Multi-Dimensional Imaging with Synthetic Aperture Radar

By Gianfranco Fornaro, Antonio Pauciullo, Vito Pascazio and 2 more

Multi-Dimensional Imaging with Synthetic Aperture Radar: Theory and Applications provides a complete description of principles, models and data processing methods, giving an introduction to the theory that underlies recent applications such as topographic mapping and natural risk situational awareness – seismic-tectonics, active volcano, landslides and subsidence monitoring - security, urban, wide area and infrastructure control. Imaging radars, specifically Synthetic Aperture Radar (SAR), generally mounted onboard satellites or airplanes, are able to provide systematic high-resolution imaging of the Earth's surface. Recent advances in the field has seen applications to natural risk monitoring and security and has driven the development of many operational systems.

• Explains the modeling and data processing involved in interferometric and tomographic SAR
• Shows the potential and limitations of using SAR technology in several applications
• Presents the link between basic signal processing concepts and state-of-the-art capabilities in imaging radars
• Explains the use of basic SAR processing tools and datasets

• Language: English
• Publisher: Academic Press
• Release date: Jan 31, 2024
• ISBN: 9780128216576

Gianfranco Fornaro

Gianfranco Fornaro received the M.S. degree (summa cum laude) in electronic engineering from the University of Naples “Federico II” in 1992 and the Ph.D. in 1997. Since 1993, he has been with IREA-CNR, where he now holds the position of Research Director, working in the area of airborne and spaceborne Synthetic Aperture Radar (SAR) processing, including SAR Interferometry and SAR Tomography. In 2013, he received the “Full Professor” habilitation in Telecommunication, and in this area, he has been Adjunct Professor at several Universities in South Italy. Dr. Fornaro has been a visiting scientist at Politecnico of Milan and DLR in Oberpfaffenhofen (Germany), also during the 1996 SIR-C/X-SAR mission. He was a NATO lecturer in the Lecture Series SET 191 and SET 235, and since 2011, he has been also a lecturer at the International Summer School on Radar/SAR of the Fraunhofer Institute. He has been a convener, tutorial lecturer, chairman, and member of the program and organizing committee at the most important IEEE conferences. He has authored more than two hundred papers on SAR (peer-review journals and proceedings of international conferences). He received the Mountbatten Premium from the IEE Society in 1997, the 2011 IEEE Geoscience and Remote Sensing Letters Best Paper award, and the 2011 best Reviewers mention of the IEEE Transactions on Geoscience and Remote Sensing journal.

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1: Introduction

Abstract

The chapter provides an introduction to the book. A brief history of radar and SAR development is presented. Then the concept or radar detection and ranging as well as imaging are briefly summarized. The remaining part of the chapter is dedicated to the description of the radar equation and to the introduction of the interferometric techniques including differential Interferometry and the more general concept of tomography.

Keywords

Radar; radar imaging; Synthetic Aperture Radar (SAR); radar equation; interferometry; differential interferometry; tomography

1.1 Brief history of radar and SAR development

At the end of the 19th century the experiments conducted first by Heinrich Hertz and after by Guglielmo Marconi, highlighted the reflection of electromagnetic waves by metallic objects, an effect predicted some decades before by James Clark Maxwell in its theoretical dissertation of electromagnetism.

The idea by Nikola Tesla to exploit the reflection of radio waves for the detection of moving targets came into reality in the 20th century. It was in fact thanks to the intuition by the German inventor Christian Hülsmeyer for the application to collision avoidance, that the telemobiloscope, i.e., the predecessor of modern radar, was patented in 1903. The concept came to the demonstration in his experiment in 1904 in Cologne, when the telemobilscope showed its capability to detect the passage of a barge some hundreds of meters away and ring a bell. However, due to the inadequacy of the high frequency electronic equipment available at that time, it took several decades for radars to make a substantial progress.

With the development related to shortwaves before, and after of microwaves, with generators as the magnetron capable to generate short high-frequency pulses, the radar experienced a huge development. Modern applications go beyond the traditional detection and tracking scopes related to surveillance, anticollision, assisted guidance, and automotive, but embrace also meteorology, altimetry, ground-penetrating and through-the-wall imaging, and finally the main topic of this book, i.e., remote sensing and Earth observation technologies. The latter have major impact for natural hazard monitoring and security as well as agriculture, forest, and urban mapping.

More specifically the early development of modern radar was carried out secretly for military uses in Germany, England, and USA, and started during and in-between the World Wars periods of the XX century.

In 1922, A. Hoyt Taylor, and Leo. C. Young at the Naval Research Laboratory (NRL) in USA demonstrated the possibility to detect the reflections of high-frequency radio waves from a wooden ships crossing a system composed by a transmitter and a portable receiver. Based on an idea suggested by Taylor, Robert Morris Page, who joined the Taylor and Young radar group at NLR, demonstrated in 1934 the possibility to operate detection of objects with a short pulse, thus giving also the possibility of target ranging, i.e., of measuring the distance of a target from the radar. Thank to this experiment, Page, Taylor, and Young are usually credited as developer and demonstrator of the world's first true radar, i.e. radar as essentially known nowadays.

Still at NRL, in June 1936, the first radar system prototype was demonstrated to government officials: it successfully tracked an aircraft at distances up to 25 miles. Similar progress was achieved also in United Kingdom by the Royal Navy where, in 1938, it was developed by the first air-warning radar system able to operate between 30 and 50 miles. The system was immediately after installed as well as on ships. Almost in the same years, in Germany there was a significant development as well. Mainly handled by private Companies, programs for development of radar systems with detection and ranging capabilities took place. The reported operating range was slightly lower than NRL and Royal Navy ones.

The term RADAR was coined in 1940 by the United States Navy as an acronym for RAdio Detection And Ranging. Radar technology experiences a sharp increase in development during World War II. It was during this period that radars were, for the first time, used as imaging systems by extending its capability to profile the backscattering along the distance (range) for bombsight systems. Ground scanning radar were built after that experiments conducted by Royal Air Force in 1941 revealed distinct returns by different land covers. H2S in 1943 was the first operative ground scanning radar system and exploiting antenna scanning capabilities to discriminate target coordinate across the range, thus producing 2D images. The development in US took place almost one year after with the AN/APQ-7, or Eagle, developed by the US Army Air Force.

Investigations to improve the Radar resolution in terms of pulse duration and sharpening of the scanning beam of the real antenna, that are the basis of modern Synthetic Aperture Radar (SAR), were however carried out in the post World War II. The resolution of a Radar is its ability to distinguish between very close targets. The smaller the distance between the targets, the better (the higher) the resolution. We could say that the resolution of a Radar is the minimum distance that two targets must have in order to be distinguished from each other. This concept will be taken up extensively in what follows.

A viable and simple solution that paved the way to the subsequent SAR development, was the Side-Looking Radar, i.e. an imaging system with a beam directed off-nadir to the ground and moving solidly with the aircraft, thus providing an additional dimension beyond the distance. The system was thus able to illuminate a strip on the ground and provide 2D resolution capabilities. This system, later called Real Aperture Radar (RAR), was in fact capable to measure the backscattering as a function of the distance (range), across the track, and of the along track position flown by the aircraft (azimuth). Range is discriminated by the pulses, whereas the azimuth is discriminated by the movement of the beam footprint. A demonstration flight of side-looking radar imaging was carried out in 1950 over the Detroit area.

Despite the important achievement to have a simple and flexible imaging system at short/microwaves, compared to systems operating in the visible and near infrared region, radar imaging was strongly hampered by the diffraction limited angular extent, and hence by the spatial resolution, of an aperture antenna. The latter is directly proportional to wavelength and inversely proportional to aperture dimension. Therefore moving from micrometer wavelengths band, associated with the visible spectral region, to the centimeter wavelengths associated with microwaves used in radar systems, the angular aperture increases by a factor roughly of the order of 10⁵. As a consequence, with the development of the electronic devices able to generate very short pulse, it was soon recognized that RAR was characterized by a rather unbalanced resolution degree, that is a high range resolution but a poor azimuth resolution.

The problem to increase the azimuth resolution was brilliantly solved by Carl A. Wiley in 1950, who was working on the idea to get high resolution by observing the Doppler shift of stationary ground targets in a moving antenna beam. What was later renamed as Synthetic Aperture Radar (SAR), that is a coherent radar able to discriminate targets along the flight direction by exploiting the Doppler effect, was reported as Doppler Beam Sharpening in a Wiley Goodyear Aircraft report in 1951. Early in 1952 Wiley got the first operational SAR. The patent application Pulsed Radar Doppler Method and Apparatus was filed on 13 August 1954, and classified from 1955 to 1964. Several years after, the SAR technology from satellite become of great interest both for military and civil aims. Spaceborne SAR conjugates synoptic view and cloud/sun independent observation features (Fig. 1.1); the latter allows regular and systematic observation capability, thus making SAR a unique Earth Observation system for emergency use as well as for the observation of dynamic phenomena.

Figure 1.1 Comparison between images collected at the same instant by multispectral (MERIS) (left image) and SAR (ASAR) (right image) sensors onboard Envisat of the European Space Agency (ESA). The images show the capability of SAR to collect data relevant to the Earth surface independently of the presence of clouds.

The first civilian spaceborne imaging radar instrument (SAR) flew on Seasat in 1978 and the whole mission was handled by NASA Jet Propulsion Laboratory (JPL). Then, starting from 1981 NASA/JPL managed a sequence of four Shuttle Imaging Radar (SIR) missions: SIR-A (1981), SIR-B (1984), and two SIR-C/X-SAR missions in April and September 1994. The latter missions were carried out by NASA/JPL in cooperation with the German and Italian Space Agencies, DLR and ASI. The SIR-C/X-SAR mission was dedicated to interferometric experiments: a technology that exploits angular diversity of images acquired with two antennas, or close repeated passes of a single antenna, to reconstruct the Digital Elevation Model (DEM) of the observed scene. The efficacy of such technique for DEM reconstructions was in fact clear since the first demonstration carried out by the exploitation of the data collected by the ERS-1 and ERS-2 satellites, in the framework of the ERS program, managed by the European Space Agency (ESA) and specifically dedicated to SAR. ERS-1 satellite was launched at mid 1991, followed by ERS-2 in 1995. The two satellites were flying in a tandem configuration with a revisit time useful for SAR Interferometry (InSAR) scopes of only one-day. As explained in the book, a revisit of one day allows achieving good interferometric coherence over repeated acquisitions but does not guarantee absence of variation of the conditions of the atmosphere, which acts as a disturbance for InSAR. ERS was a successful program with ERS-2 operating till 2011, and provided data that has been exploited for many years by scientists worldwide to test new approaches and develop the SAR technology for civilian applications.

The NASA/JPL SAR programs exploiting the Shuttle platform culminated with the Shuttle Radar Topography Mission (SRTM). Based on the interferometric experience gained wit SIR-C/X-SAR, NASA/JPL coordinated a program with the participation of DLR and ASI aimed at collecting, at C-Band and X-Band, simultaneous interferometric data with two antennas (at each band) for the generation of what was at that time the most accurate DEM available on a quasiglobal scale. The unique characteristics of SAR operated on satellites, and its application to the natural risk monitoring and security area have been the drivers for the development of what is now considered a new era for spaceborne SAR characterized by increasing investments of international Space Agencies, and more recently also by the private companies. Fig. 1.2 shows the development of space programs since 1992, when the first operational sensor for interferometric use (ERS-1 by ESA) was launched, gives a clear idea of the growth of this sector. The sensors are grouped in the three most used frequency bands: L-Band (frequency ∼1.2 GHz, wavelength ∼25 cm), C-Band (frequency ∼5.4 GHz, wavelength ∼5.6 cm), X-Band (frequency ∼9.6 GHz, wavelength ∼3.1 cm).

Figure 1.2 Evolution over the time of SAR sensors for the three most used frequency bands.

1.2 Radar: detection and ranging

The radar detection principle is depicted in Fig. 1.3. An antenna transmits a signal by concentrating the radiation, with a directive antenna, within a specific angular sector in which the radar seeks for the presence of targets. The radiation hits the targets and a percentage of the incidence radiation is scattered back to the radar. The backscattering characteristics depend on the radar cross section, a quantity dimensionally equivalent to an area and hence measured in m², that indicates the captured power which is then radiated back to radar. Returns from targets located at different distances arrive to the radar at different times. Shaping the transmitted signal to a pulse and recording the time arrivals of the different pulses allows the discrimination of different targets located at different distances (ranges). The target ranging principle is schematically depicted in Fig. 1.4.

Figure 1.3 Target detection principle used in radars.

Figure 1.4 Target ranging principle used in radars.

Consider now the case of two targets present at the same time in the illumination beam of a radar. The radar, assumed to be a pulsed radar, transmits a rectangular pulse that hits, at different times, the two scatterer located at different distances (ranges) and . The echoes from the target travel with the same velocity toward the radar and they are recorded at different instants, say and . The propagation velocity of an electromagnetic wave travelling in the air is known and well approximated by the speed of light in the vacuum c; measuring the time arrivals since the transmission allows therefore determining the different target ranges:

(1.1)

where we have considered the fact that the radiation travels forth and back thus doubling the ranges. The separation in range between the scatterers (range difference) can be evaluated from the time separation as follows:

(1.2)

In order to measure the time separation, we need that the return pulses do not overlap. Therefore, the difference of the distances between the targets and the radar ( ) must be such that the corresponding time separation ( ) given by (1.2) is larger than the pulse duration. Therefore the lower the duration of the transmitted pulses, the lower is the distance at which the scatterers can be discriminated (resolved), that is the higher is the resolution capability of the radar. Key principles of target ranging for surveillance radars, which is the one of the basic mechanism exploited to discriminate scatterers also in radar imaging, is the subject of Chapter 2.

Another characteristic of radars is the possibility to measure the component of the target velocity along the line from the target to the radar, referred to as line of sight or briefly los. The possibility is offered by the effect described in 1842 by Christian Doppler and related to the variation of the frequency in a radiation emitted by a moving source. If the radar transmits a coherent radiation, for instance a pure sinusoidal tone at frequency then, due to the Doppler effect, the radiation backscattered by a moving target and received back by the radar will be characterized by a frequency shift which is positive or negative depending on the fact that the target is approaching or moving away from the radar. The effect is depicted in Fig. 1.5.

Figure 1.5 Doppler shift for moving targets.

As explained in details in Chapter 2, if the target moves with a velocity component along the range, say with v being the velocity vector and the range versor which identifies the los. The frequency of the received signal will be where the Doppler shift is given by:

(1.3)

where λ is the wavelength associated with .

The intensity of the recorded signals provides, in addition, an indication of the characteristics of the target in scattering back the impinging radiation. Such a characteristic of the target is quantified in what is referred to as radar cross section, i.e. a parameter which is discussed in the next section in order to set the radar power budget.

1.3 Radar imaging

Since the early developments of radars, it was clear the possibility to exploit the radar for imaging scopes. This can be achieved thanks to the characteristic of a radar to sense different levels of scattering from targets and the possibility to determine their ranges. Imaging systems are characterized by 2D discrimination capabilities, at least, that is for radars range and across-range. As recalled in the first section of this chapter the first developments of radar imaging systems were associated with the possibility to exploit the across range antenna beam scanning. Actually, the radar beam can be scanned across range either mechanically or electronically to provide, at different (aspect) angles, profiles along the range of the backscattering. But more can be done: a radar can be installed on a moving platform to provide range backscattering profiles at different positions along the flight direction. This is the principle of the side looking radar imaging shown in Fig. 1.6.

Figure 1.6 Real Aperture Radar (RAR) for side-looking ground imaging.

An antenna is installed on a platform moving along a direction referred to as azimuth and illuminates the scene on the ground at broadside direction (orthogonal to the flight path). Ground targets distributed on the scene are discriminated along the range by using the pulsed mechanism discussed for the classical surveillance radars. The antenna is pointed in the side looking direction because of the need to avoid the left and right ambiguity, related to the situation in which targets symmetrically positioned with respect to the nadir and sharing the same distance provides overlapped responses.

The discrimination of targets along the azimuth is performed by the beam. Targets located at the same range within the beam will provide contributions overlapped in time. Exploiting the relation that the 3 dB angular aperture (beamwidth) of an antenna with size is given by

(1.4)

we have that the antenna azimuth footprint is given by

(1.5)

where r is the range of the target, which is the distance between the target and the trajectory.

As explained in Chapter 2 the use of pulse bandwidth ( ) of the order of hundreds of MHz and the adoption of pulse compression techniques allows easily reaching achieving (slant) range resolutions equal to:

(1.6)

and therefore of the meter or even submeter order, over a swath of width Y in ground range. The latter can be of the order of a few or tens of kilometers, depending on the exploitation of airplanes or space platforms. According to what stated above we have that the resolution along the azimuth of a RAR is:

(1.7)

Supposing to operate an airborne radar from a distance r of about 5 km and a rather large antenna with m, even by operating at high microwave frequencies such as X-band ( cm) would provide an azimuth footprint, and therefore an azimuth resolution, of more than 150 m. RAR systems are therefore characterized by an unbalancing of the resolution capabilities. C. Wiley had the idea to improve the azimuth resolution by using the movement of the antenna, which makes it possible to discriminate targets located forth or back with respect to the radar on the basis of the variation of their Doppler frequencies.

The capability of a coherent radar to improve the azimuth resolution by exploiting the Doppler discrimination principle can be also explained with the so-called aperture synthesis principle. The radar, during its motion along the azimuth, occupies positions corresponding to the elements of a hypothetical array of antennas. A large antenna, as large as the azimuth footprint, can be synthesized, thus providing the possibility to sharpen the beam via a data processing referred to as SAR Focusing. Fig. 1.7 shows the result of the SAR focusing operation for the ESA Envisat sensor over the Strait of Messina in South Italy: the two images before and after the focusing provide a comparison of RAR vs SAR.

Figure 1.7 From RAR to SAR via data focusing. The image on the left shows the data received by the radar before the azimuth focusing (azimuth is vertical). The image on the right shows the result after the azimuth focusing.

The SAR model providing a mathematical description of the received signal, its spectral characteristics and the data processing operation necessary to achieve pulse compression in range and azimuth (azimuth beam-sharpening) are described in detail in Chapter 4 and 5. In particular in Chapter 4 it will be shown that the azimuth resolution is given by:

(1.8)

and therefore a meter order resolution can be achieved as well as in azimuth. To show an example of the capability of SAR to generate images with resolutions comparable to optical images, operating at frequencies much higher that microwaves, a comparison between an image collected by one of the satellite of the Very High Resolution (VHR) COSMO-SkyMed SAR constellation, and a Google Earth image of an industrial area close to the city of Matera in South Italy is provided in Fig. 1.8.

Figure 1.8 Resolutions achieved by modern Very High Resolution systems. Left: image with 1 m resolution captured by COSMO-SkyMed SAR over an industrial area close to the city of Matera in South Italy. Right image: The corresponding optical image from Google Earth.

1.4 Radar equation

The radar equation describes the physical power budget of a given radar system configuration, providing the received power as a function of assigned transmitter power and for a specific target at a given distance, after the attenuation of the transmitted signal due to propagation. This equation ties together the radar characteristics (e.g. transmitted power, antenna aperture), the target properties (e.g. target radar cross section), the distance between target and radar (e.g. range) and the properties of the propagation medium (e.g. atmospheric attenuation). It represents a powerful tool for the radar designer, since it provides useful information about performance of the system without resorting to complex analysis and simulations. It is especially useful in early stages of the system design, when specific information about various components of the system (f.i., transmitted waveform, antenna design, signal processing algorithms, etc.) might not yet be