time difference among the acquisitions is important for interferometric processing.

Current commercial radar systems have 24-day (RADAR-SAT-2), 16-day (COSMO-SkyMed constellation of four satellites) and 11-day (TerraSAR-X and TanDEM-X) repeat cycles—i.e., they pass over the same point on the ground with the identical acquisition geometry in these time intervals.

In the case of TerraSAR-X and TanDEM-X, where the two satellites fly in a close formation with only a distance of a few hundred meters, they can acquire an interferometric data pair without any time difference. One satellite sends the signal, and both satellites record the backscatters simultaneously. This unique constellation makes it possible to perform high-quality interferometry all over the world without any limitations. The result of this unique mission will be Astrium’s WorldDEM™, a worldwide homogeneous digital elevation model. The global data-set will be available in 2014.

6. ResolutionA radar sensor’s resolution has two dimensions: range resolu-

tion and azimuth resolution. The azimuth resolution is deter-mined by built-in radar and processor constraints and depends on the length of the processed pulse, with shorter pulses result-ing in “higher” resolution. Range resolution is determined by the angular beam width of the terrain strip illuminated by the radar beam.

A SAR image’s resolution is influenced by the following parameters: wavelength, bandwidth, pulse repetition frequency (PRF), acquisition mode and incidence angle. The system pa-rameters—wavelength, PRF and bandwidth—are defined by the system acquiring the data. A smaller wavelength and a higher bandwidth result in a higher resolution.

Resolution also is influenced by the acquisition mode. ScanSAR mode results in low resolution, StripMap mode offers medium resolution, and SpotLight mode acquires images with the highest available resolution.

In addition, resolution is influenced by an acquisition’s incidence angle. A more shallow incidence angle—called far range, illuminating an area far away from the sensor—results in higher resolution. But the typical radar effects, such as layover and shadow, have to be considered when determining the right incidence angle to avoid restrictive impacts of these radar char-acteristics, particularly in areas with steep terrain.

7. ApplicationsThe intended application, from feature extraction and

change detection to ocean surveillance and elevation model-ing, strongly influences the choices to be made about radar imagery acquisition. Choice of wavelength, incidence angle, acquisition mode and polarization have to be matched to the application.

8. Processing LevelAs with most remote sensing data, different processing levels

are available for SAR images. The right processing level has to be selected, depending on the application. The following basic processing options are available for SAR data:

Slant range data. These data are delivered in a complex data format in the sensor’s geometry and include information about the received backscatter at the sensor and information about the phase of the traveled signal. Some processing—normally available in commercial products—has to be performed before viewing these datasets. Typically slant range data are used for scientific applications such as SAR interferometry.

Ground range data. After the sensor data are transformed to Earth’s surface, a ground range product is produced as an im-age file. This product isn’t georeferenced, but it can be input for orthorectification.

Geocoded and orthorectified data. Georeferenced SAR data also are available. For orthorectification, satellite orbit and height information from the ground are processed with the data. There are two options available as standard products: geocoding and orthorec-tification. Geocoding uses an average height of the acquired data. Orthorectification uses a DEM. An orthorectified dataset should be selected if an application requires high geolocation accuracy.

9. Location AccuracyA SAR dataset’s location accuracy is a result of the following

parameters: orbit information precession, incidence angle and the accuracy of the input DEM for the orthorectification.

The orbit information’s precession is the basis for a highly accurate, automatic orthorectification. If the orbit information isn’t precise, the product’s location accuracy can be optimized manually using ground control points (GCPs), but the result depends on the amount and quality of these data.

The best available orbit information should be used to retrieve the highest location accuracy. In the case of the TerraSAR-X system, the geolocational accuracy is higher than the system resolution, thus the system can be used to derive GCPs without any need for ground truthing.

The incidence angle and the input DEM’s accuracy also influence location accuracy. The higher the DEM error and the steeper the incidence angle the higher the error in location ac-curacy. If high location accuracy is required—e.g., for mapping purposes—a precise DEM and a shallow incidence angle should be selected if available.

10. Pass DirectionImages can be recorded in either ascending or descending

direction. It’s important to consider the pass direction based on the surface characteristics and ground features present in the im-aged area. Images acquired over the same area from both ascend-ing and descending orbits can be merged to achieve the optimum look direction for features on the ground. Such image merges can be particularly useful in mountainous terrain, as typical radar artifacts, such as shadow or layover, can be overcome.

Reprinted for Astrium GEO-Information Services from Earth Imaging Journal © September/October 2012.

For more information about Astrium's SAR satellite capabilities please visit: www.astrium-geo.com/terrasar