ACTIVE SENSORS RADAR

Transcription

1 ACTIVE SENSORS RADAR RADAR

2 LiDAR: Light Detection And Ranging RADAR: RAdio Detection And Ranging SONAR: SOund Navigation And Ranging Used to image the ocean floor (produce bathymetic maps) and detect objects in the ocean, perform seismic surveys (geological maps), and image our insides (ultrasound). ACTIVE SENSORS

3 Fundamental principles Real and synthetic aperture RADARs Interpreting RADAR imagery Tone, texture, polarization Shadows, foreshortening, layover Interfereometry DEMs Sensors Applications RADAR

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6 RADAR is (generally) an active system Controllable source of illumination (although not light ) Sees through cloud and rain (excluding, of course, shorterwavelength weather RADAR!), and at night Images can be high(ish) resolution (3-10 m) in (x,y) horizontal dimensions Different features are portrayed or discriminated compared to visible sensors Some surface features can be seen better in radar images: ice, ocean waves soil moisture (e.g., SMAP), vegetation mass man-made objects, e.g., buildings, ships geological structures FUNDAMENTAL PRINCIPLES

7 A Radar system has three primary components: 1. transmits microwave (radio) signals towards a scene 2. receives the portion of the transmitted energy backscattered from the scene 3. observes the strength (detection), orientation (polarization) and the time delay (ranging) of the return signals. FUNDAMENTAL PRINCIPLES

8 TIME WILL TELL

9 oradar systems work in a wide band of microwave frequencies. o The higher the frequency (shorter the wavelength) of a radar system, the more it is affected by weather conditions such as rain or clouds. o But the higher the transmitted frequency, the better is the resolution of the radar system. o Two naming schemes are currently in place. The older one (A- M) is still widely used, although it is an arbitrary naming scheme. RADAR FREQUENCIES

10 A surface s flatness is relative to the wavelength striking it The differences that make a difference

11 The across-track dimension is referred to as range. The along-track dimension is referred to as azimuth. In a radar system, resolution is defined for both the range and azimuth directions. RADAR TERMINOLOGY

12 RESOLUTION S TWO COMPONENTS

13 RADAR TERMINOLOGY

14 RADARSAT RESOLUTIONS

15 A radar system where the antenna beamwidth is controlled by the physical length of the antenna. A SLAR system in which azimuth resolution is determined by the physical length of the antenna and by the wavelength. The antenna needs to be many times longer than the wavelength to produce narrow bandwidths. (Side-Looking Airborne RADAR) The radar returns are recorded directly to produce images. REAL APERTURE RADAR

16 Advantage: Simple design and data processing. Disadvantages: Poor resolution Limited to short range, low altitude missions. Shorter wavelengths subject to atmospheric effects, scattering and dispersion. Therefore, the missions are flown at low altitudes and the coverage is small. RAR

17 A Synthetic Aperture RADAR (SAR) is a airborne or spaceborne sidelooking radar system that utilizes the flight path of the platform to simulate an extremely large antenna or aperture electronically. As an imaging sidelooking 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. From this record of signal data, processing is needed to produce radar images. SAR

18 Radar images have certain characteristics that are fundamentally different from images obtained by using optical sensors such as Landsat, SPOT or aerial photography. These specific characteristics are the consequence of the imaging radar technique, and are related to radiometry (speckle, texture or geometry) and polarization. DN levels in a radar image are related to the microwave backscattering properties of the surface. The intensity of the backscattered signal varies according to roughness, dielectric properties and local slope. Thus, the radar signal refers mainly to geometrical properties of the target and to the moisture content (relatively, wetter -> brighter). RADAR INTERPRETATION

19 A synthetic aperture radar image of Death Valley, captured from the Space Shuttle in (Image: NASA/JPL) Can you tell from which direction was the image taken? (That is, where was the space shuttle relative to Death Valley?

20 Two major types of brightness variations are observable in a radar image: variations in tone variations in texture INTERPRETING RADAR IMAGERY

21 Proportional to strength of RADAR backscatter Relatively smooth targets like calm water appear as dark tones (specular) Diffuse targets like some vegetation appear as intermediate tones Man-made targets (buildings, ships) may produce bright tones, depending on their shape, orientation and/or constituent materials (corner reflectors, or perpendicular to sensor) TONE

22 RADAR TONE

23 SAFE SAILING

24 Refers to the pattern of spatial tone variations Function of spatial uniformity of scene targets For radar images texture consists of scene texture multiplied by speckle Speckle: a grainy "salt and pepper" texture in an image caused by subpixel in-homogeneities. Texture may be described as fine, medium, or coarse RADAR TEXTURE

25 RADAR TEXTURE

26 Many radars are designed to transmit microwave radiation that is either horizontally polarized (H) or vertically polarized (V). A transmitted wave of either polarization can generate a backscattered wave with a variety of polarizations. It is the analysis of these transmit and receive polarization combinations that constitutes the science of radar polarimetry. HH - for horizontal transmit and horizontal receive VV - for vertical transmit and vertical receive HV - for horizontal transmit and vertical receive, and VH - for vertical transmit and horizontal receive. RADAR POLARIZATION

27 A missile-guidance RADAR Illustrating V and H modes Illustration of how different polarizations (HH, VV, HV & colour composite) bring out different features in an agricultural scene RADAR POLARIZATION

28 During radar image analysis, the interpreter must keep in mind the fact that the radar "sees" the scene in a very different way from the human eye or from an optical sensor; the DN levels of the scene are related to the relative strength of the microwave energy backscattered by the landscape elements. Shadows in radar image are related to the oblique incidence angle of microwave radiation emitted by the radar system and not to geometry of solar illumination. The false visual similarity between the two types of images usually leads to confusion for beginners in interpretation of radar images. RADAR INTERPRETATION

29 A major problem with RADAR images The greater the range (i.e., distance from the sensor) the greater the problem. SHADOWS

30 SHADOWS

31 Foreshortening occurs when the radar beam reaches the base of a tall feature tilted towards the radar (e.g., a mountain) before it reaches the top. Because the radar measures distance in slant-range, the slope (from point a to point b) will appear compressed and the length of the slope will be represented incorrectly (a' to b') at the image plane. FORESHORTENING

32 Layover occurs when the radar beam reaches the top of a tall feature (b) before it reaches the base (a). The return signal from the top of the feature will be received before the signal from the bottom. As a result, the top of the feature is displaced towards the radar from its true position on the ground, and lays over the base of the feature (b' to a'). The ordering of surface elements on the radar image is the reverse of the ordering on the ground. LAYOVER

33 Fundamental principles Real and synthetic aperture RADARs Interpreting RADAR imagery Tone, texture Shadows, foreshortening, layover Interfereometry DEMs Sensors Applications RADAR

34 INTERFEROMETRY

35 The Shuttle Radar Topography Mission (SRTM) obtained elevation data on a near-global scale to generate the most complete high-resolution digital topographic database of Earth. SRTM consisted of a specially modified radar system (2 SARs) that flew onboard the Space Shuttle Endeavour during an 11-day mission in Feb of RADAR AND DEMS

36 Landers earthquake (south California) of 28 June 1992 (magnitude 7.3) From several images, acquired before and after the earthquake, a differential interferogram was computed that clearly shows the seismic movements due to the earthquake. The banana-shaped fault is clearly visible. Each fringe corresponds to a co-seismic movement of 28.3 mm. The measured precision is 9 mm. INTERFEROMETRY

37 Canada: RADARSAT US: JPL AirSAR ESA: ASAR Japan: PALSAR Numerous airborne sensors Numerous aviation sensors SENSORS

38 Radar wavelength should be matched to the size of the surface features that we wish to discriminate Ice discrimination, small features, use X-band Geology mapping, large features, use L-band Foliage penetration, better at low frequencies, use K u - band In general, C-band is a good compromise APPLICATIONS AND WAVELENGTHS

39 The C-band system is designed as a medium resolution mission primarily dedicated to regular monitoring of broad geographic areas. This provides a 'big picture' overview of Canada's land mass and proximate water areas. RADARSAT

40 Geology Hydrology Oceanography Military Coast Guard (ship detection) Forestry Agriculture Sea ice Emergency response (floods, oil spills) APPLICATIONS

41 Active RADAR instruments include three broad classes: Imaging sensors (as described above, and the type of sensor most similar to the passive sensors (e.g., Landsat, SPOT) we are most familiar with. The other two classes are non-imaging microwave sensors: RADAR altimeters The microwave pulses are generally nadir pulses only, and it is the time of return that is the key piece of information. These devices are used on aircraft and satellites to produce topographic maps and maps of the ocean sea surface. RADAR Scatterometers These sensors are used to accurately measure backscatter and from that determine the different materials and the surface characteristics. These devices are used to estimate wind speeds based on sea surface roughness. RADAR

42 ERS-2 RADAR ALTIMETER QUICK-LOOK SEA SURFACE WIND SPEEDS

43 RADAR is a widely-used technology, but also generally a poorly-understood one. To properly work with RADAR data one needs to understand physics. SUMMARY

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