REMTECH SODAR TECHNICAL DESCRIPTION

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SODAR technical description.pdf

         1. - SYSTEM OVERVIEW

Our phased array Doppler Sodars continuously and reliably measure wind speed and direction, vertical motions, turbulence, thermal structure, and mixing depth at heights ranging from 15 m up to 1,000 to 5,000 meters depending on the type of antenna. Briefly, this measurement is accomplished by emitting a strong acoustic pulse in the audio band and detecting the Doppler frequency shift of the received backscattered echo. This backscattered echo signal is due to thermal turbulence in the atmosphere. The signal frequency shift (Doppler shift) and its relative strength are processed in various ways to produce far more information than previously available through conventional methods such as instrumented towers, tethersondes, etc.

All three dimensional REMTECH monostatic Doppler Sodar systems basically consist of one sole antenna (phased array type) and an electronic case. In the electronic case are the computer, transceiver, and power amplifier. Also included are interconnecting cables and a small mount for the antenna.

The system allows for full control of the antenna beams: four of the electronically steered beams are tilted (30° or 15°) or from vertical and turned 90° from each other to provide the horizontal component of wind velocity. The last beam is pointed vertically and provides that component of the wind. The system software controls the sequence and rate of operation for each beam. These are non system parameters which can be changed through keyboard input.

Different antennas are available for different ranges. The table shown below provides the specifications of the different systems.

A major advantage of the REMTECH Doppler Sodar Systems is their tremendous user flexibility. Virtually all important parameters, such as averaging and sampling times, transmitting power, antenna orientation, range gates, and maximum range, are operator selectable by simple key-in options to the minicomputer.

The exclusive signal to noise detection and comparison techniques developed by REMTECH through several years of research, permit the system to produce reliable data in a noisy environment with comparatively short data averaging times because of the discrete rejection of all spurious signals. Data which are, in fact, considered suspect, due to adverse ambient-noise, are deleted from the system. This prevents users from being misled by erroneous data. No additional data processing is required in order to achieve the system's full accuracy. Data archived can also be directly processed to provide complete statistical data summaries and climatological analyses.

System maintenance is greatly facilitated by special software diagnostic routines. If ever required, repair is accomplished by swapping the complete electronic case, vastly reducing down time.

 

 

A.   Phased Array Antenna Sodars Unique Advantages

The advantages of steered beam phased array antenna are numerous:

-   only one antenna is needed, the steering of the beam is performed by phase control among the antenna elements.

-   up to nine beams can be achieved allowing cross checking among wind components.

-   no large antenna enclosure is needed due to tapering of the amplitude resulting in a simple flat plate antenna as small as possible.

Moreover, phased array antennas offer tremendous development capabilities.

B.   Frequency Coded Transmission

Through software, REMTECH is able to provide a signature to the transmitted pulse. The basic coding consists of transmitting 9 (up to 15 optional) frequencies in the pulse. Upon reception, this coded pulse is easily detected from noise and fixed echoes within the backscattered signal. This is particularly useful for turbulence studies since it allows quicker detection for full analysis on the noise spectrum. The technique has been used for years in radar, REMTECH is still the only company to apply this to SODAR (US Patent No 5,521,883).

C.   Automatic Fixed Echo Detection

Though the antenna pattern is optimized relative to fixed echoes, some of these can still be strong enough to spoil the returned signals. In this case the software automatically detects the fixed echoes and, according to the ratio between fixed echo intensity and useful signal, either processes the data with adequate corrections or invalidates them.

D.   Unique Noise Subtraction Technique

The frequency transfer function (in phase and amplitude) between the "active antenna" and the "reference antenna" (made of 4 transducers at the 4 antenna corners) allows a very efficient noise subtraction (especially for a fixed noise source such as an air conditioner, an aspirated shield on a meteorological tower close to the Sodar). The final acoustic frequency power spectrum can be cleaned by more than 15dB’s decrease of the jamming source in the considered frequency zone.

This noise subtraction technique is also very efficient to removing spurious frequency peaks which can be generated by electrical and/or magnetic influence.