MICROW~4.PPT

Canada Centre for Remote Sensing, Natural Resources Canada Natural Resources Ressources naturelles Canada Canada TYPES OF ACTIVE MICROWAVE SYSTEMS Course Outline  Types of active microwave systems:  Circular Scanning Plan Position Indicator (PPI): Air and Naval Applications  Side Looking Radar: Remote Sensing  Real Aperture Radar  Synthetic Aperture Radar  SAR image formation Canada Centre for Remote Sensing, Natural Resources Canada Range and Azimuth Processing  SLANT RANGE: Line of sight between the radar and the illuminated target;  RANGE DIRECTION: Perpendicular to flight direction (or azimuth) of the sensor and it is also referred to as the crosstrack direction;  RANGE RESOLUTION (slant or ground): is the minimum distance on the ground at which two object points can be imaged separately; is an image characteristic determined by the system bandwidth or effective length of the pulse;  AZIMUTH: Commonly used to indicate the distance in the along track direction;  AZIMUTH DIRECTION: Direction parallel to the line of flight and also referred to as the along-track direction Canada Centre for Remote Sensing, Natural Resources Canada  AZIMUTH RESOLUTION: the smallest separation between two point targets that can be detected by the radar;  AZIMUTH COMPRESSION: In the SAR signal domain, the raw data are spread out in the range and azimuth directions and must be coherently compressed to realize the full resolution potential of the instrument. Azimuth compression consists of coherently correlating the received signal with the azimuth replica function. Resolution Cell Source: Raney, 1998 rR = range resolution Canada Centre for Remote Sensing, Natural Resources Canada rA = azimuth resolution Side Looking Airborne Radars (SLAR)  Side-looking airborne radars normally are divided into two groups:  Real Aperture Radar (RAR)  Synthetic Aperture Radar (SAR) The customary nomenclature used is "SLAR" for the real-aperture system and "SAR" for the synthetic aperture system, although the latter is also a side-looking airborne (or spaceborne) radar. Canada Centre for Remote Sensing, Natural Resources Canada Side Looking Airborne Radars (SLAR): RAR  Depends on the beamwidth determined by the actual antenna.  The SLAR imaging system uses a long, straight antenna mounted on a platform (aircraft or satellite), with its longitudinal axis parallel to the flight path;  The antenna emits microwave pulses focused by an antenna into a narrow beam that is directed perpendicular to the flight path of the platform and downward to the surface of the Earth (Slide 7); and are scattered, usually in many directions, including the direction of the antenna.  The backscatter arrive at the antenna at different times, depending on the distance from the antenna to the specific scattering object on the ground.  As the nadir is approached, the ground range resolution becomes extremely poor.  This makes sense since the objects near the nadir line are all virtually the same distance from the antenna, making them impossible to distinguish. What is Real Aperture Radar (RAR)?. Geometry of a sidelooking real aperture radar. Range resolution of a sidelooking real aperture radar Canada Centre for Remote Sensing, Natural Resources Canada Side Looking Airborne Radars (SLAR): RAR To achieve a ground range resolution small enough to be useful, the pulse duration would have to be too short to generate enough energy to produce a sufficient signal to noise ratio (SNR); To get around this, a technique known as pulse compression is often used together with a type of processing of the returned signal known as matched filtering to produce both high resolution and a high SNR. The resolution can be further improved by increasing the frequency bandwidth. Canada Centre for Remote Sensing, Natural Resources Canada Side Looking Airborne Radars (SLAR): RAR  Azimuth Resolution: Slide 7 shows that the angular spread of the radar beam is due to interference of the waves emitted from and received by the antenna;  This causes the angular spread to decrease as the aperture length increases (a longer antenna produces a finer beam); thus moderate azimuth resolution to deteriorate as the range increases;  The physical length of the antenna determines the spatial resolution of this system; the larger the antenna, the better the spatial resolution; Canada Centre for Remote Sensing, Natural Resources Canada  Two objects on the ground and with the same slant range R can only be imaged separately, if they are not both within the radar beam at the same time; Side Looking Airborne Radars (SLAR): RAR  For a specific frequency f (or wavelength ) and slant range R, the azimuth resolution is entirely dependent on the aperture length La;  However, for the altitudes at which satellite or airborne imaging sensors operate (in the 1-10 GHz region), engineering difficulties make it is impossible to achieve values of La greater than several hundred.  For example, the Canadian satellite RADARSAT, launched in 1995, orbits at 792 km and operates at a frequency of 5.3 GHz (C-Band). Canada Centre for Remote Sensing, Natural Resources Canada  The moderate to low azimuth resolution has been the main drawback of the SLAR system. What is Synthetic Aperture Radar (SAR)?  A side-looking radar system which makes a highresolution image of the Earth’s surface; The Side Looking perspective enables differentiation between targets;  As an imaging side-looking radar moves along its path, it accumulates data. In this way, continuous strips of the ground surface are “illuminated” parallel and to one side of the flight direction (slide 12 - 13).  From this record of signal data, processing is needed to produce radar images. The basic image is complex-valued and 2-dimensional;  SAR depends upon signal processing to achieve a much narrower beamwidth in the along-track direction than that attainable with the real antenna; Canada Centre for Remote Sensing, Natural Resources Canada  Digital signal processing is used to focus the image and obtain a higher resolution than achieved by conventional radar systems Concept of Synthetic Aperture Canada Centre for Remote Sensing, Natural Resources Canada Concept of Synthetic Aperture Synthetic Aperture Last time SAR senses object Distance SAR travelled while object was in view = synthetic aperture First time SAR senses object Flight path Ground Track Nadir Swath Object Canada Centre for Remote Sensing, Natural Resources Canada What is Synthetic Aperture Radar (SAR)?  During the forward motion of a SAR, a Doppler history of the backscattered signals is used to synthesize a large antenna (Slide 15);  Consequently, a high azimuth resolution is achieved, a pulse is transmitted to each position within the beamwidth and stored;  To preserve the radiometric resolution of SAR, which is primarily determined by the band width of transmitted pulse, a long pulse is generated with a linear frequency modulation (chirp);  When compressed, an optimized range resolution is achieved (slide 15) Canada Centre for Remote Sensing, Natural Resources Canada Principle of Synthetic Aperture Radar Flight direction target Source: CCRS Canada Centre for Remote Sensing, Natural Resources Canada What is SAR: Resolution  Range resolution of a SAR is determined by built-in radar and processor constraints which act in the slant range domain. Range resolution is dependent on the length of the processed pulse; shorter pulses result in “higher” resolution;  Radar data are created in the slant range domain, but usually are projected onto the ground range plane when processed into an image.  SAR gets its name from the azimuth processing and can achieve an azimuth resolution which may be hundreds of times smaller than the transmitted antenna beam width. Canada Centre for Remote Sensing, Natural Resources Canada Pulsed Radars: Focusing Radar system transmits a pulse with a long duration; ground target scatters the transmitted pulse back to the radar; Backscattered signal from a single target is spread over range and azimuth; SAR focusing collects these dispersed microwave energy into a single pixel; “Range Processing” gathers the many samples of the pulses received and combines them; “Azimuth Processing” gathers the many pulses backscattered by a target and combines them; Focusing refers to the sampling and quantization of backscattered signals into numerical values SAR PROCESSING STEPS

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