© 2005 by CRC Press 303 11 Radio Devices for Remote Sensing Two classes of devices and corresponding methods are used for remote sensing of the environment. Devices that radiate radiowaves and receive them after their inter- action with media belong to the first class and are referred to as the active devices. Devices of the second class, as a rule, receive intrinsic thermal radiation of natural objects and are referred to as passive devices. The different types of radars, altim- eters, scatterometers, and radio occultation instruments are examples of active devices. The different types of microwave radiometers are classified as the passive devices. 11.1 SOME CHARACTERISTICS OF ANTENNA SYSTEMS All radio devices for remote sensing are equipped with antenna systems for the reception of radiowaves (and for their radiation, in the case of active instruments). This is the reason why we will consider again the properties of an antenna. We have directivity and pattern . The definition of directivity was based on the angular distribution of the radiated power relative to the power at the antenna input. The energy losses (e.g., taking place in the feeding systems) were not taken into consideration; therefore, we need to include in our analysis the antenna effi- ciency: , (11.1) which reflects the ratio of radiated power P i to input power P t . So, the gain, , often figures into engineering computations. The effective area of the antenna was used to characterize the receiving properties of the antenna. Normally, this notion relates to the geometrical antenna area via the aperture efficiency : . (11.2) D(, )θϕ Ψ(, )θϕ η = P P i t GD(, ) (, )θϕ η θϕ= η a e A A = max TF1710_book.fm Page 303 Thursday, September 30, 2004 1:43 PM established already in the Chapter 1 that transmitting antenna are characterized by © 2005 by CRC Press 304 Radio Propagation and Remote Sensing of the Environment Here, the maximum effective area occurs in the numerator. Equation (1.114) helps us to define the aperture efficiency as: . (11.3) The normalized amplitude of the aperture field (the apparatus function F (r ′ )) is substituted here for the field. The utilization factor is equal to unity in the case of a uniform aperture field distribution (the apparatus function equals unity). In this case, the antenna pattern of the circular antenna can be written as shown in Equation (1.118). We must point out that a uniform distribution is an idealization of practical situations. In reality, the utilization factor is less than unity. The side lobes of both kinds of antennae (transmitting and receiving) are of interest, as very often they are sources of interference. One of the main reasons for the high side lobe level is the sharp change of apparatus function at the edge of the antenna; therefore, uniform apparatus function leads to a high level of the side lobes. The first side lobe of the circular antenna, in this case, is 17.6 dB less compared to the major one; therefore, in order to lower this level, we want the apparatus function at the edge of the antenna to tend to zero. As a modeling example, we can choose the apparatus function for the circular antenna in the view . The antenna pattern will be: , (11.4) and the first side lobe level will be 24.6 dB; 4 however, in this case, the beamwidth is whereas it is for the uniform apparatus function. The utilization factor is 0.75. 4 The stray factor: (11.5) is applied for the side lobe role general assessment. Here, Ω 0 is the solid angle of the major lobe estimated by the first zero line of the antenna pattern. In the case of the patterns represented by Equation (1.118), it is reduced to , where is the second root of the first-order Bessel function, and we have the η a A A Fd AF d = ′ () ′ ′ () ′ ∫ ∫ r r 2 2 2 2 r r Fa() ( )rr= −1 2 Ψθ θ θ () = () () 4 2 2 4 Jka ka sin sin 064. λ a 051. λ a β θϕ θϕ π = − ∫∫ ∫∫ 1 0 4 Fd Fd n n (, ) (, ) Ω Ω Ω β = Jj 0 2 12 () , j 12, TF1710_book.fm Page 304 Thursday, September 30, 2004 1:43 PM © 2005 by CRC Press Radio Devices for Remote Sensing 305 corresponding quantity . This value is defined by a similar formula as in Equation (11.4), except that in this case the root of the second-order Bessel function will be the zero-order function argument j 2,2 = 5.136 and β = 0.017. The level of the side lobes, as a rule, is not very important for radar. More important is the signal-to-noise ratio; therefore, the standard approach is to have maximal aperture efficiency and, correspondingly, maximum directivity. For radi- ometry, however, a low level of side lobes is one of the most important parameters. So, we may have to sacrifice sensitivity due to some deterioration of the spatial resolution. 11.2 APPLICATION OF RADAR DEVICES FOR ENVIRONMENTAL RESEARCH Radar devices for remote sensing of the environment are mounted on the surface of the Earth, on ships and riverboats, and on air- and spacecraft. Earth-mounted radar is used for the study of the atmosphere and ionosphere and to observe coastal sea zones and inland reservoirs, as well as various types of land and the structure of the surface layers. Ship radar is used mostly for water surface monitoring and atmos- pheric research, as well as coastal area supervision. All elements of the environment are the objects of observation for air- and spaceborne radars. The main purposes of such research are analysis of radiowave intensity and polarization, the study of the shape and time delay of signals, etc. These data are used for radar mapping, altimetry, scatterometric research, and subsurface sounding. The main areas of application are geology (e.g., structure and lithology), hydrology (e.g., determining soil moisture; mapping river basins, inner reservoirs, floods, snow- cover), farming (e.g., mapping crops, monitoring vegetation growth and maturation, determining field borders, observing soil moisture dynamics), forestry (e.g., estimat- ing merchantable wood, monitoring deforestation, detecting forest fires, assessing fire hazard situations), oceanology (e.g., sea-wave zone monitoring, measuring near- water wind velocity, determining the direction in which a pollution area is moving, studying sea currents and geoid forms), atmospheric research (e.g., sounding clouds and rain areas, determining temperature height profiles, defining atmospheric turbu- lence, detecting minor gaseous constituents), study of ionospheric dynamics (e.g., altitude profiles of electron concentrations, turbulence motion, electron and ion temperature measurement), cartography (e.g., topographical survey of regions that are difficult to access, land use mapping), polar area observations (e.g., monitoring and investigating sea ice, mapping continental ice covers, observing iceberg forma- tion and their motion, researching changes in glaciers). Generally, four types of radar devices are used for remote sensing: panoramic radars, scatterometers, radio altimeters, and subsurface radars. Panoramic radar involves sectoring or a circular scanning view and side-looking radar; the latter type includes radar with a real aperture and synthetic-aperture radars (SAR). Scatterom- eters are classified according to the methods for selecting a given area. The main applications are angle–angle (the antenna pattern is narrow on both planes), angle–time (the pattern is narrow only on the azimuthal plane, thus the signal is β = 016. TF1710_book.fm Page 305 Thursday, September 30, 2004 1:43 PM © 2005 by CRC Press 306 Radio Propagation and Remote Sensing of the Environment modulated by narrow pulses), angle–frequency (the pattern is narrow on the azi- muthal plane and is directed along the platform flight line; signal discrimination is achieved by Doppler selection), frequency–time (SAR with pulse modulation). Altimeters are classified by the modulation method: pulse, frequency, or phase modulation. Subsurface radars differ in where the antenna is mounted: remote or applied (the antenna is put on the surface of the studied object). 11.3 RADIO ALTIMETERS The radio altimeter is in theory one of the simplest radar systems. Applications of remote sensing using radio altimeters include measuring water levels, defining geoid forms, estimating seawave intensity, monitoring glacier growth, mapping land sur- face topographic, determining upper cloud levels, etc. Pulse-modulated or chirp signals are normally used for altimetric research. For the traveling time of the signal ( t r–t ) over the path, transmitter–surface–receiver serves as a device for measuring the altitude of the altimeter platform above the studied surface. The common relation is: , (11.6) where v is the mean velocity of radiowave propagation in the atmosphere of Earth. The frequency spectrum of altimetric systems is chosen in such a way that the influence of the ionosphere is small. Usually, the C, X, and Ku-bands are preferable for these devices. The error in the altitude determination is: , (11.7) where δ t r–t and δ v are errors in the delay time and mean velocity, respectively. The first summand contribution is defined by the leading edge steepness of the optimally processed pulse and the signal-to-noise ratio at the altimeter output. Generally, , (11.8) where τ i is the leading edge duration after reflection (scattering), and S/N is the signal-to-noise ratio. 89 Changes in the leading edge as a result of scattering by a The second term in Equation (11.7) depends on the propagation conditions. In the case of spaceborne altimeters, the contributions of both the troposphere and ionosphere must be taken into account. The tropospheric contribution, corresponding to Equation (4.57), can be written as: H tv = r-t 2 δδ δH v t t v=+ − − 2 rt rt 2 δ τ t SN i rt− = () 2 TF1710_book.fm Page 306 Thursday, September 30, 2004 1:43 PM rough surface are described in Chapter 6. © 2005 by CRC Press Radio Devices for Remote Sensing 307 (11.9) Equation (4.69) permits us to estimate the ionospheric contribution as: . (11.10) For the X-band, it will be about 10 cm. For greater accuracy when defining altitude, it is necessary to develop procedures for correcting the radio propagation. In the case of the troposphere, atmospheric models are used. Microwave radiometers are added to measure water vapor content, which gives us a more accurate calculation of the mean value of the refractive index. Correcting for ionospheric influences requires the design of a two-frequency altimeter or the use of very high frequencies, which generates new problems (scattering by clouds, for example). In order to overcome these, continuous analysis of signals is needed using, for example, a strobing technique. The accuracy of the altimeter also depends on the antenna beamwidth and its positional stability. These effects become most apparent in the case of strong reflec- tion from the surface edge coinciding with the footprint board. As was noted in signal. 89 Examples of backscattered signals are given in Figure 11.1. This is one reason why estimating the time delay using the arrival of the leading edge of the pulse gives us a more accurate altitude measurement. Simultaneously, pulse distor- tion analysis allows the study of the roughness parameters. In particular, this tech- nique is used for sea waves intensity monitoring. FIGURE 11.1 The approached pulse shape of normalized altimeter impulses: (1) salt desert; (2) and (3), agricultural holdings; (4) surface of lake. t v H T rt 2 m. − = − () ≅δ ε 0 1 2 25. t v f rt 2 − = ⋅ δ 12110 21 2 . 0.4 0.6 3 2 4 1 2 1 3 4 0.2 0 t 0.8 1 u TF1710_book.fm Page 307 Thursday, September 30, 2004 1:43 PM Chapter 6, surface roughness leads to a change in the duration of the scattered © 2005 by CRC Press 308 Radio Propagation and Remote Sensing of the Environment The first spaceborne altimeter was tested during the SKYLAB mission. It oper- ated at a frequency of 13.9 GHz, and the pulse duration was of the order of 25 nsec. The measurements taken showed that we can determine the topographic relief under the satellite trace with an accuracy of ±15 meters for the case of a relatively smooth surface without mountains and steep slopes. The altitude above a quiet water surface could be determined with an accuracy of about ±1 meter. 89 The investigation of ocean problems requires a more accurate altimeter. Research has shown that errors of altitude determination must be less than 10 cm. Very short pulses are needed in this case, which explains why chirp modulation is useful in modern altimeters. In particular, pulses with a duration of the order of 100 µ sec and a frequency deviation inside ∆ F = 320 MHz are used in modern altimeters. The compression procedure provides the altitude definition with an accuracy that is similar to pulses of 3.1-nsec duration. A spaceborne altimeter of very high accuracy is applied in the U.S.–French mission TOPEX/POSEIDON, which is designed to research water surface charac- teristics. 145 The C- and Ku-bands (frequencies of 5.3 and 13.6 GHz, respectively) are used. The two-frequency system allows us to correct the ionospheric influences. Microwave radiometers, operating at frequencies of 18, 21, and 37 GHz, provide the tropospheric correction. A good system of 1.5-m antenna orientation minimizes errors connected with main lobe deviation from the nadir. There is a system based on the ground for the satellite coordinates determination with heightened accuracy. A complicated procedure of corrections has been developed for altimeter data processing and interpretation. Depending on the seawave intensity and the data averaging time, the accuracy of the mean level of the oceanic surface definition lies within a range of several centimeters. 11.4 RADAR SYSTEMS FOR REMOTE SENSING OF THE ENVIRONMENT A different type of panoramic radar is commonly used for remote sensing of the environment. These radar systems differ in their principles of operation and in the technique of observation. The received pulses are distinguished by arrival time, which allows determination of the distance to the illuminated object (in the case of monostatic radar), and amplitude, both of which allow us to define the scattering properties (reflectivity) of this object. In this process, the direction of the antenna major lobe is fixed, and the direction of the investigated target is determined. The scattering properties of targets can depend on polarization of illuminated waves and scatter the waves with a polarization different from the polarization of incident waves. So, the polarization matrix or Stokes matrix are the subject of interest in more complicated systems. These data open the way for mapping the scattering properties of the environment. The radar systems applied to remote sensing are separated roughly into two classes: radar for atmospheric research (troposphere and ionosphere) and radar for monitoring the surface of Earth (land and water). Ionospheric radar will be discussed TF1710_book.fm Page 308 Thursday, September 30, 2004 1:43 PM the targets themselves, as we saw in Chapter 5 (Section 5.6). The targets usually © 2005 by CRC Press Radio Devices for Remote Sensing 309 which is used primarily for the investigation of hydrometeors. Scanning space is made possible with the help of a pencil-beam antenna. The main parameter of measurement is the reflectivity, which is determined via the intensity of the received pulses. The power of a signal received from a point scatterer is defined by the radar equation: . (11.11) Here, W is the received power; P is the pulse transmitted power; D and A e are the radar antenna directivity and its effective area, respectively; σ is the target differential cross section of backscattering; L is the distance between the radar and the target; and λ is the radiowavelength. In contrast to the traditionally applied formulae, we use the physical definition of the cross section, which is different from the radar one by the factor 4 π different engineering coefficients describing the feeder system efficiency. We also assume the absence of wave attenuation in the environment. In the future, we will generally restrict ourselves to the case of clouds as the subject of the study. They are distributed targets; therefore, the power scattered by a cloud layer of thickness l can be defined as: , (11.12) where is the differential cross section of the backscattering per unit volume of the cloud. The coordinate integration in Equation (11.12) is realized over the plane transverse to the wave propagation direction. This integration is easily trans- formed to integration over the solid angle. To calculate this integral, we can use the model antenna formula (Equation (1.123)) to obtain: . (11.13) With the result . (11.14) If we had used the antenna pattern (Equation (1.118)), then instead of Equation (11.13), we would have obtained: WL PDA L PA L ()== ee σ π σ λ4 4 2 42 WL PLl L A d L PLl L () (,) () (,) == ∫ σπ λ σπ d 0 e 2 d r 22 2 0 2 r λλ 2 2 Ad e ()ΩΩ ∫ σπ d 0 (,)L IAd A d A d A == ≅ = ee 2 e 2 e 2 2 22 0 () ()sin ()ΩΩ π θ θθ πθθθ λ (() ∞ ∫∫∫ 2 00 2π WL PLlA L () = ()() σπ de 0 2 0 2 , TF1710_book.fm Page 309 Thursday, September 30, 2004 1:43 PM (see Chapter 5). Among other factors, we do not take into account in Chapter 13; here, let us describe briefly tropospheric radar, or weather radar (WR), © 2005 by CRC Press 310 Radio Propagation and Remote Sensing of the Environment . One can easily see that the difference is insignificant, and that Equation (11.14) is quite good for future analysis. The cloud drops are very small compared to radiowavelengths, and the Rayleigh approximation for the cross section of scattering can be used. Equation (5.45) is suitable for this case and gives us: , (11.15) where N is the drop concentration. Equation (11.15) is valid on the assumption that all the drops are similar. In reality, their radius a is the stochastic value, and we need to apply to the main value: , (11.16) where f ( a ) is the distribution function of drop radius. The parameter: (11.17) is referred to as the reflectivity, which we discussed before; 89 hence, . (11.18) The layer thickness is determined by the pulse duration τ p : . (11.19) The relations obtained represent the method of defining the reflectivity of the clouds. IA Jxdx x A= ≅ ∞ ∫ 80 0456 0 2 1 4 3 2 0 λλ ee () () .() σπ ε ε d 046 2 1 2 (,)LNka= − + aafada 66 0 = ∞ ∫ () Θ == ∞ ∫ Na N afada 66 0 () WL PA l L L() () ()= − + 801 2 4 42 2 π λ ε ε e Θ l c = τ p 2 TF1710_book.fm Page 310 Thursday, September 30, 2004 1:43 PM © 2005 by CRC Press Radio Devices for Remote Sensing 311 Different distribution functions are used for calculating the . 39 One of the most universal is the distribution proposed by Deirmendjian: 86 . (11.20) Here, a m is the modal radius (i.e., most probable one). The parameters α and γ depend on the type of hydrometeors. The averaged value is: . (11.21) for various types of hydrometeors. Spectral analysis of the scattered signals allows us to study the internal motion in clouds. Similar formulae can be used for snow and hail clouds. It is sometimes necessary to use more exact formulae for the scattering cross section, especially in the case of hail, whose size can be comparable with the wavelength. The reflectivities of precipitation are connected with its inten- sity J (mm/hour) by the empirical formula: 89 . (11.22) Parameters A = 200 and b = 1.6 are used for rain, and A = 2000 and b = 2.0 are used for snow in moderate latitudes. 89 Weather radar applications are used in mete- orological services and aviation. Recently, they have also found application in space research along with the help of so-called rain radars. 91,92 Radar is sometimes applied to tropospheric turbulence research. The specific cross section of the backward scattering is, in this case (accordingly to Equation (5.173)): . (11.23) One can see that in the case of Kolmogorov–Obukhov turbulence. The calculations show that this cross section is small, and powerful radar is needed for the detection of tropospheric turbulence. 89 Pencil-beam antennae are seldom used for remote sensing of land, but they do find application in airborne navigation systems; however, they have poor space resolution due to difficulties encountered by having a large-size antennae on board. This is the reason why side-looking radar (SLR) and synthetic aperture radar are widely used. We will first discuss SLR. An antenna with a narrow pattern in one a 6 fa a a a a m () exp= () − γ β α γ α γ γ β α γ Γ m = + , β α γ 1 aa 6 6 6 6 = + () () Γ Γ βγ β γ α γ m Θ = AJ b σπ ππ ν ε ν ν d K2 0 4 0 22 12 42 2 2 () ( )= ≅ + − k k qC k σπ λ d 0 ()∝ −13 TF1710_book.fm Page 311 Thursday, September 30, 2004 1:43 PM The data represented in Table 12.2 allow us to determine combinations of parameters © 2005 by CRC Press 312 Radio Propagation and Remote Sensing of the Environment direction can be rather easily mounted on the flying platform — for example, a leaky waveguide antenna mounted along an airplane fuselage. So, the spatial resolution of the radar along the flight direction can be written as: 90 , (11.24) where d is the horizontal antenna size, and R is the slant range to the illuminated element of the land surface. The radar radiates short pulses which permit discrimi- nation of the reflections from the land elements separated by distance: , (11.25) where θ is the angle between vertical and the direction to the illuminated element (see Figure 11.2a). Equation (11.25) determines the spatial resolution across the direction of flight. More exactly, lines of similar ranges are circles and lines of similar angle positions are lines (see Figure 11.2b). Thus, it can be said that Equation (11.24) defines the resolution by azimuth and Equation (11.25) by one-in range. The SLR antenna looks sideways, thus its name. The operation of such radar is based on backward scattering of rough surfaces; for example, SLR does not “see” smooth water surfaces. The backward scattering depends on inclinations of the land The latter one is realized during the platform flight when the range selection gives the scan line, and the frame scan is realized in the flight process. It is similar to formation of a television image. In the first airborne SLR, the radar image was displayed by an electron-ray tube and then photographed on film. The film served as the memory for information storage. Now, such storage is a function of the computer and associated devices. δ λ βX d RR== h δ τ θ Y c = p 2sin TF1710_book.fm Page 312 Thursday, September 30, 2004 1:43 PM or water elements (see Chapter 6) and is the basis of mapping a landscape by SLR. FIGURE 11.2 Side-looking radar operation. Y h z θ β h β h R cτ p /2 cτ p 2 sinθ x Y SW h R β h θ 0 antenna a) b) [...]... characteristics of the antenna become important One of the antenna variants is the so-called bow-tie dipole (Figure 11. 6), and another type of wide-band antenna is the horn We will not go into detail here regarding the types of antennae but refer the reader to special publications These antennae often play the role of a GPR transmitter, and the method of shocking excitation is used for this purpose The short... 2004 1:43 PM 318 Radio Propagation and Remote Sensing of the Environment SAT, and others.146 The range resolution in this technique is determined by the relation: δY ≅ c , 2∆f sin θ (11. 38) where ∆f is the radar signal bandwidth Equation (11. 29) can be used, as before, to define the swath In order to extend the observation band, the technique of switching the antenna beam direction in the vertical plane... is the noise temperature of the system, and ∆F is the pass-band of the lowpass filter The last formula defines the potential fluctuation sensitivity of the radiometer It cannot be realized because of the receiver gain coefficient fluctuations The gain fluctuation influence can be reflected by reducing the last formula to the form: δT = Tn δG 1 + ∆f τ av G 2 , (11. 61) where G is the main value of the. .. Tb (Ω) is the brightness temperature of the observed scene as a function of the solid body, T0 is the averaged temperature of the antenna and the feeder, Tbm is the averaged brightness temperature within the antenna’s major lobe, and Tos is the same temperature averaged over the side lobes (see Figure 11. 8) The radiometer output in the temperature scale is realized by the square-law detector The main... fluctuation in the noise temperature The standard deviation of the noise fluctuation intensity is equal to the mean value of the intensity itself due to the Gaussian character of the thermal fluctuations Hence, the radiometer must have a unit to determine the average of the noise fluctuations in order to decrease their level This unit can be an integrator or low-pass filter Thus, the main elements of MWR are the. .. temperature of the environment Usually, the 0. 5- to 600-GHz band is used for MWR Industrial sources of interference and space noises influence the microwave measurements in the lower part of this region The upper part of the region is exposed to electronic noises, thermal radiation of the atmosphere, and so on At their output, MWRs form voltage Uout, which is linearly related to the power at the MWR antenna... of this radiometer is represented by Figure 11. 10 Several new elements appear in this scheme The first one is the switch for the periodic, sequential connection of the receiver to the antenna and to the reference load transmitting the noise of the calibrated level The switch realizes the modulation of the input signals, and the synchronous detector selects the signal before the detector input The estimation... operated during a SIR-C mission by one of the Shuttle flights The objects of interest are components of the polarization matrix: σ0 = σ0 hh σ0 vh σ0 hv , σ0 vv (11. 51) which describes the intensity of the matched and cross-polarized components of the scattered field More detailed analysis is conducted using the Stokes matrix Thus, we have two matrices; one of them describes the components of the scattered field... from the upper and bottom boundary of the layer, gives information about the layer thickness The thickness l is determined by the formula: l= vτ , 2 (11. 52) where v is the velocity of the pulse propagation in the layer The permittivity dispersion is negligible in these cases, and the pulse velocity coincides with the wave phase velocity: v = c ε′ Sounding of the ground presents many difficulties The. .. must increase the 3-cm antenna to a length of about 3 km! It is not feasible to have such an antenna, but the problem can be overcame with the help of so-called synthetic-aperture radar (SAR) Let us explain the principles of this kind of radar operation Figure 11. 4 shows the geometry of the problem The radar platform (P) moves with velocity v in the x-direction The considered elements of illuminated . REMOTE SENSING OF THE ENVIRONMENT A different type of panoramic radar is commonly used for remote sensing of the environment. These radar systems differ in their principles of operation and in the technique. Press 318 Radio Propagation and Remote Sensing of the Environment SAT, and others. 146 The range resolution in this technique is determined by the relation: , (11. 38) where ∆f is the radar signal bandwidth measurement of σ 0 permits us to estimate the height of the sea waves and then draw conclusions about the velocity of the near-surface winds. Knowledge of the forest area σ 0 at the C-band leads