Results from Testing a System Calibration Transponder with Two Different SODARs Benjamin Piper1, Sabine von Hünerbein1 1
University of Salford, Newton Building, Salford, M5 4WT, UK (
[email protected])
ABSTRACT SODAR measurements are subject to several uncertainties. One method for exploring uncertainties created within the SODAR system involves the use of a transponder system where the SODARs signal is recorded by a microphone and used to create an echo signal which is played back to the SODAR. This system has been tested with two different SODARs in an outdoor environment with methods employed to reduce the affect of background noise and real atmospheric echoes. Comparisons are made between the two sets of results. This allows exploration of how the different processing features of the SODARs affect the measurements made.
INTRODUCTION A technique for exploring the affect of a SODAR's internal processing on the measurement being made has been created based upon a transponder system consisting of several speakers and microphones and a laptop computer. The transponder acts as a virtual atmosphere by recording the sound emitted by a SODAR and creating a return echo based upon a set of variables. An early version of this transponder system was presented in Piper [1] in which two methods of return echo generation were explored. This work was based upon work carried out in Baxter [2]. The transponder system has increased in complexity to include individual speakers for each SODAR beam and more accurate processing. Some work has been carried out in order to allow for the application of the transponder system in a field situation. Two sets of preliminary tests have been made at using different SODARs at two separate sites. In these measurements the effectiveness of the transponder in a field situation is explored. OVERVIEW OF TRANSPONDER SYSTEM The transponder system consists of three loudspeakers, a microphone and a laptop computer with a multichannel soundcard. It is designed to imitate the atmosphere. Signals from a SODAR are recorded and then processed to create a Doppler shifted return echo which has a frequency content based on a wind profile model. Data recorded by the SODAR can then be compared to this profile and differences explored. Physically it is set-up so that echoes are returned to SODAR from the direction in which they were emitted and that the sonic characteristics of the echoes are as close to real SODAR echoes as possible. To achieve this individual loudspeakers are used
for each of the beams emitted by the SODAR. The speakers used are mid-high frequency tweeters that have a broad directionality as this is the closest approximation to the near plane wave behaviour of atmospheric SODAR echoes that is achievable from a practical distance above the SODAR. Figure 1 shows a diagram of the set physical set-up.
Figure 1. System
Physical Set-up of Transponder
The laptop computer is used to create a return echo based on a set of variables which can be compared to the data recorded by the SODAR. Within this processing the SODAR signal is analyzed to extract frequency and time information and then modulated to give a desired Doppler shift and multiplied by an amplitude envelope that is dependent upon atmospheric absorption and scattering equations. The analysis uses a fast Fourier transform (FFT) to find the frequency with the maximum amplitude in the SODAR signal and a Hilbert transform to estimate the length of the signal. The modulation is performed using a single side band (SSB) modulation method that is detailed in Piper [1]. The amplitude envelope is calculated using Equation 1 where σs is the scattering cross section, c is the speed of sound in air, τ is the pulse duration, α is the absorption of the sound in air and z is the height of the echo source.
cτ e −2αz E =σs 2 z2
(1)
σs is calculated using Equation 2 where λ is the signals wavelength CT is the temperature structure func-
(2)
α is calculated using a series of empirical equations that can be found in Salomons [3]. Once a set of echoes has been created the transponder is put into return mode. Each time the SODAR emits a pulse the transponder is triggered and an echo signal is played back to the SODAR through the speakers. The transponder is manually synced to the beam cycle of the SODAR so that the correct signal is played for each beam. REDUCING THE AFFECT OF WIND AND BACKGROUND NOISE AND REAL ATMOSPHERIC ECHOES Previous experiments using the transponder system with SODARs have all taken place in a semianechoic chamber where no atmospheric reflections occur, there is no wind and the background noise is at a very low level in comparison to a typical SODAR measurement site. In order to carry out transponderSODAR measurements in a field situation these issues need to be considered. Wind at the level of the SODAR presents two problems. Firstly noise is generated by the wind as it passes the edges of the SODAR baffle and the frame on which the transponder is mounted. This adds to the level of the background noise. Secondly wind that is incident upon the transponder microphone can result in high amplitude transient noise occurring which sets off false triggers within the transponder software. Wind noise is avoided by using a two layer wind shield around the microphone, which is similar to those employed by broadcast engineers in on-location television work. This consists of an absorbent inner foam ball, which is attached to the end of the microphone, and a loose fibrous outer material, which disperses gust energy before it reaches the microphone. Background noise limits the height range that can be tested in the same way that it limits the normal operation of a SODAR at a site. Excessive levels mask the return echo preventing the SODAR from making the correct analysis. The amplitude of the echo signal played by the transponder could be increased to overcome this but this can lead to unrealistically high signal to noise ratios in the lower range gates. It is found than some amplitude increase is necessary to deal with the real atmospheric echo. Extra acoustic foam is attached to the frame on which the transponder is mounted to mitigate the increased background noise caused by wind incident on the frame. The real atmospheric echo is a considerable problem in outdoor conditions. If the signal from the transponder is at a comparable level then the two signals compete. Depending on the peak detection method employed by the SODAR, this results in either one of the two echoes being selected as correct or an average of the two. In order to overcome this problem the transponder echo needs to have an amplitude significantly higher than the atmospheric echo. This
RESULTS WITH METEK SODAR AT SUBERB CARRINGTON A set of measurements was carried out with the transponder at the SUBERB test facility in Carrington using a METEK PCS-2000/24 SODAR. The testing carried out was using wind profiles that had a constant speed at all heights. Results were recorded at a number of horizontal input velocities between -30ms-1 and -1 30ms . The results recorded by the SODAR were then analysed and correct for known differences between the SODAR and the transponder such as temperature and frequency changes. Figure 2 shows the mean differences for the different speeds (a) and heights (b) tested. a
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σ s = 6e − 4 λ−1 / 3
can be achieved by reducing the volume of the atmospheric echo or increasing the amplitude of the transponder echo or both. It could also be possible to trigger the transponder electronically but this may not be possible with some SODARs and therefore the use of a microphone is preferred for consistency. A layer of acoustic foam is fitted above the transponder to reduce the amplitude of the signal emitted by the SODAR and all reflections returning to the SODAR. The transponder output is set 6 dB higher than the realistic amplitude found using the envelope equations. This level of increase is chosen as higher levels were found to distort the signal. The combination results in the transponder echo having an amplitude several times larger than the atmospheric echo.
Input Speed (ms )
tion, typically in the order of 10e-4, and T is the lapse rate corrected temperature at the echo source.
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Figure 2. Mean differences between Transponder input and METEK SODAR data for a) each input velocity and b) each height The results show a large asymmetry when analysing transponder input speed against SODAR output. This is due to differences in the way Doppler shift is calculated. The transponder uses a commonly found approximation shown in Equation 3 whilst the METEK SODAR uses a method found in Ostashev [4] which is shown in Equation 4.
V =
∆ωc 2ω1
(3)
V =
∆ωc (ω 2 + ω1 )
(4)
Figure 3 shows the mean differences after corrections have been applied.
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Figure 3. Mean differences between Transponder input and METEK SODAR data for a) each input velocity and b) each height with corrections applied for difference in Doppler shift calculation.. The mean differences in terms of input velocity are less than 0.1 ms-1 for all but the highest positive ve-1 locities. There is a peak for 2ms that is likely to have been caused by the transponder going out of synchronisation with the beam cycle of the SODAR for part of the measurement. Examining the mean differences in terms of height shows that the performance is consistent up to 215m. Above this point the signal to noise ratio becomes too low for a reliable measurement and this is shown by the larger deviations that occur. These are promising results. It should be noted that during these measurements it was found that the input amplifiers in the SODAR were not functioning correctly and therefore the amplitude of the signal from the transponder was unrealistically high in order to generate realistic signal to noise ratios. The resultant data is only responding to the input from the transponder and problems with the real atmospheric echo are minimized. This is a convenient result but can only give some information about the SODAR as the SODAR was not functioning correctly. Tests with the SODAR functioning correctly are needed to make further conclusions. RESULTS WITH ASC SODAR AT WINDTEST GREVENBROICH Measurements were carried out using the transponder with an ASC 4000 SODAR at the Windtest turbine testing field at Grevenbroich. At the time of the testing the SODAR was known to be fully functional. These measurements are the first in which a SODAR other than the METEK PCS-2000/24 has been used to test the transponder. The motivation behind this set of measurements was to explore how adaptable the transponder system is and whether reasonable conclusions could be made with little knowledge of the processing of the SODAR in question. The measurements were made in sub-zero temperatures that reduced the performance of the components from which the transponder is built and therefore only a limited height range was effectively tested. Two sets of data were recorded but the first set was rejected as the influence of a strong fixed echo was adding significant bias to all of the SODARs measurements. The
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SODAR was repositioned before the second set of data was recorded and the influence of the fixed echo was shown to be significantly reduced. Figure 3 shows the second set of results recorded with the ASC4000 after corrections have been made for shifts in temperature as this affects the speed of sound and therefore the amount of Doppler shift.
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Figure 4. Mean differences between Transponder input and ASC4000 SODAR data for each a) input velocity for heights of 100-140m and each b) height. The differences have a dependency on the transponder input velocity. The trend shows that the SODAR data is consistently below the input velocity. This is suggestive of influence from the real atmospheric echo as well as a fixed echo. Only data from 100m to 140m is used as it is clear data above and below this is incorrect due to performance issues with the transponder. Below 100m the influence of the atmospheric echo is too strong. Above 140m the signal to noise ratio is too low due to the reduced performance of the speakers and sound card in the cold conditions.
DISCUSSION Two sets of measurements have been made using the transponder system with two different SODARS. In both cases significant problems occur. The first set shows artificially good rejection of background noise and the real atmospheric echo whilst the second case shows high influence from these external influences. This makes it difficult to compare the two. It is possible to conclude that the two SODARs use different equations to calculate Doppler shift. Both these equations are estimations and further work is required to create a unified approach to the complex problem of Doppler shift in a moving and inhomogeneous atmosphere. The flaws in both sets of measurements highlight the difficulty in using the transponder in an outside environment. The measurements made with the ASC4000 highlight to possible use of the transponder to identify the magnitude of fixed echo influence in a SODAR measurement although this is so far unquantified.
SYSTEM IMPROVEMENTS AND FURTHER WORK Currently the problems of wind noise and background noise have been effectively dealt with through the use of acoustic shielding. If this was not the case then the SODAR would not have been able to record any data that correlated with the transponder signal such as those presented here. The problems which need to be overcome in order for the transponder to be useful in field conditions are how to remove influence from the atmospheric echo and fixed echoes. This should be dealt with through the use more complete acoustic barriers mounted above and around the transponder. Further work is needed in interpreting the results of measurements made using the transponder. Currently only knowledge of the Doppler shift equation used and some indication of the presence of fixed echo are gained from these experiments. Fixed echoes should be eliminated to effectively carry out the type of measurements for which the transponder was initially designed. However their presence in the results presented here suggest that the transponder could be adapted to explore the performance of a SODAR when a fixed echo is artificially generated in a known and repeatable manner. Future versions of the transponder system for in field use need to be more robust and weather proof than the current version as the cold conditions found in Grevenbroich significantly reduced the measurement height range. CONCLUSIONS A transponder system that has been designed for use in a semi-anechoic chamber has been adapted to allow for measurements to be made in a field environment. The adaptation is focussed on reducing the influence of wind noise, background noise and atmospheric echoes. Two sets of measurements are presented that show that it is possible to make these measurements in a field environment but with significant difficulties. Further work is required to successfully adapt the transponder system for use in a field environment. REFERENCES [1] Piper B., S. von Hünerbein, 2008: Development of a Transponder Based Technique for the Acoustic Calibration of SODARs, IOP Conference Series: Earth and Environmental Science, 1 012044 doi: 10.1088/1755-1315/1/1/012044 [2] Baxter R., 1994: Development of a Universal Acoustic Pulse Transponding System for Performance Auditing SODARs. Presented at the 7th International Symposium on Acoustic Remote Sensing and Associated Techniques of the Atmosphere and Oceans, Boulder, Colorado, October. [3] Salomons E. M., 2001: Computational Atmospheric Acoustics. Kluwer Academic Publishers, Dordrecht, pp 109-110 [4] Ostashev V. E., 1997: Acoustics in moving homogeneous media. E & FN Spon, London, pp 161-162
ACKNOWLEDGMENTS Kenneth Underwood Corporation
Atmospheric
Systems
Monika Krämer, Qi Wang – Windtest Grevenbroich Günter Warmbier – GWU Group
