Microwave Remote Sensing PDF

BASICS OF MICROWAVE REMOTE SENSING P. Mukhopadhyay EES 407  Remote Sensing Fundamental  The entire range of EM radiation constitute the EM Spectrum  Microwave sensors sense electromagnetic radiations in the microwave region of the EM Spectr...

BASICS OF MICROWAVE REMOTE SENSING P. Mukhopadhyay EES 407  Remote Sensing Fundamental  The entire range of EM radiation constitute the EM Spectrum  Microwave sensors sense electromagnetic radiations in the microwave region of the EM Spectrum Radar wavelengths Radar wavelengths  Microwave remote sensing Passi Activ ve e Emitted Reflected energy energy Imagi Imagi Non- ng ng Imaging Sea surface height Low spatial (SSH) Wind Low spatial High spatial vectors (Speed & resolution direction) Land Ocean Atmosphe resolution resolution applicatio applicatio re Land Ocean Atmosphe ns ns applicatio applicatio applicatio re ns ns ns applicatio Soil SST Cloud liquid ns Topographic Oil spills moisture Wind water Ocean Glacier speed Water vapor elevations waves studies Rain rate Cloud micro physics Polarization When discussing microwave energy, the polarization of the radiation is also important. Polarization refers to the orientation of the electric field. Most radars are designed to transmit microwave radiation either horizontally polarized (H) or vertically polarized (V). Similarly, the antenna receives either the horizontally or vertically polarized backscattered energy, and some radars can receive both. VV H H VH HV Passive microwave sensing All objects emit microwave energy of some magnitude, but the amounts are generally very small. A passive microwave sensor detects the naturally emitted microwave energy within its field of view. This emitted energy is related to the temperature and moisture properties of the emitting object or surface. Passive microwave sensors are typically radiometers or scanners and operate in much the same manner as systems discussed previously except that an antenna is used to detect and record the microwave energy. The microwave energy recorded by a passive sensor can be emitted by the atmosphere, emitted from the surface, or transmitted from the subsurface. Because the wavelengths are so long, the energy available is quite small compared to optical wavelengths. Thus, the fields of view must be large to detect enough energy to record a signal. Most passive microwave sensors are therefore characterized by low spatial resolution.  Passive Microwave Remote Sensing from Space Disadvantages Advantages Penetration through Larger field of views non- precipitating (10-50 km) clouds compared to VIS/IR Radiance is linearly sensors related to temperature Variable emissivity (i.e. the retrieval is over land nearly linear) Polar orbiting Highly stable instrument satellites provide calibration discontinuous Global coverage and temporal coverage wide swath at low latitudes (need to create weekly composites)  Passive Microwave Applications Soil moisture Snow water equivalent Sea/lake ice extent, concentration and type Sea surface temperature Atmospheric water vapor only over the oceans Surface wind speed Cloud liquid water Rainfall rate (TIRS) measures land surface temperature in two thermal bands with a new technology that applies quantum physics to detect heat.) will relate to the “spectral radiance” (measured in W m −2 sr−1 μm−1) that reaches the sensor for a certain wavelength band. We know that the amount of radiation from an object depends on its temperature T and emissivity \epsilon. That means that a cold object with high emissivity can radiate just as much radiation as a considerably hotter one with low emissivity. Often the emissivity of the object is unknown. If we assume that the emissivity of the object is equal to 1.0, then with the help of Planck’s law we can calculate directly the ground temperature that is needed to create this amount of radiance in the specified wavelength band of the sensor for the object with a perfect emissivity. The temperature calculated in this way is the radiant temperature or Trad. The terms brightness or “top-of-the-atmosphere” temperature are also frequently used. The radiant temperature calculated from the emitted radiation is in most cases lower than the true, kinetic temperature (Tkin) that we could measure on the ground with a contact thermometer. The reason for this is that most objects have an emissivity lower kinetic temperature than 1.0 and radiate incompletely. To calculate the true Tkin from the Trad, we need to know or estimate the emissivity. The relationship between Tkin and Trad is: The Rayleigh–Jeans law is an approximation to the spectral radiance of electromagnetic radiation as a function of wavelength from a black body at a given temperature. For wavelength λ, is Where is the spectral radiance (the power emitted per unit emitting area, per steradian, per unit wavelength), c is the speed of light , is the Boltzmann Constant, and T is the temperature in kelvins. For frequency ν, the expression is instead The Rayleigh–Jeans law agrees with experimental results at large wavelengths (low frequencies) but strongly disagrees at short wavelengths (high frequencies). This inconsistency between observations and the predictions of classical physics is commonly known as the ultraviolet catastrophe. Planck’s law gives the correct radiation at all frequencies, has the Rayleigh–Jeans law as its low-frequency limit. A solid angle is a three-dimensional analog of a circular angle that relates a portion of the volume of a sphere to the surface area it subtends. If that area equals the sphere’s radius squared, the solid angle is one steradian. This diagram displays two Max Planck empirically obtained an expression for black-body radiation expressed in terms of wavelength λ = c/ν (Planck's law): where h is the Planck constant, and kB is the Boltzmann constant. Planck's law does not suffer from an ultraviolet catastrophe and agrees well with the experimental data In the limit of high temperatures or long wavelengths, the term in the exponential becomes small, and the exponential is well approximated with the Taylor polynomial's first-order term: Therefore Planck's blackbody formula reducing to which is identical to the classically derived Rayleigh–Jeans expression When comparing the frequency- and wavelength-dependent expressions of the Rayleigh–Jeans law and these two expressions then have different units, as a step in wavelength is not equivalent to a step in frequency. Therefore, Even after substituting the value because has units of energy emitted per unit time per unit area of emitting surface, per unit solid angle, per unit wavelength, whereas has units of energy emitted per unit time per unit area of emitting surface, per unit solid angle, per unit frequency. To be consistent, we must use the equality where both sides now have units of power (energy emitted per unit time) per unit area of emitting Starting with the Rayleigh–Jeans law in terms of wavelength, we get Where Bν(T)= B λ (T) dλ /dν where (dλ /dν )=- c/ν2 |Bν(T)|= (-c/ν2) Put λ=c/ ν (T)= * (T)= * Bν(T) Microwave Brightness Temperature Microwave radiometers can measure the emitted spectral radiance received (L This is called the brightness temperature (TB) and is linearly related to the kinetic temperature (T)of the surface The Rayleigh-Jeans approximation provides a simple linear relationship between measured spectral radiance temperature and emissivity 4  TB  L 2kc Rayleigh-Jeans Approximation a constant 2kcT spectral radiance is a linear L   function of kinetic temperature 4  k is Planck’s constant, c is the speed of light,  is emissivity, T is kinetic temperature This approximation only holds for  >> max (e.g.  > 2.57mm @300 K) 4 T = TB  Tk in TB   L TB 2kc T is also called the “brightness temperature” B typically temperature Brightness shown as T can be related to kinetic temperature through emissivity passive microwave brightness temperatures can be used to monitor temperature as well as properties related to emissivity. Emissivity (ε) The emissivity of a material is the ratio of energy radiated by a particular material to energy radiated by a black body at the same temperature. Radiated energy by a ε= target Radiated energy by a black body A true black body would have an ε = 1, Real object would have ε < 1. Emissivity is a dimensionless quantity (does not have units). It is a measure of a material's ability to radiate absorbed energy The more reflective a material is, the lower its emissivity. Water 0.95 - 0.963 Sand 0.76 Highly polished silver has an emissivity of about 0.02 Band [GHz] Polarization 10.65 V,H 19.35 V,H 21.3 V 37.0 V,H 85.5 V,H Active Microwave Remote sensing Rayleigh Criterion for Roughness 3 5 cm cm 25 cm Radar Backscatter Coefficient  Active microwave sensors Active microwave sensors provide their own source of microwave radiation to illuminate the target. Active microwave sensors are generally divided into two distinct categories: Imaging Non-imaging Imaging: The most common form of imaging active microwave sensors is RADAR. RADAR is an acronym for RAdio Detection And Ranging, which essentially characterizes the function and operation of a radar sensor. The sensor transmits a microwave (radio) signal towards the target and detects the backscattered portion of the signal. The strength of the backscattered signal is measured to discriminate between different targets and the time delay between the transmitted and reflected signals determines the distance (or range) to the target. Radar range – R C* R t The distance, R, from the antenna = to 2 the target is calculated as where c is the speed of light (3 x 10-8 m sec - 1) t is the time between the Viewing Geometry and Spatial Resolution The imaging geometry of a radar system is different from the framing and scanning systems commonly employed for optical remote sensing. Similar to optical systems, the platform travels forward in the flight direction (A) with the nadir (B) directly beneath the platform. The microwave beam is transmitted obliquely at right angles to the direction of flight illuminating a swath (C) which is offset from nadir. Range (D) refers to the across-track dimension perpendicular to the flight direction, while azimuth (E) refers to the along-track dimension parallel to the flight direction. This side-looking viewing geometry is typical of imaging radar systems (airborne or spaceborne).  The portion of the image swath closest to the nadir track of the radar platform is called the near range (A) while the portion of the swath farthest from the nadir is called the far range (B). If a Real Aperture Radar (RAR ) is used for image formation (as in Side-Looking Airborne Radar) a single transmit pulse and the backscattered signal are used to form the image. In this case, the resolution is dependent on the effective length of the pulse in the slant range direction and on the width of the illumination in the azimuth direction. The range or across-track resolution is dependent on the length of the pulse (P). Two distinct targets on the surface will be resolved in the range dimension if their separation is greater than half the pulse length. (The half width of a pulse length is the full width at half maximum (FWHM)) For example, targets 1 and 2 will not be separable while targets 3 and 4 will. Slant range resolution remains constant, independent of range. However, when projected into ground range coordinates, the resolution in ground range will be dependent of the incidence angle.  The azimuth or along-track resolution is determined by the angular width of the radiated microwave beam and the slant range distance. This beam width (A) is a measure of the width of the illumination pattern. As the radar illumination propagates to increasing distance from the sensor, the azimuth resolution increases (becomes coarser). In this illustration, targets 1 and 2 in the near range would be separable, but targets 3 and 4 at further range would not. The radar beam width is inversely proportional to the antenna length (also referred to as the aperture) which means that a longer antenna (or aperture) will produce a narrower beam and finer resolution. 1. Finer range resolution can be achieved by using a shorter pulse length, which can be done within certain engineering design restrictions. 2. Finer azimuth resolution can be achieved by increasing the antenna length. However, the actual length of the antenna is limited by what can be carried on an airborne or space borne platform. For airborne radars, antennas are usually limited to one to two meters; for satellites they can be 10 to 15 meters in length. 3. To overcome this size limitation, the forward motion of the platform and special recording and processing of the backscattered echoes are used to simulate a very long antenna and thus increase azimuth resolution.  synthetic aperture radar or SAR As a target (A) first enters the radar beam (1), the backscattered echoes from each transmitted pulse begin to be recorded. As the platform continues to move forward, all echoes from the target for each pulse are recorded during the entire time that the target is within the beam. The point at which the target leaves the view of the radar beam (2) some time later, determines the length of the simulated or synthesized antenna (B). Targets at far range, where the beam is widest will be illuminated for a longer period of time than objects at near range. The expanding beam width, combined with the increased time a target is within the beam as ground range increases, balance each other, such that the resolution remains constant across the entire swath. This method of achieving uniform, fine azimuth resolution across the entire imaging swath is called synthetic aperture radar, or SAR. Most airborne and spaceborne radars employ this type of radar. C-band SAR  Satellite Altimeters (sea level measurement Very accurate, ) satellite-altimeter systems are needed for measuring the oceanic topography, surface circulation of the oceans, and the variability of gyre-scale currents. Altimeters emit a microwave pulse Measurement = time required for the pulse to travel from the satellite to the sea surface and back (“2-way travel time”) Altimeter system can measure: 1.Changes in the global mean volume of the ocean 2.Seasonal heating and cooling of the ocean 3.Tides 4. surface Geostrophic current system Current and planned Instruments ALT POSEIDON-3 ++ POSEIDON- 3B AltiKa RA-2 Ka-band Radar SIRAL POSEIDON-2 (SSALT-2) SRAL  Ocean’s Temperature via Satellite Sea surface temperature (SST) is measured by passive sensing of emitted thermal radiation Thermal – IR sensors (Clouds/ haze/fog are major obstruction to acquire the data) AVHRR (NOAA-18, 19 & Metop-A, B) Advanced Very High Resolution Radiometer MODIS ( AQUA/TERRA) Moderate Resolution Imaging Spectroradiometer. VIIR S (Suomi NPP, JPSS-1/NOAA-20) Visible Infrared Imaging Radiometer Suite alternative to acquire the data, passive Microwave remote sensing Passive microwave radiometers Scanning Multichannel Microwave Radiometer (SMMR ) Special Sensor Microwave/Imager (SSM/I) Tropical Rainfall Measuring Mission Imager (TRMM-TMI) Advanced Microwave Scanning Radiometer (AMSR-E) SST images help quantify changes in: fisheries management climate and seasonal monitoring/forecasting validation of atmospheric models evaluation of coral bleaching ocean heat storage ocean currents temperature-sensitive biological activity AVHR TMI  Thermal bands of AVHRR and MODIS Water vapor (WV) correction: If no WV (single Water vapor exists to remove the WV absorption (Two bands) band) SST = a0 + a1T i + a2(Ti - T j) SST = a + a T 0 1 i With a single band, we don’t know whether the drop is due to increased WV or a drop in temperature. If we use two bands, absorption by WV is grater at 11.5 -12.5µm than 10.5-11.5 µm, so the difference in observed radiance is a measure of the amount of WV. Atmospheric Atmospheric corrections in the infrared are simpler than for ocen corrections: colour. Sun glint (Day time) Emission from clouds water vapor AVHRR (MCSST) = c1+ (c2*T4) + (c3*(T4-T5)) + (c4*(sec (ө)- 1)*(T4-T5)) MODIS(MCSST) = c1+ (c2*T31) + (c3*(T31-T32)) + (c4*(sec (ө)-1)*(T31-T32)) Night MCSST Triple = c1+ (c2*T4) + (c3*(T3-T5)) + (c4*(sec (ө)-1)*(T3-T5))  Monitoring Ocean Winds Scatterometers are unique among satellite remote sensors in their ability to determine the wind velocity & direction over water. Applications weather forecasting, marine safety, fishing, long term climate studies.  Satellite Scatterometers Short Intended Spatial Product Scan Operatio Background Period of Resolution Grid Characteristics nal Service Spacin Frequen g cy HY-2B 2015 - 25 km 12.5 km Conical scan Ku band Scatterometer 2018+ one wide swath SCATSAT 2016 - 25 km 12.5 km Conical scan Ku band 2021 one wide swath FY-3E 2016 - 25 km 12.5 km Conical scan C and Ku band 2020 one wide swath Meteor-M N3 2017 - 25 km 12.5 km Conical scan Ku band 2020 one wide swath METOP-C 2017 - 50 km 12.5 km Two sided C band Scatterometer 2023+ Double swath Oceansat-3 2018 - 25 km 12.5 km Conical scan Ku band 2023 one wide swath CFOSAT 2018 - 25 km 12.5 km Conical scan Ku band 2021 one wide swath