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Intelligent control, measurement and data processing in microwave scatterometer

B. Q. Zheng,H. Qin, X. Ao
Center for space science and Applied research
Academia Sinica, P.O. Box 8701,Beijing 100080 China


Abstract
This paper will describe a intellectualized airborne microwave scatterometer, Model LS-J, which adopts the computer control technique of the high speed microprocessor TMS 320 C25, to control measurement and data processing feasibly. It makes the scatterometer operation both simple and reliable, and the data are also accurate.

Introduction
Microwave remote sensing is a new subject, it has been more and more widely applied in army, agriculture, mining industry and ocean science. The microwave scatterometer is an active microwave sensor, it is used to measure the scattering conditions of the various terrain, the ocean wind, ice cover, soil humidity and so on. So, it provides important information to estimate the crop yield and to forecast the calamities. It can also be used to set up the scattering coefficient data rate to serve for the image radar.

First of all, we will briefly review the principle of the scatterometer, then discuss the necessity of intelligent control, measurement and data processing of the scatterometer, and lastly, introduce the implement of the intellectualized scatterometer Model LS-J.

Principal of Scatterometer
  1. Transmitter antenna sends a microwave beam to the detected terrain materials.
  2. Receiver antenna receives the backscattering factor of the terrain materials with the information of the detected terrain materials.
  3. This scatter factor is processed by the internal circuit to extract the information signal.
  4. The procedure of extracting a sample signal relating to transmitted wavebeam as the measured reference signal is called the internal calibration.
  5. Calculating the ratio of the information signal energy to the calibration signal energy and taking other system parameters into account to get the scattering coefficient of detected terrain materials.
According to the different working board, scatterometers are divided in two types: static and movable ones. The former is fixed on the static ground, motionless ship or helicopters, and adopts linear FM mode. The measured objects are relatively static. The latter is fixed on the moving automobiles, planes or satellites, adopts continuous-wave Doppler mode, measure the Doppler frequency shift caused by the relative moving objects.

They can be used to measure a wide area continuously. These two kinds of the scatterometers are developed by CAS/CSSAR have continuously and widely been used in practice. Among them Model RS-2 and RS-3 are fixed ground based one’s. The model LS-J is movable airborne one.

Control measurment and data processing of scatterometer
This section includes two aspects of the work: the control of the working state and data processing. The control of the working state is divided into three: the control of the transmitted wavebeam, the control of the measurement signal and the control of internal calibration state transfer.

When an object is irradiated with wavebeam in different incident angle, The scattering coefficient is different. There are two methods to get the scattering information of the different incident angle: one of the them uses fan beam and separate the different incident angle scattering information a part. Another one makes use of the pen shape beam and get the different incident angle scattering information by changing the angle of the antenna. In order to get the accurate different incident angle scattering coefficient, computer must be used to control and discern wavebeam angle and to process the data.

The dynamic range of various terrain scattering coefficient is between –40 db to +20 db. The intensity of reflex signal is also the same range. Therefore, in the data processing, a certain amount of attenuation must be considered according to the difference of signal intensity and the attenuation value is considered in the calculation of the scattering coefficient. It is necessary to use computer to control and record the attenuation value.

Data processing has a large amount of work to do especially for the movable model. It has to complete a complex calculation in a very short time. In the realtime data processing, the high speed microprocessor is needed.

Realization of the intelligence
The model LS-J is a airborne scatterometer, it uses mainly a high speed microprocessor TMS320C25 and is supported by an IBM computer to realize the intelligent control, measurement and data processing. Its operation is introduced as follows:
  1. The control of the Antenna Angle
    LS-J radiates pen shape beam, the incident angle is changed by turning the transmitter antenna. The antenna is driven by a step motor. The processor gives four kinds of signal: phase, direction, switching and step. The angle of the antenna can be controled by counting the pulses and interrogating the step motor. Ome degree accuracy of the antenna locatio is required.

  2. The Acquisition of Flight Parameter
    LS-J is a CW-Doppler system. The scattering information is obtained from the Dopple frequency shift. In order to process the data conveniently, the sampling rate is related to the frequency shift, so that the spectrum are relatively stable through FFT. The flight altitude and velocity must be Known accurately.

    The serial port of the processor is connected to the aircraft INU to get the flight parameter. If the aircraft has no INU, this parameter is provided manually.

  3. The Control Of Data Sampling
    The data sampling is realized by timing interruption service, the sampling interval is decided by the flight velocity and the cycle of timing interrupt.

  4. The Exciting of Internal Calibration Signal
    The processor gives a square wave (internal calibration square wave). Its frequency is related to sampling rate. The square wave produces the internal calibration signal after frequency division. So, the internal calibration signal is related to sampling rate, and its spectrum is stable relatively. This design makes the information frequency, internal calibration frequency and sampling rate interrelated and not confused.

  5. Transition of Internal Calibration and Measurement State
    The control of the internal calibration is carried out via a NAND gate switches on or off the internal calibration square wave. Because the internal calibration is very intensive, it can’t be submerged. So long as the internal calibration signal appears, the internal calibration is state bound to completed, which is internal calibration state, otherwise the NAND switches off, the internal calibration disappear, the measurement state is present.

  6. The control of Signal Attenuation Value
    Control circuit of the attenuator is connected to the processor. When the signal amplitude exceeds the sampled range, a signal of attenuator is connected to the processor to make an appropriate adjustment. However, frequent adjusting is not suitable.

  7. Real time Data Processing
    The sampled data is processed via FFT, filtering, and four operation (logarithm, triangle function, multiplication and division) with other parameters. Then, the scattering coefficient in dB is obtained.

  8. Data Record
    The calculation result is recorded on floppy disk and will be printed out when the aircraft returns to ground.
Conclusion
The intelligent design is the operating center of LS-J. It makes the scatterometer operation both simple and reliable, and the data are also accurate.