UNERSITY OF NAIROBI UNIT: PRICIPLES AND APPLICATIONS OF REMOTE SENSING AND APLLIED CLIMATOLOGY

Transcription

1 UNERSITY OF NAIROBI DEPARTMENT OF METEOROLOGY UNIT: PRICIPLES AND APPLICATIONS OF REMOTE SENSING AND APLLIED CLIMATOLOGY COURSE CODE: SMR 308 GROUP TWO: SENSORS

2 MEMBERS OF GROUP TWO 1. MUTISYA J.M I10/2784/ KATIKU J.M I10/3941/ EVANS K. MARU I10/2993/ JOSEPH VALAKA M. I10/3013/ PURITY MUENI J. I10/3017/ MUINDA FRANCIS M. I10/3924/ RAMADHANI J. MUNGA I10/3948/ CHANZU E.M I10/3959/2009

3 SENSORS Sensors are the devices used to acquire images in remote sensing. In remote sensing, the sensors are able to acquire information that the human eye cannot normally see. This is done using radiation in other parts of the electromagnetic spectrum than in the visible portion. Classification of sensors (1)In respect to the type of Energy Resources: a) Passive sensors: These are Sensors that do not use their own source of electromagnetic illumination and depend upon the radiation emitted or reflected from the object of interest. b) Active sensor: These are sensors that use their own source of electromagnetic radiation to illuminate the target and in most cases use properties of reflected radiation. a) Passive sensing

4 The sun's energy is either reflected, as it is for visible wavelengths, or absorbed and then reemitted, as it is for thermal infrared wavelengths. Remote sensing systems which measure energy that is naturally available are called passive sensors. Passive sensors can only be used to detect energy when the naturally occurring energy is available. For all reflected energy, this can only take place during the time when the sun is illuminating the Earth. There is no reflected energy available from the sun at night. Energy that is naturally emitted (such as thermal infrared) can be detected day or night, as long as the amount of energy is large enough to be recorded. Examples includes: Visible/IR Radiometer (AVHRR)Microwave Radiometer(SSM/I,TMI,MSMR) Microwave Radiometer\Attached with Nadir looking instruments( e.g. in TOPEX,ERS ) The Advanced Very High Resolution Radiometer (AVHRR) AVHRR is a five channel scanning radiometer in visible, near infra-red and infra-red wavelengths for analysis of hydrological, oceanographic and meteorological parameters such as vegetation index (i.e. greenness), clouds, snow and ice cover and sea surface temperatures. Data are obtained by all the five channels with a resolution of 1 km. The digital AVHRR data is transmitted from the satellite in real time (High Resolution Picture Transmission or HRPT) as well as selectively recorded on board the satellite for subsequent playback when the satellite is in communication range of the ground control station. This high resolution data is called Local Area Coverage (LAC). AVHRR data is also sampled on real-time to produce lower resolution Global Area Coverage (GAC) data. b) Active sensing

5 Active sensors provide their own energy source for illumination. The sensor emits radiation which is directed toward the target to be investigated. The radiation reflected from that target is detected and measured by the sensor. Advantages for active sensors include (i)the ability to obtain measurements anytime, regardless of the time of day or season, (ii)can be used for examining wavelengths that are not sufficiently provided by the sun, such as microwaves, or to better control the way a target is illuminated. However, active systems require the generation of a fairly large amount of energy to adequately illuminate targets. Some examples of active sensors are a laser fluorosensor and synthetic aperture radar (SAR), Altimeter, (ERS /\TOPEX), Laser Water Depth Meter, Scatterometer (ERS, ADEOS, QuikScat) Synthetic/Realapertureradars (e.g. SAR, PR onboard (TRMM)

6 2) In respect to Wavelength Regions: a) Visible b) Thermal Infrared c) Microwave Under this category of classification, it gives sensors depending on the wavelength of the electromagnetic radiation under which they operate at, for example we have sensors that operate under: a) Visible range( µm) Have short wavelengths of radiation as compared to microwave radiation Have high intensity, which makes visible sensors to have high spectral resolution Visible sensors can be located in different positions i.e. i. At the surface

7 ii. iii. At the space looking down Limb sensing(sounding) Example of applications Distribution of plants Forest and farm fields Rivers Lakes Urban areas Limitations for surface conditions Not good at night Not good in cloud conditions a) Thermal infrared ( µm) This is the emitted radiation from land surface and from the constituents of the atmosphere. Examples of application of these types of sensors are used to monitor Sea surface temperatures Land surface temperatures Status of volcanic activities Status of forest fires Urban heat islands Advantage is that it is good at night and disadvantage is that not good in cloudy conditions b) Microwave ( cm) There are sensors that operate under this wavelength. This generates active and passive microwaves sensors

8 An active microwave sensor emits microwaves and observes those reflected by land surface while a passive microwave sensor observes microwaves naturally from land surface. Examples of application Active-Suitable to observe mountains and valley topography Passive sea surface temperature snow accumulation thickness of ice Advantages Disadvantage not affected by day, night or weather(clouds or moisture in air) using this wavelength range one is able to identify water content of cloud Though it can gives the water content of a cloud it does give the amount that is evaporating. These are more examples of sensors and how they function: Passive sensors a) Gamma ray spectrometer This measures the amount of gamma rays emitted by the upper soil or rock layers due to radioactive decay. The energy measured in specific wavelength band provides information of abundance of (radio isotope that relate to specific mineral. Therefore the main application is found in mineral exploration. Gamma rays have very short wavelength on the order of picometer (pm). Because of large atmospheric absorption of these waves this type of energy can only be measured up to the few hundred meters above the earth s surface. b) Aerial camera The film types used in the camera enable electromagnetic energy in the range between 400nm and 900nm to be recorded. Aerial photographs are used in wide range of applications.

9 Aerial photos are used in medium and large scale (topographic) mapping and cadastral mapping. Frequently used to record data. c) Video camera Most video images serve to provide low cost image data for qualitative purposes i.e. to provide additional visual information about an area captured with another sensor e.g. laser scanner or radar. Most image processing and information extraction methods useful for individual images can be applied to video frames. d) Multispectral scanner A scanner is an instrument that obtains observation in point-by-point and line-by-line manner. A scanner differs from an aerial camera, which record an entire image in only one exposure. A multispectral scanner is an instrument that measures the reflected sunlight in the visible and infrared. A sensor systematically scans the earth s surface, thereby measuring the energy reflected by the viewed area. This is done simultaneously for several wavelength bands hence the name multispectral scanner. A wavelength band or spectral band is an interval of the electromagnetic spectrum for which average reflected energy is measured. Typically, a number of distinct wavelength bands are recorded, because these bands are related to specific characteristics of the earth s surface for example, Reflection characteristics in the range 2µm to 2.5µm (for instance landsat TM band 7) may give information about mineral composition of the soil, whereas the combined reflection characteristic of the red and near infrared bands may tell something about vegetation such as biomass and heath. The definition of wave bands of a scanner therefore depends on the applications for which the sensor has been design. e) Imaging spectrometer or hyper spectral imager The principle of imaging spectrometer is similar to that of multispectral scanner, except that spectrometer measures many (64-256), very narrow (5nm to 10nm) spectral bands.

10 This results in an almost continuous reflectance curve per pixel rather than the limited number values for relatively broad spectral bands of multispectral scanner. Imaging spectrometer data therefore can be used for instance: i. Determination of mineral composition of the earth s surface. ii. iii. Chlorophyll content of surface water. Total suspended matter concentration of surface water. f) Thermal scanner They measure thermal data in the range of 8µm to 14µm. Wavelength in this range are directly related to object s temperature i.e. data on cloud, land and sea surface temperature are indispensable for weather forecasting. For this reason, most remote sensing system designed for meteorology includes thermal scanner. They can also be used to study the effect of drought on agricultural crop (water stress) and to monitor the temperature of cooling water discharge from thermal power plant. Detections of underground coal fires. g) Microwaves radiometers Long wavelength electromagnetic energy (1cm to 100cm) is emitted from objects on or just below the earth s surface. Natural materials may emit radiation that is somewhat lower than the ideal case of a blackbody, which is demonstrated by emissivity smaller than one. A Microwave radiometer records this emitted radiation of objects. The depth from which the emitted energy can be recorded depends on the properties of the specific materials, such as the water content. The recorded signal is called the brightness temperature. Because blackbody radiation is weak, the energy must be measured over relatively large areas, and consequently passive microwave radiometers are characterized by low spatial resolution. Passive microwave radiometer data can be used in:

11 i. Mineral exploration ii. iii. iv. Soil mapping Soil moisture estimation Snow and ice detection Active sensors a) Laser scanner They are typically mounted on the aircraft or helicopter and use a laser beam to measure the distance from the sensor to points located on the ground. The distance measured is combined with exact information on the sensors position, using a satellite position system and an inertial navigation system (INS), to calculate terrain elevation Laser scanning produces detailed, high resolution, Digital Terrain Models (DTM) for topographic mapping. Laser scanning can also be used for oblique and transverse measurements. b) Imaging radar Radar (Radio detection and ranging) instruments operate in the (1cm -100cm) wavelength range. Since radar is an active sensor system and the applied wavelengths are able to penetrate clouds, it can acquire images day and night and under all weather conditions, although the images may be affected somewhat by heavy rainfall. The combination of two stereo radar images of the same area can provide information about terrain heights (radar grammetry). SAR interferometry (INSAR) combines two radar images acquired at almost the same locations which can either be used to assess changes in height or vertical deformations with great precision which may be caused by oil and gas exploitation (land subsidence or crustal deformation relation earthquakes. c) Radar altimeter They are used to measure the topographic profile parallel to the satellite orbit. They provide profiles i.e. single lines of measurements, rather than image data.

12 They operate in the 1cm to 6cm wavelength range and are able to determine height with a precision of 2cm to 5cm. They are useful for measuring relatively smooth surfaces such as oceans and for small scale mapping of continental terrain models. d) Bathymetry and side scan sonar Sonar (Sounding Navigation Ranging) It is a process used to map sea floor topography or to observe obstacle under water. It works by emitting a small burst of sound from a ship. The sound is reflected off the bottom of the body of water. The time that it takes for the reflected pulse to be received corresponds to the depth of the water. Move advanced system record the intensity of the return signal, thus giving information about the material on sea floor. Sonar looks straight down and operates very much like a radar altimeter. The body of water will be traversed in paths like a grid and not every point below the surface will be monitored. The distance between data points depends on speed of the ship, the frequency, the measurement and the distance between the adjustment paths. Side scan sonar is one of the most accurate systems for imaging large area of ocean floor. This is a towed system. It is normally moved in a straight line somewhat similar to side looking airborne radar (SLAR). Side scan sonar transmits a specially shaped acoustic beam perpendicular to the ship path and out to the left and right side. This beam propagates in to the water and across the sea bed. The roughness of the floor of the ocean and any object lying upon it, reflects some of the incident sound energy back in the direction of the sonar. The sonar is the sensitive enough to receive these reflections amplify them and send them to sonar data processor and display. Image produce by side sonar system are highly accurate and can be used to delineate even very small (<1cm) objects.

13 The shape of the beam in side scan is crucial to the formation of the final image. Typically the acoustic beam, aside scan sonar is very narrow in the horizontal dimension (about 0.1 degree) and much wider (40-60degrees) n the vertical dimension. Using sonar data, contour data maps of the bottom of bodies of water are made. Bathymetric maps- These are maps that show contours under bodies of through depth soundings. They are analogous to topographic maps that are made to show contours of terrestrial areas. e) LIDAR (Light Detection and Ranging) A Lidar transmits coherent laser light, at certain visible or near infrared wavelength, as a series of pulses (thousands per second) to the surface, from which some of the light reflects. Travel time for the round trip and the returned intensity of the reflected pulses are measured parameters. Lidar instruments can be operated as profilers and as scanners on airborne and space borne platforms, day and night. Lidar can serve either as a ranging device to determine altitudes and measure speeds or as particle analyzer for air. Light penetrates certain targets, which makes it possible to use it for accessing it for assessing tree height (biomass) and canopy conditions or for measuring depths of shallow waters such as tidal flats. f) Synthetic Aperture Radar (SAR) - The most common active remote sensing system, which emits radar pulses from under an aircraft or satellite onto a given area. The reflected or back-scattered radar signals form an image. REFERENCE Drury, S. A. (1993). Image Interpretation in Geology (3rd ed.). London: Taylor& Francis. 290 p. Lillesand, Thomas M. and Kiefer, Ralph W. (1994). Remote Sensing and Image Interpretation (3rd ed.). New York: John Wiley and Sons. 750 p. Sabins, Floyd F. (1997). Remote Sensing: Principles and Interpretation (3rded.). New York: W. H. Freeman and Company. 494 p. Internet Resources The WWW Virtual Library: Remote Sensing Extensive links to satellite data sources, journals and on-line publications, societies, and companies and other organizations engaged in remote sensing. Remote Sensing Tutorial created by the Goddard Space Flight Center or

14 An application-oriented on-line tutorial covering all aspects of remote sensing, including thermal images and radar, with many sample images. Remote Sensing Tutorials created by the Canada Centre for Remote Sensing On-line tutorials in remote sensing fundamentals, radar and stereoscopy, and digital image analysis.