Davide Travaglini is researcher at the University of Florence

Transcription

1 Davide Travaglini is researcher at the University of Florence University of Tuscia geolab is a Laboratory of Geomatics for the development of applications of Earth Observation (EO) and Geographical Information Systems (GIS) in the environmental sector. niversity of Florence forestlab.net is a network of four laboratories of forest geomatics

2 alian Journal of Remote ensing is the official journal f the Italian Remote Sensing ociety RS is currently abstracted nd indexed in Science itation Index Expanded e eb of Science Since 2009 geolab is the Editorial Office of the Rivista Italiana di Telerilevamento/Italian Journal of Remote Sensing

3 Master's Degree GEOMATICS AND NATURAL RESOURCES EVALUATION PRINCIPLES OF REMOTE SENSING - Part I - Davide Travaglini (davide.travaglini@unifi.it)

4 Contents Introduction Definition of remote sensing History Application Advantages and drawbacks The Remote Sensing process Electromagnetic radiation Electromagnetic spectrum Instruments for data acquisition: Platforms and Sensors Scanning systems Examples of Satellites Missions

5 Definition of Remote Sensing "the science of deriving information about the earth's land and water areas from images acquired at a distance. It usually relies on measurement of electromagnetic energy reflected or emitted from the features of interest (Source: Campbell J.B., Introduction to Remote Sensing. New York: Guildford, pg. 2)

6 Example of remotely sensed image

7 History of Remote Sensing 1849: Hot-air balloons took first aerial photographs of the land : Aerial photographs were used for reconnaissance purposes : Aerial photos were used extensively for mapping, reconnaissance and intelligence : Development of photogrammetry for peaceful purposes in a variety of disciplines 1957: First satellite launched - Sputnik (USSR) Hot-air balloon Aerial photograph

8 From the 1970s new instruments were developed onboard satellites, forming images in the infrared as well as in visible light, produced false color images Launch of Environmental satellites started with ERTS-1 (Earth Resources Technology Satellite) Later named Landsat 1

9 Land observation satellites: Meteo satellites: Landsat (1972), NASA/USA Seasat (1978), JPL HCMM (1978), NASA Spot (1986), France RESURS (1985), Russia IRS (1986), India ERS (1991), ESA JERS (1992), Japan Radarsat (1995), Canada ADEOS (1996), Japan IKONOS (1999), Quick Bird (2001), ALOS (2006) Japan TIROS (1960), Ninbus (1964), ESSA (1966), ATS (1966), DMSP (1966), Kosmos (1968), Meteor (1969), ITOS (1970), GEOS (1975), NOAA (1976), GMS (1977), Meteosat (1978), TIROS-N (1978), Bhaskura (1979), NOAA (1982), Insat (1983), ERBS (1984), MOS (1987), UARS (1991), TRMM (1997)

10 Application of remote sensing technology griculture: e.g. crop type mapping crop monitoring orestry: e.g. clear cut and burn mapping tree species classification eology: e.g. structural mapping geologic units ydrology: e.g. flood delineation soil moisture ea ice: e.g. type and concentration ice motion and cover: e.g. rural/urban change biomass mapping cean & Coastal: e.g. ocean feature/colour oil spill detection tmospheric chemistry: e.g. atmospheric composition - air pollution rchaeology: e.g. landscape archaeology

11 Main advantages and drawbacks Synoptic view Data Acquisition Remote sensing techniques are complementary to ground systems Multispectral ranges Multiresolution data sets Multitemporal data sets Local conditions influences data quality Ratio cost/benefits has to be checked Multidisciplinary applications

12 The Remote Sensing process The Remote Sensing process is a system with input (e.g. the solar energy) and output (information). Each component of the system modifies or adds to the signal

13 Step involved in Remote Sensing process Remote sensing involves the interaction between incident radiation and targets of interest that requires the systems and involvement of 7 specific elements. It also involves the sensing of emitted energy and use of non-imaging sensors. Source: James A. Tindall, Deconvolution of Plant Type(s) for Homeland Security Enforcement Using Remote Sensing on a UAV

14 1. Energy Source or Illumination: the first requirement for remote sensing is to have an energy source which illuminates or provides electromagnetic energy to the target of interest. a. Active sensor: detect reflected responses from objects that are irradiated from artificiallygenerated energy sources such as radar b. Passive sensor: detect the reflected or emitted electromagnetic radiation from natural sources 2. Radiation and the Atmosphere: as the energy travels from its source to the target, it will come in contact and interact with the atmosphere it passes through. This interaction may take place a second time as the energy travels from the target to the sensor.

15 3. Interaction with the Target: once the energy makes its way to the target through the atmosphere, it interacts with the target depending on the properties of both the target and the radiation. 4. Recording of Energy by the Sensor: a sensor is required (remote, not in contact with the target) to collect and record the electromagnetic radiation after the energy has been scattered by, or emitted from, the target.

16 5. Transmission, Reception, and Processing: the energy recorded by the sensor has to be transmitted, often in electronic form, to a receiving station where the data are processed by computer software into an image (hardcopy and/or digital). 6. Interpretation and Analysis: the processed image is interpreted, visually and/or digitally or electronically, to extract information about the illuminated target.

17 7. Application: the final element of the remote sensing process is achieved when we apply the information we have been able to extract (the data) from the imagery about the target to better understand, reveal some new information about, or assist in solving a specific problem.

18 Electromagnetic radiation (EMR) Visible light is only one of many forms of electromegnetic energy The electromagnetic spectrum stretches from radio waves to gamma rays Source: All this energy radiates in accordance with basic wave theory

19 Basic wave theory Electromagnetic radiation consist of: - an electrical field (E) which varies in magnitude in a direction perpendicular to the direction in which the radiation is travelling - a magnetic field (M) oriented at right angles to the electrical field Both these field travel at the speed of light: c=λv=(3x10 8 m/s) (Source Lillesand T.M., Kiefer R.W, 2000)

20 The two characteristics of electromagnetic radiation that are particularly important for understanding remote sensing are: Wavelength: the length in µm of one wave cycle measured as the distance between successive wave crests (1µm=1x10-6 m) Frequency: number of cycles per second passing of a fixed point, Hz Wavelength and frequency are related by the following formula: c = λv where: c = speed of light (3x10 8 m/s) λ = wavelength v = frequency

21 Quantistic theory of electromagnetic radiation The quantistic theory suggest that electromagnetic radiation is composed of many particles called photons or quanta. The energy of a qauntum is given as: Q = hv Where: Q = energy of a quantum (or photon), in Joules (J) h = Planck s costant (6.626 x J sec) v = frequency = c/λ Q = hc/ λ The energy of a quantum is inversely proportional to its wavelength (λ) The low energy content of long wavelengths (e.g microwaves or thermal IR emission from terrain feature) means that system operating at long wavelengths must view large areas of the earth in order to obtain a detectable energy signal

22 Electromagnetic spectrum The electromagnetic spectrum ranges from the shorter wavelengths (ultraviolet including x rays) to the longer wavelengths (microwaves, radar waves) Visible: Near infrared: Middle infrared: Thermal infrared: Microwaves: µm µm µm µm 1 mm 1 m (1µm=1x10-6 m)

23 There are several regions of the electromagnetic spectrum which are useful for remote sensing. (Source Canada Centre for Remote Sensing

24 (Source Canada Centre for Remote Sensing Ultraviolet (UV) light has shorter wavelengths than visible light The ultraviolet has the shortest wavelengths which are practical for remote sensing Some Earth surface materials, primarily rocks and minerals, fluoresce or emit visible light when illuminated by UV radiation Astronomers have to put ultraviolet telescopes on satellites to measure the ultraviolet light from stars and galaxies

25 The visible wavelenghts cover a range from approximately 0.4 to 0.7 µm The longest visible wavelength is red and the shortest is violet Red: µm Orange: µm Yellow: µm Green: µm Blue: µm Violet: µm The visible portion can be shown in its component colours when sunlight is passed through a prism Blue, green, and red are the primary colours all other colours can be formed by combining blue, green, and red in various proportions

26 The next portion of the spectrum of interest is the infrared (IR) region which covers the wavelength range from approximately 0.7 µm to 100 µm The infrared region can be divided into two categories based on their radiation properties: Reflected IR: it is used for remote sensing purposes in ways very similar to radiation in the visible portion Source Canada Centre for Remote Sensing ) The reflected IR covers wavelengths from approximately 0.7 µm to 3.0 µm (near IR and middle IR) Thermal IR: its is quite different than the visible and reflected IR portions, as this energy is essentially the radiation that is emitted from the Earth's surface in the form of heat. The thermal IR covers wavelengths from approximately 3.0 µm to 100 µm

27 The portion of the spectrum of more recent interest to remote sensing is the microwave region from about 1 mm to 1 m. This covers the longest wavelengths used for remote sensing The shorter wavelengths have properties similar to the thermal infrared region while the longer wavelengths approach the wavelengths used for radio broadcasts. (Source Canada Centre for Remote Sensing )

28 Radiant energy very material with temperature above 0 K (-273 C) emits electromagnetic energ Terrestrial objects are source of radiation How mach energy any object radiates is expressed by the Stafan-Boltzmann law: M = εσt 4 M = total radiant energy (emittance) from the material (W/m 2 ) ε = emissivity coefficient of the material σ = Stafan-Boltzmann costant ( W/m 2 K 4 ) T= absolute temperature (K) of the material Note that: - The total energy of a material increases with increases in temperature -The Stefan-Boltzmann law is expressed for an energy source that behaves as a blackbody

29 Blackbody - Wien s law - Emissivity A blackbody is hypothetical, ideal radiator that totally absorbs and reemits the incident energy at every wave length without reflection Spectral distribution of energy radiated from blackbodies of various temperature (from 200 to 6000 K) Total radiant exitance M is given by the area under the spectral radiant exitance curves The wavelenght at which a blackbody radiation curve reaches a maximum is related to its temperature by Wien s law: λ max = A/T Source: Lillesand and Kiefer, 2000 λ max = wavelength of maximum spectral radiant exitance, µm A = 2898 µm K T = temperature, K Emissivity (ε) describes how well a body emits energy comparing with the blackbody energy emitted from a body at a certain temperature ε = energy emitted from a blackbody at the same temperature

30 The sun emits radiation in the same manner as a blackbody whose temperature is about 6000 K From Wien s law, sun has a peak that occur at about 0.5 µm (in the range of visible wavelenghts) Thus when the sun is present we can observe earth features by reflected solar energy Source: Lillesand and Kiefer, 2000 The earth s features temperature (e.g soil, water, vegetation) is about 300 K (27 C) From Wien s law, earth features have a peak that occur at about 9.7 µm (in the range of thermal IR wavelenghts) Thermal IR energy can not be seen by human eyes, but it can be sensed by thermal radiometers and scanners

31 Interactions with the atmosfere Particles and gases in the atmosphere can affect the incoming light and radiation. These effects are caused by the mechanisms of scattering, absorption and emission. Source: James A. Tindall, 2006.

32 Scattering Scattering occurs when particles or large gas molecules present in the atmosphere interact with and cause the electromagnetic radiation to be redirected from its original path. There are 3 types of scattering which take place. 1. Rayleigh scattering occurs when particles are very small compared to the wavelength of the radiation these could be particles such as small specks of dust or nitrogen and oxygen molecules rayleigh scattering causes shorter wavelengths of energy to be scattered much more than longer wavelengths 2. Mie scattering occurs when the particles are just about the same size as the wavelength of the radiation dust, pollen, smoke and water vapour are common causes of Mie scattering Mie scattering tends to affect longer wavelengths than those affected by Rayleigh scattering. Non selective scattering this occurs when the particles are much larger than the wavelength of the radiation water droplets and large dust particles can cause this type of scattering nonselective scattering gets its name from the fact that all wavelengths are scattered about equally.

33 Absorption. Emission from the atmosphere occurs in the mid infrared region In contrast to scattering, absorption causes molecules in the atmosphere to absorb energy at various wavelengths. Ozone serves to absorb the harmful (to most living things) ultraviolet radiation from the sun without this protective layer in the atmosphere our skin would burn when exposed to sunlight Carbon dioxide it tends to absorb radiation strongly in the far infrared portion of the spectrum - that area associated with thermal heating - which serves to trap this heat inside the atmosphere. Water vapour water vapour in the atmosphere absorbs much of the incoming longwave infrared and shortwave microwave radiation the presence of water vapour in the lower atmosphere varies greatly from location to location and at different times of the year very little water vapour to absorb energy high concentrations of water vapour

34 Atmospheric windows Large window at wavelengths beyond 1 mm is associated with the microwave region. Those areas of the spectrum which are not severely influenced by atmospheric absorption and thus, are useful to remote sensors, are called atmospheric windows. The visible portion of the spectrum corresponds to both an atmospheric window and the peak energy level of the sun Heat energy emitted by the Earth corresponds to a window around 10 µm in the thermal IR portion of the spectrum,

35 Target interactions (I) Incident energy - E I (λ) Radiation that is not absorbed or scattered in the atmosphere can reach and interact with the Earth's surface. Source: James A. Tindall, There are 3 forms of interaction that can take place: (A) Absorption - E A (λ) (T) Transmission - E T (λ) (R) Reflection - E R (λ)

36 Energy balance equation E I (λ)= E A (λ) + E T (λ) + E R (λ) Incident energy Transmitted energy Absorbed energy Reflected energy The proportions of E A (λ), E T (λ) and E R (λ) will depend on: the wavelength of the energy for a given feature type the proportion of absorbed, transmitted and reflected energy will vary at different wavelength for example, two materials may be not distinguishable in one spectral range and be very different in another wavelength band the material type and condition of the feature these differences permit us to distinguish different feature on an image

37 Radiation reflected from a target Many remote sensing system are interested in measuring the radiation reflected from targets Energy balance equation can be expressed as: E R (λ) = E I (λ) [E A (λ) + E T (λ)] Incident energy Transmitted energy Reflected energy Absorbed energy There are two types of reflection, which represent the two extreme ends of the way in which energy is reflected from a target: a. specular reflection; b. diffuse reflection The geometry manner in which an object reflects energy is a function of the surface roughness of the object pecular reflectors are lat surface; the angle of eflection equals the angle f incidence diffuse reflectors are rough surface that reflec uniformly in all direction

38 pecular reflectors are lat surface; the angle of eflection equals the angle f incidence diffuse reflectors are rough surface that reflec uniformly in all direction In addition, the category that characterizes any object dictated by the surface s roughness in comparison to the wavelength of the energy incident In short, when the wavelength of energy incident is much smaller than the surface height variation of the object, the reflection from the surface is diffuse For example, at long wavelength (e.g. radio range), a sandy beach can appear flat to incident energy, whereas at short wavelength (e.g. visible range) it appear rough

39 Spectral reflectance of targets hlorophyll strongly absorbs radiation in the red nd blue wavelengths but reflects green avelengths. The internal structure of healthy aves act as excellent diffuse reflectors of nearfrared wavelengths. Longer wavelength visible and near infrared radiation is absorbed more by water than shorter visible wavelengths. Thus water typically looks blue or blue-green at shorter wavelengths, and darker if viewed at red or near infrared wavelengths The reflectance characteristics of targets may be quantified by measuring the portion of incident energy that is reflected This is measured as a function of wavelength and it is called Spectral reflectance = E R (λ) / E I (λ) x 100 A couple of examples of targets and how energy at the visible and infrared wavelengths interacts with them. 1. Leaves 2. Water

40 What do we measure with Remote Sensing? A graph of the spectral reflectance of an object as a function of wavelength is named Spectral reflectance curve Spectral reflectance curve help us on the choice of wavelength regions in which remote sensing data are acquired for a particular application Example: How can we distinguish between broadleaved and coniferous forests? broadleaved have higher near-ir reflectance than coniferous broadleaved appear lighter than coniferous broadleaved coniferous Example of generalized spectral reflectance for deciduous (broadleaved) and coniferous trees (source: Lillesand and Kiefer, 2000)

41 Typical spectral reflectance curves for three basic types of earth surfaces (source: Lillesand and Kiefer, 2000) Vegetation High absorption of blue and red energy by plants and high reflection of green energy High reflection in near IR region ( µm) Water absorption band (dips at 1.4, 1.9, 2.7 µm): water in the leaf absorb greately at these wavelengths Reflectance peaks occur at about 1.6 and 2.2 µm Water Energy absorption at near-ir wavelengths and beyond Reflection in visible wavelengths. The reflectance properties of a water body are a function of the water per se but also the material in the water

42 Typical spectral reflectance curves for three basic types of earth surfaces (source: Lillesand and Kiefer, 2000) Soil Factor affecting soil reflectance are: Moisture content Soil texture Surface roughness Organic matter content

43 Thermal radiation Thermal windows of the electromagnetic spectrum are: 3-5 µm 8-14 µm Sensor detect radiant energy emitted from a surface Example: aerial thermal photograph of a residental distric (source: Warmer streets and hot water appear lighter than cooler vegetation and cooling pond

44 Microwave Microwave - from approximately 1cm to 1m in wavelength Microwave radiation can penetrate through cloud cover, haze, dust and rainfall Microwave are not susceptible to atmospheric scattering which affects shorter optical wavelengths Microwaves can penetrate dry soils and rocks for few dm Active microwave sensors provide their own source of microwave radiation to illuminate the target. The most common form of imaging active microwave sensors is RADAR (RAdio Detection And Ranging) RADAR sensor transmits a microwave (radio) signal towards the target and detects the backscattered portion of the signal radar Transmitted signal backscattered signal

45 The amount of energy backscattered depends on: surfaces properties angle at which the microwave energy strikes the target The strength of the backscattered signal is measured to discriminate between different targets The time delay between the transmitted and reflected signals determines the distance (or range) to the target Etna, 22 september the COSMO-SkyMed image, with 1-metre spatial resolution (Spotlight mode), shows the summit of Etna, the most famous and intensively studied volcano in the world. Copyright ASI 2007, all right reserved.