Synthetic Aperture Radar

A Synthetic Aperture Radar ( acronym: SAR; German as: " Synthetic Aperture Radar " ) belongs to the class of imaging radars and is used as a sensor for remote sensing. There is a Side -Looking Airborne Radar - used as from aircraft or satellites and supplies this as a two-dimensional representation of a terrain cut by scanning the Earth's surface with electromagnetic waves, but with a much higher resolution. ( :; " Radar real aperture " shortcut German RAR ) refers to all radars that do not apply the method for SAR are as real aperture radar.

The images generated by a SAR are relatively easy to interpret because of their similarity with photographic images and are used for Erderkundungs ​​, mapping and reconnaissance purposes. A SAR is in contrast to optical sensors in almost all weather conditions, operational, since turbidity of the atmosphere due to fog, rain or snow weaken the microwave radiation compared with rays of light far less. In addition, an SAR, as each active radar sensor can also be used at night. One speaks in this respect also from an active remote sensing system which illuminates the observed objects themselves.

While the geometric resolution of a RAR deteriorated because of the diverging beam antenna with rising distance to object, can independent spatial resolution can be achieved even down to the meter and decimeter range with a SAR under certain conditions of the slant range and wavelength.

• 2.1 pulse compression
• 2.2 antenna
• 2.3 SAR processor

• 5.1 Aircraft SAR
• 5.2 Satellite SAR

The synthetic aperture

When spoken by a synthetic aperture radar, as is usually the so-called focused SAR meant: an additional focus of the individual signals is achieved by phase differences caused by time differences between the einzelnene antenna positions are compensated by the signal processor. If no phase corrections made ​​to the echo signals, one speaks of unfocused SAR.

Operation of a SAR

The SAR principle requires a moving perpendicular to the beam direction antenna, whose position is exactly known at any time. The direction of movement is commonly referred to as Along Track or azimuth (German: flight direction or azimuth ') and the transverse coordinate to as cross track or Range ( German: , transverse direction or distance ') respectively. In the literature, Along Track is also called crossrange. Footprint is called the range that currently captures the real antenna, Swath (German swath ) the strip of land to the footprint sweeps through the advancement of the real antenna. The geometry is that of a simple Side -Looking Airborne - radar.

The principle of the synthetic aperture ( here unfocused ) is to replace the snapshot of a large antenna by many images of a small moving antenna. In the course of this movement, each object is illuminated in the target area at a variable angle of view and recorded accordingly. If the path of the real antenna sufficient detailed knowledge and the scenery is immovable, can be synthesized aperture a large antenna from intensity and phase of the received radar echoes, and so a high spatial resolution can be obtained in the direction of the antenna. In practice, one can think of this as a very large phased array antenna whose individual radiators are not connected in parallel, but their positions are taken up by a small antenna sequentially in time. By the radar signal processor, the individual amplitudes and phase positions are connected to each other as if a phased array antenna would have been used with a very large aperture. The azimuthal resolution is still dependent on distance and is ½ ( λ ∙ R) ½ where R is the distance and λ as the wavelength used.

Modern computer technology allows for each mapped pixel, the phase of the received signal from that location can be changed. The SAR can then correct the time differences between the different antenna positions for each distance. Places that are closer to radar, have due to the trigonometric ratios larger running time differences, as places that are further away. This travel time difference is measured as a phase difference. From the recorded echo data own synthetic antenna is illuminated for each site calculated the angular resolution in azimuth is chosen so that the geometric along-track resolution is the same for all considered distances.

In this case, the following phenomenon is observed: For the same angular resolution, a synthetic aperture requires only half the length of a real aperture.

A clear explanation for this is: For a real aperture to changes in distance and thus measurable phase shifts of the radar echoes from the aerial view of a parallel past rover always refer to the location of the antenna center. With a synthetic aperture, in addition contribute to removal and phase changes due to the different position in succession along the real antenna of the synthetic aperture.

In order for a synthetic aperture can be realized, it is imperative that the radar system is fully coherent. That is, be known exactly, the phase relationship between the transmit and receive signal and transmit pulse to transmit pulse must. These usually one uses a high constant frequency source required by the all mixing and sampling as well as all time-periodic processes are derived.

Derivation of the geometric resolution in azimuth

The achievable with an optimal resolution SAR is equal to half the length of the real antenna in azimuth and flight direction, ie at a reduction of the azimuthal antenna length Laz ( in the figure referred to above with L) of the real antenna, the resolution capabilities of δ Az improved according to:

To derive the above three flight positions 1, 2 and 3 of the moving in azimuth antenna are shown in the diagram. As with the RAR the azimuthal angular resolution at the wavelength λ is:

Position 2 marks the location of the minimum distance from an object at the point P on the trajectory. If S0 is the corresponding slant range, the axis Daz the irradiated surface has length:

The point P is irradiated not only from the average flight position 2, but also from any position 1 to 3. The distance M of positions 1-3 thus corresponds exactly to the diameter of the antenna Daz luminous spot in the relevant distance S0. The SAR uses all information received from the object at point P, which come from all recordings in the range M = DAZ. Technically computing an antenna with the azimuthal length DAZ is simulated by uptake and storage of all values ​​. According to the above -mentioned property of the synthetic aperture with a according to equation ( 3) halved resolution:

But Daz is from Eq. (3 ) are known. Replace by Daz in Eq. (4 ) leads to Eq. (1):

Thus, the resolution of the synthetic aperture is independent of wavelength and object distance.

Alternative description of the SAR principle

Another description of the SAR principle provides the analysis of the Doppler shift of echo signals reflected from an object: Upon entry into the cone of rays of the antenna by an object the reflected echoes are displaced in the direction of higher frequencies due to the decreasing range. After passing the minimum distance (miss distance, just in Querabposition ) the distance increases again and the received signals are shifted to lower frequencies.

In the receiver, the center frequency of the echo signal is brought ( or superhet principle of superposition ) by mixing with the center frequency of the transmission signal to zero. The remaining deviations from zero is called the Doppler frequency or Doppler shortly. The Doppler profile of the echoes of an object from an initial positive values ​​through zero to negative values ​​is called Doppler history.

Each object with the same distance to the trajectory also has the same Doppler history, but shifted in time, as the arrangement along the flight path and airspeed corresponds.

Objects in other distances not mind either if they are closer, a shorter time or when they are removed, a longer history with the same Doppler frequency range, which is called the Doppler bandwidth.

Is not too large angle of the real antenna the Doppler history can be viewed as a linear course of the frequency versus time, i.e., the echo signal is mixed down to zero center frequency of an object having zero represents a linear frequency-modulated signal

These designated as ( down ) chirped waveform is due to the pulsed and coherent transmission signal before as a result of complex individual values. Multiplying these individual values ​​with corresponding values ​​of a similar chirp, but with increasing frequency ( up-chirp ), then the changes in the frequency underlying phase shifts cancel. The addition of the resulting individual values ​​then supplies the result of the synthetic aperture for the specific object under consideration.

This process is called correlation. The match to be generated for each range correlation function is called replica. It corresponds in the ideal case, the complex conjugate of a point target response values ​​.

While a customized correlation function causes a constructive addition of all individual contributions, an unmatched function has only an accidental result of addition result. In this way, the echo of the observed object, which is received concurrently with the echo of others, also the illuminated objects in the radar receiver is filtered out from the composite signal.

Alternative derivation of the geometric resolution in azimuth

The radar antenna is moving uniformly and unaccelerated v0 with speed. When the ever- changing is azimuthal angle formed by the direction of the object P at the antenna axis, the angle associated with the Doppler shift of the echo signal of this object is given by:

The approximation is valid for not too large angular apertures of the real antenna. The entire BD Doppler bandwidth of the echo signal is obtained if one uses the maximum azimuth angle used and the values ​​subtracted from each other:

The frequency of a signal of duration T can, at best, with a frequency resolution? F = 1 / T can be determined. When applied to the SAR signal, this means that the best frequency resolution is determined by the available observation period. However this is equal to the time it takes for the radar to traverse the distance M = DAZ:

It is referred to as aperture time. Consequently, the frequency resolution is by:

Limited. According to Eq. (6 ) corresponds to this Doppler frequency resolution of a spatial angular resolution of:

This corresponds to a spatial distance in azimuth by:

Consequently, this is the best possible resolution of a SAR in azimuth.

For the formation of the synthetic aperture can be thought of a filter bank, wherein:

Filters strung together cover the entire Doppler bandwidth. The echoes of an object appear, according to their instantaneous Doppler shift, one after the other at the output of each filter. If these detected signals and adds them together in time and in correct phase, the result is a K times have higher amplitude as compared with a signal at the output of a filter. The energy of the useful signal that is climbs the K ² times the value, the power of unwanted signal components, such as noise or echo from neighboring objects, however, due to the random nature of the additions, only K times. Thus, the signal to noise ratio (SNR = signal -to -noise ratio) improved - which is the ratio of useful energy to noise energy - also to K times.

The value K = TSAR BD is called time - bandwidth product. As one can easily recalculate, the resolution is the same as the synthetic aperture length divided by the time - bandwidth product and at the same airspeed divided by the Doppler bandwidth:

SAR example

To achieve an azimuth resolution of 1 m at 10 km distance, when using a real antenna an aperture length of 10 km / 1 m = 10,000 wavelengths is required. At 10 GHz transmission frequency, corresponding to 3 cm wavelength, which is around 300 m, which is a practically unrealizable size. As mentioned above, requires a corresponding synthetic aperture to be only half as long. Thus, the same resolution is accomplished with echo data recorded along a length of 5.000 wavelengths, or 150 m. But the real antenna must ensure that the object in question during the whole path can be illuminated. For this purpose, m is a real aperture length in azimuth of 10 km / 5,000 = 2 is required.

From the length of the synthetic aperture (in this example L = 150 m), a virtual near and far field of the synthetic aperture of the antenna can be calculated. The boundary between the two regions is rfern ≈ 2 · L2 / λ and here at about 1500 km. Only then, the electromagnetic waves of the different source locations would form a plane wave front. Most satellites have their orbits within this distance, so they are in the near field of the synthetic aperture. The distance to the target differs between the positions of the platform. If the target is located on the central axis of the real aperture, the distance is less than when the real antenna has to squint from a peripheral position toward the target. This expresses itself in a phase difference Δφ. Thus, not a simple summation of the real proportions of the individual charts are made ​​but it must be necessary in the near field as well as the imaginary part are considered. It follows that, in the image processing software for each pulse period to effect a phase correction is to produce a sharp image, resulting in the term "focused SAR".

The time - bandwidth product is according to Eq. (12) Then, 2 × 3 cm × 10 km / (2 m × 2 m) = 150, as according to Eq. (13 ) also needs to be. At a speed of 100 m / s, the Doppler bandwidth is 100 Hz, the aperture time 1.5 s and the best frequency resolution 0.67 Hz

Resolution in range

The image coordinate perpendicular to the flight direction (range ) is just as in RAR (also: Side -Looking Airborne - radar, SLAR ) generated by distance measurement. This is done by evaluating the different signal propagation times of the echoes differently from distant objects. Such a measurement can be carried out only in the radial direction ( = the direction of propagation of the transmitted signal ). So that a bottom surface in the transverse direction can be imaged by means of a distance measurement, the antenna look direction must have a lateral component. Thus, the ( projected onto the floor ) flight path of an SAR always at a certain distance parallel to the near edge of the swath.

The resolution in the radial direction ( Slant Range) is basically determined by the signal bandwidth of the transmitted signal used. At steep incidence angles, the achievable range resolution (Resolution Ground - Range) deteriorated in the plane corresponding to the projection of the radial distance resolution on the flat ground. At 45 ° angle of incidence, it is therefore around 1.4 times worse than in the radial direction. At normal incidence, a range resolution in the plane is no longer defined.

Essential elements of a SAR

Pulse compression

This gives a pictorial representation of the terrain flown, it makes sense to choose the ground range resolution comparable to the azimuth resolution. With respect to the slant -range resolution is initially the bandwidth of the transmitted radar signal:

C is the speed of light. For 1 m resolution ie 150 MHz signal bandwidth are required.

Compared with the Slant -range resolution, the ground range resolution is reduced more strongly due to the projection, the steeper is the grazing angle ε of the incident beam relative to the ground:

Therefore, the range resolution is often correspondingly fine selected as the azimuth resolution (at 45 ° or about 70 % of the azimuth value ).

In the first decades of the development of radar was used unmodulated pulses, ie signals that have been ' cut out ' for example from a continuous wave (CW from engl. Continuous Wave) by briefly keying the transmitter tube. Such a signal has a bandwidth that is inversely proportional to its length:

Increasing resolution requirements led therefore to shorter pulses; the resulting reduction in energy content they tried to compensate through ever higher transmission powers. Depending on the frequency range was 10 MW or higher pulse power can be realized. An increase in the pulse repetition frequency ( PRF Pulse Repetition Frequency) to improve the energy balance often are other aspects, such as, inter alia, the distance uniqueness contrary.

Because the pulse power for technical reasons ( dielectric strength of components) can not be increased arbitrarily, they increasingly went over to the pulse compression method in the 60s. For this purpose, a relatively long pulse is changed during the transmission in its frequency. Most commonly a linear frequency modulation ( LFM) is applied, in which the transmitter frequency changes linearly from a lower limit to an upper limit ( up-chirp ), or vice versa ( down-chirp ). The term chirp is because an acoustic LFM signal as chirping sounds. Incidentally bats use this waveform in the ultrasonic range.

The receiver side, the signal is transformed through the appropriate machinery in a bandwidth corresponding short pulse.

At the beginning we used analog SAW components (SAW surface acoustic wave, dt surface acoustic wave ) for pulse expansion and compression. A short pulse excites a surface acoustic wave traveling across a substrate having dispersive characteristics. At the other end of the substrate, the different frequency components arrive at different times, thus forming the desired LFM pulse. For compressing a similar SAW device is used with complementary characteristics and the stretched pulse time compresses again while maintaining its bandwidth to its original length.

Since the mid- 1980s, digital technology has advanced in frequency ranges beyond 100 MHz, digital signal processors are practically only used. These high-speed digital -to-analog converter, the signal from the pre-calculated data - if necessary in a plurality of frequency segments to be assembled - produce synthetically. When receiving the echo is uncompressed digitized and made ​​the pulse compression by a correlation method in the computer. The advantage of digital technology is that the replica can be obtained for the compression directly from the broadcast signal by looping into the receiver. The deviations contained in the broadcast signal from the ideal form, for example by distortions in high-frequency transmission amplifier (HPA of Engl. High Power Amplifier) ​​, are thus recognized immediately. By forming the complex conjugate function from the sampled data, the replica is created. A compression with this reference function corresponds to filtering with a matched filter ( engl. matched filter ), which, under white background noise, the output signal with the highest signal / noise ratio ( engl. Signal/Noise-, short S / N Ratio) supplies.

The characteristics of pulse compression are similar to those of the SAR signal in the Doppler domain. So here is the time - bandwidth product (often greater than 1000), the velocity factor of the chirp signal, as the gain in signal to noise ratio.

Finally it should be noted that the required for a given resolution bandwidth can be distributed over several pulses (frequency -step procedure). This reduces costly bandwidth requirements on the radar components. At the same time, however, the complexity of radar internal control and the SAR processor increases.

Antenna

Of the many known antenna types are only three commonly used for SAR applications:

• Reflector antenna: This antenna type is similar to satellite - TV receiving antennas widely used. The properties such as size, collection efficiency, sidelobe behavior, inter alia, are invariably set in the design. For a tilt in space ( for example, elevation or azimuth) a mechanical rotary device and / or multiple feed elements must be provided. The advantage of this type of antenna is its suitability for large bandwidth with cost-effective implementation. The reflector antenna requires an RF power amplifier (HPA, High Power Amplifier) ​​as a source of the transmission signal. The practically required RF power in the range of about 1 to 10 kW can be provided at present only by tube amplifiers, usually traveling wave tubes ( Traveling Wave Tube Amplifier, short TWTA ).
• Passive array antenna: a phased array antenna is comprised of many individual radiators which are arranged on a flat surface in a regular grid. Each of these lamps or an antenna element group is connected via a phase shifter to a feed network. The viewing direction of the antenna can be electronically scanned ( for fixed installations up to ± 60 ° ) by changing the phase shifter settings in a wide range. The advantage is the virtually instantaneous active beam control, as is often required with multi-mode radars and special SAR modes. Disadvantages are the high cost relative to the reflector antenna cost. Big swing angle and high signal bandwidth require special feed networks with real-time controllable runtime (English True Time Delay shortly TTD) to address the dispersion of the signals. The passive array antenna requires a central power source in the form of HPA.
• Active array antenna: This realizable only recently antenna is an array antenna in which each radiator or small groups of radiators each have their own transmission amplifier and a private reception part own ( Active Electronically Scanned Array). The agility of this antenna type corresponds to that of the passive array antenna with an additional degree of freedom is added by selective deactivation function of individual transmission amplifier. The high cost is justified by several advantages. To allow the distributed generation of the transmit power to use semiconductor amplifier with low operating voltage. In addition, the failure of individual amplifier not to the uselessness of the entire system (redundancy).

SAR processor

At the beginning of the SAR technology in the 1950 to 1960s, there were only the analog signal processing. For pulse compression is used SAW techniques and SAR focusing optical processors in the form of conical and cylindrical lenses ground. The drawback: the lenses were only respect for a well-defined geometry height and lateral distance used. With this method did manage to realize resolutions in the micron range, however, led the lack of motion compensation only in exceptional cases optimal results.

Only with the introduction of faster computers and analog to digital converter to the early 1980s witnessed the SAR principle the expected breakthrough. Due to the initially meager computational power were sought computationally efficient algorithms for SAR processing. The first time application of principle was the range-Doppler processor in which the focusing in two coordinates have been done mainly by the fast Fourier transform (FFT ENGL. Fast Fourier Transformation). These processors have worked with off-line data recording and the results provided only after each aerial survey. Other algorithms (chirp scaling, frequency scaling and v. a ) In the meantime available. These now allow a real-time SAR processing for very specific SAR modes ( see below).

A SAR focusing only has a good result for the sequence when the location of the antenna deviates less than about λ/16 from the ideal trajectory. At 10 GHz transmission frequency that is less than 2 mm! One of the important tasks of a SAR processor for systems in the air use is therefore today the motion compensation. On the one hand, the position and movement of highly sensitive data, GPS -based gyro platforms are recorded and evaluated and applied in addition autofocus computation method to detect the inevitable deviations from an ideal trajectory and eliminate them. Autofocus methods require a multiple calculating SAR image sections, in order to determine the motion error. Therefore, the required processing capacity for real-time requirements is considerably higher than in systems with no auto-focus capability.

Special SAR method

Special features of imaging by SAR

The images obtained by means of SAR have certain special characteristics that must be considered in the evaluation:

• Shortening ( foreshortening ): Under Foreshortening is called a condensed representation of actual distances ( compression of distances). Imagine a mountain, which is scanned by the radar beam of a SAR. The base of the mountain ( in the picture: a) first, reflects the radar beam, then the top (in the picture: b). If the two time points of reflection very close to each other, the actual distance (a - b ) is compressed between the base and top of the mountain (a ' - b' ) is reproduced. This effect complicates the interpretation of a mountain landscape.
• Superposition ( lay- over): At a high object, such as a tower, the spire has a smaller distance to the radar as the base point. The top of the tower is earlier, ie, closer mapped. As the impression of the overhang is formed in a radar image: the point b ' was before the point a ' are shown. This can result as in Foreshortening at pictures of mountainous terrains to interpretation difficulties.
• Shadow: Due to the illumination means entrained " light source ", the pictures shadow, so places without echoes reflected on. These arise, such as in optical imaging, where areas are shaded by higher objects of the radar beam. The effect is more pronounced the flatter the grazing angle and the higher is the shadow-casting object. On the other hand, allow the shadow also a good interpretation of virtually sculpted figures. A grazing angle of 5 ° is the lower limit for well- evaluated SAR images.
• Moving target displacement: A moving object is imaged in the wrong place. This is done because of the Doppler history is added to a solid object of the Doppler displacement of the moving object or subtracted. However, this corresponds to the history of a later or earlier disposed object. An object that moves seen from the satellite, appears closer in azimuth. The SAR photo on the right was taken from a satellite, which is to the north, that is from the bottom to the top of the screen, and its SAR sensor flown to the east, ie to the right, had done. Ships can be seen as bright reflections. Oil secretions on their travel path dampen surface waves. From there, the radar beam is only slightly reflected the lane appears black. Distinct bow waves are visible. Ships to move from right to left on the satellite, appear displaced upward in flight direction of the satellite. The ships that are above their lane. Accordingly, the moving to the right ships appear in the lower part below the dark line of travel.
• Speckle: Under Speckle one understands the nature of a coherent imaging that surface objects, such as ordered fields, from pixel to pixel, due to the random composition of the echoes from individual contributions, completely different values ​​can accept. Images with speckle effect, therefore, torn and grainy. Speckle can be reduced at the expense of resolution, by using the multi- touch method. To this end, several poor -resolution SAR images are calculated from different areas and then incoherent Doppler ( energetically ) added. The random distribution of the values ​​of an area - pixel provides for a reduction of speckle.

SAR applications

Due to its versatile applications, especially in remote sensing, SAR has a global role in the study so that the establishment of its own, specifically focused on SAR meeting was deemed necessary. The EUSAR is focused on the radar sensor, dedicated its technologies including image-forming signal processing and image processing, but also provides a forum for users of SAR data. EUSAR is still the only specialized SAR conference worldwide.

Aircraft SAR

Airborne SAR systems, due to their all-weather capability, mainly used for military reconnaissance. The present (2005 ) technically recoverable geometric resolution is below 15 cm, which makes an RF bandwidth of more than 1 GHz required. Since surveillance radar has multiple modes of operation, this system is always working with electronically pivoting passive or active growing array antennas with lengths of 1-4 m in azimuth. On motion compensation and real-time capability, great emphasis is placed, that is, the systems produce high resolution images on board and send it to the evaluating points on the bottom. The required processing capacity requires both the installation space, as well as the primary energy, the most of the available on-board resources. See also SOSTAR -X.

Another class provide mini -SARs for use on board of cruise missiles (drones ) dar. here asked (1-3 km ) is the smallest utmost unit volume at high resolution (< 1 m) and moderate strip width. In the meantime, can be installed on board even in these applications the required processor capacity, so that only already reclaimed end results must be transmitted to the ground via telemetry. At high carrier velocities, the necessary measures for motion compensation are low, so that the processor is loaded on board by this sub - task only relatively little.

Civil SAR is almost always used almost exclusively dedicated to mapping in the form of interferometric SAR on board of turboprops from. Their geometric resolution is usually in the range 0.5-2 m. The Jet Propulsion Laboratory used at the time a Gulfstream III, inter alia, to explore the consequences of the oil spill in the Gulf of Mexico.

Satellite SAR

Initially satellite SAR has been realized as a pure research projects currently in the phase of increasing military and civilian use, it enters. Militarily, the elucidation of any point on earth within given periods is required resolutions in the range of less than 1 m. To this end, several satellites with the same equipment and coordinated trajectories are required. To keep the cost down, cutting back on the equipment are inevitable. Thus the original years ago still required active array antenna (for example, SAR-Lupe ) long given in practical systems, a simple reflector antenna.

On the civilian side, the research-based SAR systems of the past are gradually being replaced by commercial offers for mapping custom areas. Again, the constraint leads to lower costs to favor the simplest possible systems.

The photo on the right shows a SAR image example. The small white dots are oil stations, the black areas of thin oil layers. The long-period water waves above are called internal waves, small waves with wavelengths around 100 m, see bottom arrow, are surface waves generated by wind. The detailed resolution of 25 x 34 km ² area is better than 100 m.

From the space probes Venera 15, 16, and Magellan was mapped by the SAR method, the planet Venus. The Cassini -Huygens mapped with the SAR method Saturn's moon Titan.

More information about already realized or conceived in the emergence of satellite SAR systems can be found on pages listed here:

In Earth Orbit

• Seasat
• European Remote Sensing Satellite 1 & 2
• Envisat
• Shuttle Radar Topography Mission
• RADARSAT -1
• RADARSAT -2
• TerraSAR -X
• SAR-Lupe
• COSMO- Skymed
• Active Electronically Scanned Array
• Lacrosse (satellite)
• Yaogan Weixing -1

To other celestial bodies

• Venera 15 and 16
• Magellan
• Cassini -Huygens