Remote sensing for resources development and environmental management: Remote sensing for resources development and environmental management

Damen, M. C. J.

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Multivolume work

Persistent identifier:: 856342815

Title:: Remote sensing for resources development and environmental management

Sub title:: proceedings of the 7th international Symposium, Enschede, 25 - 29 August 1986

Year of publication:: 1986

Place of publication:: Rotterdam; Boston

Publisher of the original:: A. A. Balkema

Identifier (digital):: 856342815

Language:: English

Additional Notes:: Volume 1-3 erschienen von 1986-1988

Editor:: Damen, M. C. J.

Document type:: Multivolume work

Volume

Persistent identifier:: 856343064

Title:: Remote sensing for resources development and environmental management

Sub title:: proceedings of the 7th international Symposium, Enschede, 25 - 29 August 1986

Scope:: XV, 547 Seiten

Year of publication:: 1986

Place of publication:: Rotterdam; Boston

Publisher of the original:: A. A. Balkema

Identifier (digital):: 856343064

Illustration:: Illustrationen, Diagramme

Signature of the source:: ZS 312(26,7,1)

Language:: English

Usage licence:: Attribution 4.0 International (CC BY 4.0)

Editor:: Damen, M. C. J.

Publisher of the digital copy:: Technische Informationsbibliothek Hannover

Place of publication of the digital copy:: Hannover

Year of publication of the original:: 2016

Document type:: Volume

Collection:: Earth sciences

Chapter

Title:: 2 Microwave data. Chairman: N. Lannelongue, Liaison: L. Krul

Document type:: Multivolume work

Structure type:: Chapter

Chapter

Title:: Relating L-band scatterometer data with soil moisture content and roughness. P. J. F. Swart

Document type:: Multivolume work

Structure type:: Chapter

184 (4) OQ For “O the I □ UJ ID Z < CL 1 From eq. (2) and (3) the resolution cell follows as: Aa = Ax.Ay = cxB a h/sin20 Which obviously has a minimum at 0 = 45 degre example at a flight altitude of 300 m as used SIR-B experiment we find that for DUTSCAT: T = 100 ns 0a =0.22 rad ^^min = ^980 m^ In the search for relations between surface proper ties and radar backscatter characteristics it must be noted that the irregular nature of surfaces in general causes the scattered electromagnetic field components to be random functions. This means that the scattered power as measured by the radar is different for each resolution cell as it depends on phase relations with in the resolution cell itself. Therefore the estimated average of this scattered power, or the radar cross section per unit area is the most commonly used para meter in radar studies. After an incoherent averaging known as "speckle reduction" the standard deviation of the average received power P is: SCAN 665 0 = 63° a = p//n (5) Figure 2. Averaged received power for scan 665 with an incidence angle of 63 degrees. 3 DATA PROCESSING From the NLR we receive preprocessed data of both the scatterometer and the airplane's inertial naviagion system. These two types of data are time tagged so that they can be combined and in the mean time correc ted e.g. for the spatial spreading loss and the an tenna weighing. This so called radiometric correction is expressed by the radar equation: with N the number of independent measurements caused e.g. by the movement of the airplane. For N we find the next expression (Ulaby e.a. 1982): ,3 4 P (4tt) R r (Ay.G./g 2 ) 1 (8) N = Ax-(L a /2) (6) With L a the antennalength which is approximately: L a A/6 a (7) with A the free space wavelength which is equal to 0.25 m for the frequency of 1.2 GHz used by DUTSCAT. From (6) and (7) we see that an independent measure ment is done after each movement of half the antenna length. The received power appears to be exponentially distributed and it can be calculated that for N=200 the averaged power is known with an uncertainty of 1 decibel within 90% of the time (Smit 1978). The along- track distance required in our case to obtain this accuracy is 120 m or 1.3 along-track resolution cells at an incidence angle of 45 degrees (h=300m). 2.2 DUTSCAT data The DUTSCAT scatterometer has a rather high internal pulse repetition frequency of 78.125 kHz so that every 12.8 ys a pulse is transmitted. The received signal is digitized with a sampling rate of 20 MHz which means that the sample time is equal to 50 ns. Then a coherent averaging is applied to the signals of sub sequent pulses to improve the signal to noise ratio. After this the previously described speckle reduction is performed by averaging incoherently. Additional averaging is done to reduce the data rate of the digital output. Finally we have a pulse repetition frequency of 4.77 Hz, so that effectively the across- track direction is scanned once every 210 ms. An example of an arbitrary resulting scan is given in fig. 2. From the velocity of the airplane, that carries the scatterometer, which is known to be about 50 m/s it directly follows that this particular scan (665) is taken at about 7 km after the start of the flight track. The information that at the time of this scan the incidence angle was 63 degrees is ex tracted from flight parameter data that simultaneously has been recorded on tape along with the radar data. The nominal incidence angle can be selected by the operator inside the airplane and varied between 10 and 80 degrees. During the SIR-B experiment the flight track was flown 8 times for one series of measurements at intervals of 10 degrees. This was done on two days, and in total three times so 24 recordings were made. P : received power (averaged) P : transmitted power G ^ : maximum antenna gain Jg : Jg 2 (x,y)dx or R.Jg 2 (a,£)da g : antenna gain function a,£ : azimuth & elevation angle (antenna coordi nates) Not explicitely written out in this formula is the fact that the resulting 0 O or the radar cross section per unit area depends on the sample number to which it belongs. Variations of the transmitted power P-t can be monitored with the aid of the system's inter nal calibration. The antennagain G in the direction of maximum radiation has been measured. For the DUTSCAT antenna (a 0.9 m parabolic dish) a value was found of 17 dB. As another part of the external cali bration the antenna gain function has been measured over 60 degrees. According to eq. (8) this function has to be squared and integrated with respect to the azimuth angle (along-track or x-direction). The re sult (see fig. 3) is a function of the elevation angle in antenna coordinates. For the example given in fig. 2 the calculated radar cross section 0 Q is presented in fig. 4 where of course the most accurate values are expected to be around the main direction of the antenna. The vertical line in the last figure corresponds to an elevation angle in antenna coordi nates of -30 degrees below which the gain function has not been measured. F* 1.2 GHz Figure 3. Squared antenna gain function integrated with respect to the azimuth angle (-30..+ 30 deg.). 1«. ». -n. -2«. -3«. 5«. Figure 4. i 4 SIR-B ex: As already SIR-B expe: inally, thi 5 days wit! in early Se delayed tw: and 12 Octi ground dati field cond: the preced: failed to ] terometer ( ameters anc facilitate The test agricultur; Figure 5. 1 The homoger soiltype ar impression literature The fligl area and he measurement fields were resulted ir ness and ve The radai soils deper surface pai relative in radar paran and polari2 which dat soil moistr tering mode incidence a An exampl by the cosi tering coef

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