MODULE 9 LECTURE NOTES 1 PASSIVE MICROWAVE REMOTE SENSING

Transcription

1 MODULE 9 LECTURE NOTES 1 PASSIVE MICROWAVE REMOTE SENSING 1. Introduction The microwave portion of the electromagnetic spectrum involves wavelengths within a range of 1 mm to 1 m. Microwaves possess all weather operability, i.e., they are capable of penetrating clouds, haze, light rain, smoke depending on their wavelengths and hence are suitable for providing information at all times of day. This property of microwaves enables it as the best choice for atmospheric studies. Different physical principles are followed to capture images in microwave / infrared/ visible spectrum. Microwave reflections/emissions provide a different view of the earth s surface that are entirely different from those observed using visible/infrared wavelengths. In addition, unique information like sea wind/wave direction, polarization, backscattering etc are obtained which cannot be observed by sensors operating in visible/infra red regions. The only disadvantage is coarse resolution and requirement for sophisticated data analysis techniques. The portion of energy scattered to a sensor operating in microwave spectrum will depend on many factors such as dielectric constant of surface materials, type of land use/land cover, surface roughness, slopes, orientation of objects, microwave frequency, polarization, incident angle etc. Observations using microwave sensors at selected frequencies are capable of providing information regarding atmospheric structure (i.e., profiles of temperature, water vapor), liquid water content etc which make it an invaluable source for meteorological observations. The primary factors that influence the transmission of microwave signals are the wavelength and polarization of the energy pulse. These bands within the microwave spectrum were named using names originally to ensure military security during the early stages of development of radar. These traditional names have been adopted by the Institute of Electrical and Electronics Engineers and internationally by the International Telecommunication Union. These are listed below: D Nagesh Kumar, IISc, Bangalore 1 M9L1

2 Table 1: Designation of various radar bands with their wavelengths Band Designation Wavelength (cm) K a K K u X C S L P Microwave radiations are essentially characterized using intensity and polarization. Intensity is provided by the thermal emissions reaching the sensor reflected from the earth s surface after passing through the atmosphere. Intensity can be either backscatter (for active sensor) or brightness temperature (for passive sensor). Polarization defines a group of simple waves comprising of a beam of radiation. Electromagnetic radiation is comprised of electric and magnetic radiations oscillating in mutually perpendicular directions in such a way that at any point in space, the electric field vector of a simple electromagnetic radiation will always trace an ellipse. Polarization describes the eccentricity and orientation of this ellipse which can either be linearly polarized (i.e., in the horizontal and vertical directions) or circularly polarized. Figure 1: Electromagnetic Wave D Nagesh Kumar, IISc, Bangalore 2 M9L1

3 Microwave remote sensing can be of two types: active and passive. The term active refers to sensor which transmits its own source of energy and then receives the backscattered energy from the surface of earth. Radar stands for radio detection and ranging. Radars like synthetic aperture radar, scatterometers, radar altimeters, ground based weather radars etc are all active instruments. Passive sensors like microwave radiometers receive the naturally emanating electromagnetic energy from the earth-atmosphere system. These sensors detect very low levels of microwave energy. The typical passive and active microwave sensors used are tabulated below Table 2: Active and Passive sensor Application Sensor Type Instrument Applications Passive Sensor Microwave Sea surface Temperature (SST), Radiometer Salinity, sea ice, rainfall intensity, air temperature, Active Sensor Microwave Scatterometer Microwave Altimeter Imaging Radar Near sea surface wind, ozone water vapor etc Soil moisture content, water vapor, rainfall Intensity, near sea surface wind, ocean wave, biomass, sea ice, snow Sea surface topography, wind velocity, geoid, tide Ocean wave, topography, ice, sea surface wind, geology 2. Passive remote sensing All natural materials emit electromagnetic radiation that is complex functions of emitting surfaces. A passive sensor operating in the microwave spectrum will usually rely on the naturally available microwave energy within their field of view instead of supplying their own source of illumination and measuring the reflected radiation like an active sensor. Passive systems utilize the electromagnetic energy that is reflected or emitted from the Earth s surface and atmosphere. Their advantages are manifold like penetration through nonprecipitating clouds that aids in meteorological studies, global coverage and wide swath, D Nagesh Kumar, IISc, Bangalore 3 M9L1

4 highly stable instrument calibration etc. These systems suffer from large field of views (10-50 km) when compared to systems operating in the visible or infrared wavelengths. The sensors operating in passive systems use an antenna ( horn ) to detect photons at microwave frequencies which are then converted to voltages in a circuit. 2.1 Principle The basic principle governing signal detection by passive sensors is Rayleigh Jeans s approximation of Planck s law. Conceptually, in order to understand passive microwave remote sensing, the idea of blackbody radiation theory is essential. As per thermodynamic principles, all material (gases, liquids or solids) tend to both emit as well as absorb incoherent electromagnetic energy at absolute temperature. Figure 2: Plot of Relative radiance energy vs wavelength D Nagesh Kumar, IISc, Bangalore 4 M9L1

5 If B correspond to the Planck blackbody function, I be the magnitude of thermal emission and denote the emissivity, then thermal emission can be expressed using the relation: I * B ( T) [ (2 c 2 5 h / )]/( e hc/ kt 1) where h is Planck s constant, k is Boltzmann s constant, c the speed of light and T is thermal temperature. Once we approximate the thermal emission from Planck function using Rayleigh Jeans formula, then the microwave brightness temperature can be conveniently expressed as a linear function of physical temperature and emissivity as: Tb *T PhysicalTemperature where is a complex function of dielectric constant whose values are quite well known for gases and calm water but not so well understood for the complicated case of rough water and land surfaces. A space borne radiometer viewing the earth senses the electromagnetic energy emanating from the land surface that reaches the top of atmosphere into the antenna after undergoing atmospheric attenuation (dampening of signal due to atmospheric components). It depends on the absorption/scattering properties of the land surface and atmosphere that tends to vary with respect to frequency and polarization. The antenna receives radiation from regions defined by the antenna pattern which is usually strongly peaked along its beam axis. Radiative transfer models are generally used to interpret the Tb received by antenna. This topic will not be discussed in this module. More details can be obtained in Chandrasekhar, 1950; Wilheit et al., 1977; Volchok and Chernyak (1968) etc. Just like thermal radiometers, microwave radiometers are non imaging devices whose output get digitally recorded on a magnetic medium. Normally, the radiometer output refers to the apparent antenna temperature. The total noise power resulting from the thermal radiation incident on the antenna, also known as antenna temperature is expressed as a function of the antenna gain pattern ( G (, ) ) and the brightness temperature distribution incident ( Tb (, ) ), as: T a Tb(, ) G(, ) d D Nagesh Kumar, IISc, Bangalore 5 M9L1

6 Upwelling Atmospheric Tb ATMOSPHERE Downwelling Atmospheric Tb Reflected Atmospheric Tb Tb due to surface emission OCEAN Figure 3: Space borne radiometer observing ocean at a nadir angle This system is calibrated using the temperature that a blackbody located at the antenna must reach so that the same energy is radiated as is collected from the ground scene. D Nagesh Kumar, IISc, Bangalore 6 M9L1

7 2.2 Examples of Passive microwave radiometers Advanced Microwave Sounding Unit (AMSU) 1978 present Scanning Multichannel Microwave Radiometer (SMMR) Special Sensor Microwave/Imager (SSM/I) 1987-present Tropical Rainfall Measuring Mission (TRMM) 1997-present Advanced Microwave Scanning Radiometer (AMSR-E)2002-present 2.3 Passive microwave applications a) Soil Moisture For passive microwave signals, in the absence of significant vegetation cover, soil moisture provides the dominant effect on the received signal. The spatial resolution of satellite borne passive microwave systems designed to study soil moisture range from km. The low frequency ranges of 1-3 GHz is usually considered for soil moisture sensing. This is because of the reduced atmospheric attenuation and higher vegetation penetration in these wavelengths. The dielectric constant of water tends to significantly increase depending on an increase in volume fraction of water in the soil. This relationship varies with respect to different types of soil. Different soil types have different percentages of water bound on it. The bound water adhering to soil particles will exhibit rotation less freely at microwave frequencies thereby leading to a lower dielectric effect than the free water in the pore spaces. In this context, penetration depth of microwave frequencies is important as it indicates the thickness of surface layer within which moisture and temperature variations can significantly affect the emitted radiation. b) Snow water equivalent While viewing snow covered areas, microwave emission will comprise of contributions from snow as well as from the underlying ground. Crystals of snow will scatter part of the radiation thereby reducing the upwelling radiation measured with a radiometer. Deeper snow is indicative of greater snow crystals to scatter the upwelling microwave energy. This characteristic property is used to estimate snow mass. Usually frequencies greater than 25 GHz is utilized for detecting snow. The strength of scattering signals are proportional to the D Nagesh Kumar, IISc, Bangalore 7 M9L1

8 snow water equivalent. Melting water from a snow covered surface will tend to increase the microwave brightness temperature at frequencies above 30 GHz as water droplets will emit rather than scatter microwave radiation. Under these conditions, it will be difficult to extract information regarding the snow water equivalent. c) Atmospheric water vapor An extensive study of microwave absorption of atmospheric gases (both theoretically and experimentally) shows that, emission/absorption in gaseous atmosphere is dominated by the presence of water vapor and oxygen [Waters, 1976; Ulaby et al., 1981]. Absorption characteristics of these gases are summarized by Staelin [1969], Paris [1971], Derr [1972], Waters [1976], Fraser [1975]. Microwaves undergo resonant absorption and emission at certain frequencies due to the quantum energy states of the water vapor/oxygen molecules. Within microwave spectrum, these molecules are subjected to rotational transition wherein, a molecule changes rotational energy states. This causes a peak in Tb measured by a radiometer. The magnitude of increase in Tb depends on the total number of water vapor/oxygen molecules along the propagation path through the atmosphere. An increasing altitude gets accompanied with a decrease in the number of water vapor/oxygen molecules per unit volume. This in turn reduces the bandwidth of water vapor/oxygen emission (absorption) leading to an increase in absorption at the peak of resonance. The rotational lines of water and oxygen are pressure broadened in the atmosphere owing to the presence of other gases; there is also a slight dependence on temperature [Kidder and Haar, 1995]. Water vapor has a weak absorption line at GHz and a strong line at 183 GHz. All sensors currently used for precipitation make a measurement near this channel (SSM/I at GHz; TMI at 21.3 GHz and AMSR at 23.8 GHz). d) Rainfall rate At microwave wavelengths, precipitation sized drops interact strongly with microwave radiation [Kidder and Haar, 1995]. Interaction of electromagnetic (EM) waves with a spherical dielectric causes scattering (redirecting) or absorption (conversion to mechanical energy) of radiation depending on size of precipitation particles [Barrett and Martin, 1981]. One of the earlier studies by Mie [1908] introduced the general mathematical solution for scattering and absorption of electromagnetic waves by a dielectric sphere of arbitrary radius. Later on, this was applied to the context of rain by Gunn and East [1954]. If we consider a D Nagesh Kumar, IISc, Bangalore 8 M9L1

9 single raindrop particle with size < < of electromagnetic wave, the absorption cross section will be proportional to the volume and mass of rain drop while scattering cross section will be negligible. When cloud drop coalesce into raindrops with dimensions comparable to microwave wavelengths, absorption per unit mass increases and scattering can no longer be ignored. D Nagesh Kumar, IISc, Bangalore 9 M9L1