Advanced Synthetic Aperture Radar (asar)

Richard Wrigley

Advanced Synthetic Aperture Radar (ASAR) Introduction In this essay I intend to describe the technical specifics of ASAR, describe its remote sensing technique, examine measurements made by ASAR as well as discuss future measurements, and discuss the importance of ASAR’s measurements. Advanced Synthetic Aperture Radar (ASAR) is an advanced model of the Synthetic Aperture Radar (SAR) instrument. ASAR was launched on the European Space Agency (ESA) mission ENVISAT in 2002. ENVISAT is used for Earth observation. The ASAR instrument is an active remote sensor used to study features of the Earth such as sea surface winds, topography of the Earth and natural hazards.

Technical Concept of ASAR and SAR ASAR uses synthetic aperture radar to obtain images over its observation area. Synthetic aperture radar involves using pulsed-Doppler radar that emits short bursts of radio pulses to obtain an image of the targeted area. ASAR is able to transmit several hundred pulses while ENVISAT passes over a certain object. These pulses backscatter (reflect) in many different directions due to the shape and composition of the object that is being studied. The backscattered pulses can be manipulated using signal processing to produce an image that would have only previously been obtained by placing a large antenna in orbit, which would be too expensive. ASAR therefore provides a cheaper alternative for high resolution imaging of the Earth by creating a synthetic aperture. However, if the processor onboard ASAR used all the backscattered pulses to produce an image of the object being studied, this said image would contain speckle. Speckle is a type of interference or ‘noise’ caused by ASAR’s fine resolution and how coherent the pulse signals are. It is created by particular objects on the Earth’s surface that reflect the pulsed signal well when ASAR is viewing these objects from a particular angle. This speckle can be minimised by processing the data obtained over a certain object in stages. If for example a quarter of the returned samples were processed first to obtain a prediction for the cross-section of the object, a basic image of the object can be produced. Then another quarter of the samples are processed to obtain another prediction of the cross-section it will produce another basic image, which can be averaged with the first quarter’s sample. This would be combined with the third and fourth quarters’ samples to produce a final image. This method is called the ‘4-looks’ method, as ASAR effectively looks at the same object four times to produce its’ final image (See fig 1.1). This method however, is not restricted to four looks. The speckle of an image is reduced more by more looks. The range of ASAR is described as ‘the distance along the view of the radar system.’ (Source: PA2604 lecture 9 slides, J. J. Remedios). As ASAR’s radar view is 1

Richard Wrigley perpendicular to the velocity of ENVISAT, it means that it ASAR views along the path that ENVISAT is travelling. (See fig 1.2) The slanted range of the radar is equivalent to the ASAR’s radar distance. Also, the ground range of ASAR refers to distance along the surface of the Earth. The spatial resolution of ASAR is achieved by gating the signal for range as well as measuring the Doppler shift. The Doppler shift is the change in the velocity of the transmitted radio wave. ASAR can achieve a spatial resolution generally around 30m. Although when ASAR is at certain angles it has been able to image long roads, and oil pipelines. The range resolution (δy) can be expressed as:

δy = ∆r/(2sinθ) = cτp/(2sinθ) [1.1] (Source: PA2604 lecture 9 slides, J. J. Remedios). Where ∆r is the pulse length of the radar. θ is the incidence angle (which in ASAR’s case is between 15 and 45 degrees (Source Kongsberg Satellite Services)). τp is the duration of the pulse. Finally, c is the speed of light. The azimuth resolution (where the azimuth is perpendicular to the track of ENVISAT) is expressed as:

δx = D/2 [1.2] (Source: PA2604 lecture 9 slides, J. J. Remedios). Where δx is the azimuth resolution, D is the width of the antenna. D is affected by the swath width, which can extend up to 400km. ASAR is calibrated by the European Space Agency. ESA calculates an external scaling factor, which uses three transponders. ASAR is calibrated by taking estimates from the elevation pattern, which are obtained by scanning targets on the Earth’s surface that already have known properties. For example vegetation such as grassland, forests, rain forests etc. The value for the elevation pattern taken by ASAR is then compared to the estimates, which allows for calibration of the instrument. (Source: ASAR Product Handbook, ESA). ASAR operates in two modes, as a conventional stripmap SAR (also known as the image mode) and also as a ScanSAR (See fig 1.3). When operating in the image mode, the antenna array has the flexibility to allow ASAR to alter the swath width by varying the size of the pulsed-Doppler beam and the angle of incidence. (The angle at which the beam hits the Earth). When ASAR is operating in the image mode it generally has a swath width of between 56 to 100km and a resolution of about 30m. (Source: Eurimage Products & Services). Although the image mode allows a precise measurement of an area, its swath is small. This means that the area studied by ASAR be small as well. Although precision is useful to study small areas, ENVISAT is intended to study the entire Earth’s surface. Therefore ASAR needs to be able to scan larger areas. This is achieved by using the ScanSAR principle. The swath width of ASAR is increased by electronically steering the elevation of the antenna beam of ASAR. The images are created by scanning the angle of incidence, as well as synthesising images for the multiple beam positions. Each area scanned is known as a

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Richard Wrigley sub-swath. One sub-swath is scanned by the radar, then the following, until all the swaths have been scanned in order to achieve full image coverage area of the swath. ASAR has five determined sub-swaths that overlap each other to effectively cover the entire swath. This is able to achieve a swath of 400km, but has a resolution of only 150m, meaning that it is not effective at imaging areas in depth, unlike stripmap SAR. In addition, ASAR utilizes another measurement mode, the Alternating Polarisation Mode (See fig 1.3). This is a modified version of the ScanSAR technique. Instead of scanning at multiple angles of incidence, it scans a single swath at two polarisations (HH and VV). This produces two images of the same area that can be combined to produce a more detailed image of the area than what could have been achieved with a single polarity.

Remote Sensing Technique Used ASAR is an active sensor, as it uses a man-made radiation source to study the surface of the Earth. This has several advantages over passive sensing, which depends on natural sources of radiation e.g. visible light. ASAR can operate at night, as it is not dependant on visible light. It can study objects that are obscured by cloud cover. As one of ASAR’s main objectives is to study the Earth’s oceans, it has to be use short radio waves otherwise there would not be a decent amount of backscatter in order produce a decent image. ENVISAT has a polar orbit. The orbit is also sun synchronous, meaning that ENVISAT stays still relative to the position of the sun. This means that as ENVISAT stays ‘still’ as the Earth rotates, meaning that ENVISAT can fly over any position on the Earth’s longitude. Its polar orbit means that ENVISAT can fly over any position along the Earth’s latitude. This means that over a given amount of time ENVISAT will be able to cover the entire surface of the Earth, and therefore so can ASAR. The data obtained by ASAR is downloaded in real time to its ground receiving station. If ENVISAT is out of range of its receiving stations, then the data is stored on ENVISAT’s onboard tape recorder until it is in range of a station. (Source Kongsberg Satellite Services.) However, its high data rate of ASAR limits it operational time to approximately 30 minutes per orbit. One orbit of ENVISAT takes 101 minutes to complete, meaning that ASAR is only operational approximately 30% of the time. The frequency used by ASAR is in the C-band, which has a frequency range from 4 to 8GHz, and a wavelength range of 3.7 to 7.5cm. (See Table 1) This is important, as one of ASAR’s main goals is to study the ocean surfaces. A small wavelength is needed to produce a decent amount of backscatter. If ASAR used a long wavelength, then it would not produce a large amount of backscatter from the ocean, meaning that there would only be reflection over water and the oceans would appear dark on images produced by ASAR. ASAR also uses Interferometric processing where two views of the same location on the Earth are combined together to create an image showing the change of phase of the backscattered waves from both viewing positions (See fig 1.4). This is a similar to the effect in a Michelson Interferometer, and is used to study shifts in the Earth’s terrain. (E.g. earthquakes) 3

Richard Wrigley

Applications, Measurements and Future Measurements Due to the fact that ASAR can still observe the Earth at night, and through clouds, added to the fact that it is very precise at measuring distances means that ASAR’s primary measurements and observations include sea ice monitoring. This involves studying the movements of icebergs, and studying the amount and rate of ice breaking away from the polar icecaps due to climate change and other factors. Studying the movements of ice can also allow safer navigation of the oceans by ships, real time data is relayed to icebreakers and allows redirection of sea vessels. The measurement for the flow of ice can also be used to forecast the weather. Cartography (mapping) is another application of ASAR. This can be used to produce up to date maps for navigation as well as Earth Observation Science. Surface deformation detection uses the interferometer on ASAR to study the effects from the Earth’s tectonic activity. A prime example of this is the study the shift of land after an earthquake. Another example will be the distortion caused by an eruption of a volcano. Interferometric testing conducted by ASAR can be used hopefully in the future to be able to predict an earthquake. Glacier monitoring is also one of the main applications of ASAR. ASAR detects the movements and shrinking of glaciers. This provides vital information on the affect climate change is having to the Earth. The method used to monitor the movements of glaciers is similar to the method used to measure the surface deformation. ASAR also monitors the change in snowfall over one location over a given amount of time. Again, this is vital to research into climate change, but also has more practical purposes for human habitation of that area. It can warn against a build up of snow that could lead to an avalanche, and possibly even provide weather and snow conditions for ski resorts. Other applications include crop production forecasting and forest cover mapping. When an image produced by ASAR contains a higher level of backscatter than usual, it means that the terrain being observed is ‘rougher’ than usual. This most likely is due to vegetation. ASAR can therefore can over time study the change in vegetation over particular areas, by studying the change in ‘roughness’ over an area. Two examples of this could be imaging the destruction of the rain forest by manmade on non-manmade influences, and the decrease in crop production over areas that have been affected by long periods of drought. One of the most important uses of ASAR is to study the wave spectra of oceans. As mentioned in the section ‘Basic Sensing Technique Used’ this requires a short wavelength to produce good quality images of the ocean surface, as larger wavelengths would not result in enough backscatter for ASAR to image the oceans without looking black. Other applications and measurements made by ASAR include urban planning, and the study of coastal erosion. ASAR has also been used to study the extent of damages caused by natural disasters. 4

Richard Wrigley Two of these that caught the attention most of the public would be the damaged caused to South East Asia by the tsunami in December 2004, and the disaster in Louisiana by Hurricane Katrina in the summer of 2005. Current and future measurements conducted by ASAR include the mapping of Antarctica, the Amazon Rainforests, and creating ‘snapshots’ or the Artic ice.

Importance of Measurements ASAR’s numerous measurements and applications are vital for many applications back on Earth. Its ability to accurately map the movements of ice flows is vital for safe navigation of the oceans. It allows icebreaker ships to clear paths through unexpected ice flows, as well as being able to redirect ships about to enter these flows, thus preventing numerous maritime disasters. The monitoring of glacier movements, changes in snowfall, and the reduction of the size of the polar ice caps provides further evidence to support the theory that climate change is having a negative impact on the Earth’s environment. This evidence is helping to persuade many international statesmen to reform their environmental policies, and helps support the idea of lowering carbon emissions to reduce the impact of climate change. Its imaging of the rainforests and the reduction of its area by manmade factors (logging, farming, etc.) again will help to persuade politicians to enforce stricter controls on companies operating in the rainforests. ASAR also provides vital images and measurements of disaster areas. These images will aid in the future redevelopment of these areas, as they will they can be used to show areas where it would be safer to rebuild to avoid such damage if a similar disaster were to happen in the area. Its interferometer, which is able to study the impact of earthquakes and volcano eruptions, is being used to be try to predict the location and date of future earthquakes. The measurements made of the oceans, allows us to study their wave and tidal characteristics, study ocean fronts, the dynamics water at coastal areas, and detect oil slicks.

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Richard Wrigley

To summarize, ASAR is an active sensor with a polar orbit. It uses pulsed-Doppler radar to transmit short radio pulses in order to obtain images of the surface of the Earth. Its synthetic aperture is created by combining of samples the radar received, and combining multiple readings together. The ASAR instrument is used to carry out numerous measurements and observations. It is primarily used for studying the ocean currents and surfaces, studying the change in vegetation (e.g. rainforests, crops), the study of earthquakes and volcanoes, as well as the study of climate change. This means that ASAR is vital to for the study of climate change, its measurements and evidence gathered can be vital to help persuade politicians to implement schemes to reduce human impact on the Earth.

References Books L. Marelli, “SAR Image Quality” 1st Edition, ESA Scientific & Technical Publications Branch, 1980. R. H. Stewart, “Methods of Satellite Oceanography” 1st Edition, University of California Press, 1985. “A Dictionary of Space Exploration (Oxford Paperback Reference S.” 1st Edition, Oxford University Press, 2005. Websites 2604 Lecture Slides (Lecture 9): http://leos.le.ac.uk/home/group/jjrlect/rs2603.html ASAR Product Handbook (ESA): http://envisat.esa.int/handbooks/asar/ ESA Missions: http://envisat.esa.int Kongsberg Satellite Services: http://www.ksat.no/Downloads/Envisat%20Products.pdf Eurimage Products and Services: http://www.eurimage.com/product/envisat.html

Appendix: Images, Diagrams and Tables

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Richard Wrigley

Fig 1.1

Source: http://envisat.esa.int/handbooks/asar/aux-files/ephimg-14467725.jpg

Source: http://envisat.esa.int/handbooks/asar/aux-files/ephimg-23641229.jpg

First image taken with one look. Second image is of same object, but with multiple looks. Fig 1.2

ENVISAT

VELOCITY

GROUND TRACK OF ENVISAT EARTHS SURFACE VIEW OF ASAR Fig 1.3

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Image Mode

ScanSAR Mode

Alternating Polarisation Mode (Source: www.eurimage.com)

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Richard Wrigley Table 1 Band Frequency Range / GHz Wavelength Range / cm P-band 0.4-1 30-75 L-band 1-2 15-30 S-band 2-4 7.5-15 C-band 4-8 3.7-5 X-band 8-1 2.5-3.7 Table of frequency bands. (Source: PA2604 lecture 9 slides, J. J. Remedios). Fig 1.4

Interferogram of around Istanbul, which shows a ground displacement of 28 mm (Source: www.envisat.esa.int/handbooks/asar/)

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