Evaluation of AQ500 sodar performance, Sweden

Evaluation of AQ500 sodar 

performance, Sweden 

Report number: KVT/YY/2011/043 – Rev. 1

KVT/YY/2011/R043 

Content 

1 INTRODUCTION .................................................................................................. 3 

2 SITE INFORMATION ............................................................................................. 4 

3 SODAR DATA ..................................................................................................... 6 

3.1 AVAILABILITY OF STANDARD DEVIATION MEASUREMENTS 6 

3.2 AVAILABILITY OF WIND SPEED MEASUREMENTS WITH VARYING WIND SPEED 7 

3.3 OPERATION OF THE SODAR 8 

4 COMPARISON BETWEEN SODAR AND MAST ................................................................ 9 

4.1 FILTERING OF MEASUREMENT DATA 9 

4.2 COMPARISON OF MEASURED WIND SPEED AND DIRECTION 10 

4.3 COMPARISON OF MEASURED TURBULENCE INTENSITY (TI) 12 

4.4 COMPARISON OF MEASURED WIND SHEAR 13 

5 SUMMARY AND CONCLUSIONS ............................................................................... 15 

6 REFERENCES .................................................................................................... 16 

APPENDIX A ................................................................................................................. 17 

APPENDIX B ................................................................................................................. 19 

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1 Introduction 

In the recent years the use of remote sensing equipment for assessing the wind resources when 

developing wind energy projects has increased significantly. For Swedish wind farm projects, in 

particular, the deployment of the sodar AQ500 manufactured by AQ System AS has become 

widespread. In this report Kjeller Vindteknikk (KVT) evaluates the performance of one AQ500 

sodar deployed for 6 months (mid-October to mid-April) at a forested site in Sweden. The 

measurements from the sodar are compared to those of a nearby 100m meteorological mast 

equipped with cup- and ultrasonic anemometers. The work presented in this report is ordered 

by Statkraft Development AS. 

This analysis offers a comparison of the basic wind energy climate parameters measured with 

sodar and anemometry. The scope of this work is to assess the accuracy of the data output from 

the AQ500 sodar presented in the result files. We have, therefore, not applied any additional 

filtering routines to the sodar data nor investigated the underlying raw data (that is usually not 

accessible for the average sodar operator). There are a wide range of dependencies and 

questions to be answered on the subject of sodar measurements. With limited time available, 

we have prioritised to investigate the relations and parameters we believe to be of most 

importance for wind resource assessments carried out for similar sites today. 

The sodar site, henceforward simply called Sodar, is situated in a clear-fell area. The site can 

be defined as complex with regard to the forest. However the area where the mast and sodar 

are located is relatively flat. The elevation of the area in the vicinity of the site is between 490 

and 500 m.a.s.l. The height variations in the area around the sodar are displayed in Figure 1-1. 

Figure 1-1: Map with height variations at the Sodar site and the Mast site. 

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2 Site information 

The sodar measurement campaign at the Sodar site extends from late October 2010 to mid- 

April 2011. Statkraft has carried out measurements in a 100 m lattice mast in the vicinity of the 

Sodar. There is mature forest of about 10-15 m height a couple of hundred meters away from 

the sodar and the mast in several directions. The sodar site is a typical Swedish forested area 

with a mix of clear-fell areas, young forest, and mature forest. With respect to the surface 

roughness (forest) the site can be characterized as somewhat complex, however the area is 

relatively flat with no hills. Aerial images with an overview of the sodar site and the exact 

position of the Sodar and the Mast are shown in Figure 2-1 and Figure 2-2 respectively. 

Figure 2-1: Aerial photo of the area surrounding the Sodar site. The black frame indicates the borders of the cut 

out shown in Figure 2-2. The width of the black square is approximately 650 m. 

Figure 2-2: Aerial photo with the position of the Sodar site indicated with a blue dot and the position of the 

Mast site with a red dot. The distance between the sodar and mast is 200 m. 

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The Mast is located 200 meters from the Sodar in the direction 154° (from sodar to mast where 

north is 0°). The 100 m lattice mast is equipped with cup-anemometers in several heights and a 

heated ice-free ultrasonic sensor close to the top mounted cup-anemometers. This makes the 

measurements from the mast well suited for comparing the wind speeds measured in 100 m 

height, and the wind shear measured by the sodar and mast. 

The details regarding the measurement period, heights, and placing of the sodar and mast are 

outlined in Table 2-1. 

Table 2-1: Details regarding the sodar and mast measurements. 

Campaign name Meas. heights [m] Campaign length 

Site elevation 

[m.a.s.l] 

Sodar [50,55,...,200] 19.10.2010-18.04.2011 ~495 

Mast 100,99, 96, 72, 47 9.4.2010 - present ~496 

The technical details regarding the type and settings of the sodar can be found in Appendix A 

together with an instrumentation overview for the Mast. 

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3 Sodar data 

The average wind speed measured by the sodar at different heights throughout the 

measurement campaign is shown in Table 3-1. The underlying dataset is only filtered for 

erroneous (9999) values in the wind speed. Other than that the data is used as output from the 

sodar. The availability of the wind speed measurements between 50 and 140 meters is very 

satisfying. One should also observe that the sodar was deployed during the winter season when 

we generally expect the availability of measurements campaigns to be lowest. 

Table 3-1: Measured wind speed and data availability for various heights at the Sodar site. All valid 10-min wind 

speed measurements from the entire sodar campaign are included, i.e. only wind speed records with 9999 are 

removed. 

Meas. Height [m] 50 60 70 80 90 100 110 120 130 140 150 160 180 200 

Wind speed [m/s] 6.8 7.4 7.9 8.3 8.8 9.1 9.4 9.7 10.1 10.3 10.5 10.9 11.4 11.9 

Availability [%] 98.6 98.6 98.5 98.0 97.6 97.0 96.2 94.9 92.9 90.1 86.7 82.3 73.1 63.2 

3.1 Availability of standard deviation measurements 

The AQ500 sodar can output valid wind speed measurements while the corresponding standard 

deviation (STD) measurement is rendered erroneous (9999). The latter is possible because there 

is a stricter signal requirement (sound-to-noise ratio) in the sodar software for standard 

deviation values compared to the requirement for a wind speed measurement. The reason for 

this is that the sodar uses vector average measurements from the entire 10-min period to 

calculate the average wind speed, while the standard deviation is calculated every 

measurement cycle. 

For the average wind speed values presented in Table 3-2 we have removed all wind speed 

measurements where the wind speed and/or the corresponding STD measurement are 

erroneous. This obviously leads to a lower availability, but it also increases the wind speed with 

3.2 % in average over all heights. The increase in average wind speed ranges from 1.7 % in 50 m 

height to 4.1 % in 200 m height. In 100 m height the increase is 3.2 %. This result suggests that 

the probability of the sodar to acquire STD measurements is not independent of wind speed, 

but that there is an increasing probability for loosing STD data with decreasing wind speed. One 

should be careful not to exclude wind speed measurements based on the availability of the 

corresponding STD measurement. The latter makes it, for instance, difficult to calculate the 

average wind speed distribution and turbulence intensity from the same dataset without 

introducing a bias in one of the measures. 

Table 3-2: Measured wind speed and data availability for various heights at the Sodar site. Only 10-min wind 

records that have both valid wind speed and standard deviation measurements are included, i.e. 10-min records 

with either 9999 in the wind speed and/or the standard deviation are excluded. 

Meas. height 50 60 70 80 90 100 110 120 130 140 150 160 180 200 

Wind speed 6.9 7.5 8.1 8.6 9.0 9.4 9.7 10.1 10.4 10.7 11.0 11.3 11.8 12.5 

Availability 85.9 95.5 83.2 80.1 77.3 74.2 71.0 68.0 64.3 60.7 56.8 52.4 43.7 35.3 

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3.2 Availability of wind speed measurements with varying wind speed 

We have in Figure 3-1 plotted the availability in 200 m as a function of the wind speed 

measured in 50 m height. The figure indicates how the availability of the wind speed 

measurement varies with wind speed. The figure shows that failure of retrieving valid wind 

speed measurements (for this example in 200 m) is more likely to occur at low and high wind 

speeds compared to medium wind speeds (6-11 m/s) . The trend is also the same for lower 

heights. For lower heights, however, the availability is significantly better than in 200 m and 

the problem is therefore of less importance. The result shows that there probably is introduced 

a bias in the wind speed measurements because the availability depends on wind speed. Note 

that we here only consider errors in the mean wind speed measurements, and not the influence 

of the more frequent errors in the STD measurements. 

Figure 3-1: The availability in 200 meters as a function of the wind speed measured in 50 m height. 

The average wind speeds calculated in 50 m height when concurrent measurements in different 

heights are erroneous is shown in Table 3-3. The average wind speed in 50 is lower when there 

are errors at higher heights. And correspondingly we see that the average wind speed in 50 m is 

higher when only measurements that have valid concurrent measurements at higher heights are 

included. The effect of this error diminishes with decreasing height since the availability then 

improves. 

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Table 3-3: Average wind speed measured in 50 m height for different subsets of measurements. 

Wind speed in 50 m height based on: Wind speed [m/s] # measurements 

all measurements in 50 m : 6.8 25783 

measurements when erroneous measurement in 200 m: 6.5 9290 

measurements when valid measurement in 200 m: 7.0 16493 





The crude analysis shown here suggests that the wind speed can be overestimated because the 

failure to acquire measurements occurs for climate conditions with low wind speeds. When 

computing the wind shear, the problem is to some degree evaded since only concurrent wind 

speed measurements from heights the wind shear is computed between are used. If the wind 

shear at the site is dependent of the wind speed, which is often the case, on must be careful 

not to use data for heights with reduced availability. 

3.3 Operation of the Sodar 

The Sodar was deployed 19.10.2010. About a month later, 16.11.2010, AQ System installed 

their cold climate option on the sodar. This mainly consists of a heating fan for the diesel 

generator that is also used for directing hot air on the parabolic dishes in the sodar antenna 

unit. There have been episodes when the data availability has been low/zero. This is probably 

caused by snow on the reflective dishes. The system has, however, autonomously removed the 

snow by melting, and returned to normal operation again. The detailed logbook for the Sodar 

with an overview of the availability each day and all site visits is supplied in Appendix B. All 

personnel involved in the maintenance and operation of the sodar has been instructed to report 

their site visits to Kjeller Vindteknikk. We believe that there also has been removed snow from 

the unit during the site visits. Unfortunately this preventive action is not specified in the sodar 

logbook, nor has it been reported to us. During the 6 months the campaign lasted the system 

has been refilled with diesel 3 times. 

The internal clock in the AQ500 sodar has been drifting during the measurement campaign. This 

is a known problem with the AQ500 sodar system that we have reported to the manufacturer 

several times in the past. Typically the clock on an AQ500 sodar drifts about 5 minutes per 

month. We have mended the issue by adjusting the sodar time via the manual remote 

connection software SODWIN.NET whenever the discrepancy between the sodar and our server 

time is 5 minutes or larger. A more appropriate solution to the problem would have been to 

automatically synchronize the sodar clock through the automatic software SODWIN-MCOM that 

normally communicates and downloads data from the sodar once every 24 hours. Currently the 

latter program does not have this option. 

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4 Comparison between sodar and mast 

Measurement data from the Mast has been used for comparison with measurements from the 

Sodar. The wind speed measurements in 100 m from the sodar have been compared with the 

top mounted Thies First Class Advanced cup-anemometer at 100.0 m in the mast. The three 

side mounted Thies First Class cup-anemometers at heights 96.2 m, 71.6 m, and 47.1 m have 

been utilized for comparing the wind shear measured in the mast and by the sodar. 

4.1 Filtering of measurement data 

The internal filtering routines developed by AQ system aims at only releasing quality controlled 

data in the sodar output files and their customers are encouraged to use the data as it appears 

and only screen the dataset for obvious errors. The sodar data is, therefore, screened and used 

as it is reported in the raw files from the sodar without further data conditioning. We have only 

removed invalid measurements recorded as 9999. We have attempted to use the data both with 

and without removing wind speed measurements with corresponding errors in the STD 

measurements. Our manual screening of the sodar data, including the comparison with the 

mast data, reveals no obvious errors in the dataset recorded by the sodar. 

All the cup-anemometer data used in this analysis has been filtered for ice influenced periods 

by comparing the measurements to those of the Gill heated ultrasonic anemometer mounted in 

the mast and the sodar data. For the particular campaign at the Mast site the heated ultrasonic 

sensor was used for identifying the ice periods through an automatic routine. We emphasise, 

however, that ice filtering the cup-anemometers based on deviation from the sodar 

measurements would be possible as the sodar and ultrasonic anemometer agrees very well. It is 

integral to identify and remove ice influenced measurements from the dataset used for 

calculating the long term wind speed. Figure 4-1 shows a typical example from the 

measurement time series where there is ice on the cup-anemometer while the agreement 

between the ice and snow free sodar and ultrasonic anemometer measurements is very good. 

The sodar and ultrasonic anemometer measurements are both valuable as they verify that ice 

and snow do not influence either of the instruments. 

Figure 4-1: Example of an icing episode on the unheated Thies cup anemometer (red). The sodar (green) and 

the heated sonic anemometer in the mast (blue) continue to measure. 

We have filtered out all anemometer measurements when the wind comes from sector 1 – 6 

(346°-165°). The latter filtration is carried out to ensure minimal mast influence on the top 

mounted cup-anemometer used in the comparison with the sodar. The mast has two cup- 

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anemometers mounted in 100 m height, denoted Ano_Thies_A and Ano_Thies_B in the 

instrumentation overview supplied in Appendix A. The two anemometers show excellent 

correlation (10-min) and have a discrepancy of less than 0.1 % for the data used in the 

comparison between the mast and sodar. 

Only concurrent data from the Sodar and the Mast have been used in the comparison presented 

in the report. The comparison is carried out based on 10-min averaged wind speed and STD 

measurements in 100 m. The direction measurements used in the analysis are recorded in 95 m 

and 97.4 m height for the sodar and mast respectively. 

4.2 Comparison of measured wind speed and direction 

The measured wind speed and correlation between the anemometers and the sodar is shown in 

Table 4-1. The availability of concurrent measurements after ice and sector filtering is about 

50% (or 12969 10-min records) for the 6 months long sodar campaign. The table shows that the 

average wind speed measured by the sodar and in the mast deviate with 0.3 m/s or 3.5 %, 

where the anemometers in the Mast display the lower wind speed. 

Table 4-1: Mean wind speed measured by the Sodar and in the Mast. All measurements are concurrent, 

amounting to 12969 measurements (total availability of about 50 %). 

Measurement 

Meas. 

height [m] 

Mean wind 

speed [m/s] Correlation: Sodar Ano A Ano B 

Sodar 100 8.89 

Mast Ano_Thies_A 100 8.59 0.98 

Mast Ano_thies_B 100 8.60 0.98 1.00 

The monthly average wind speeds measured in 100 m height by the Sodar and in the Mast are 

shown in Table 4-2. As was the case for the mean of all measurements, the discrepancy 

between the monthly average wind speeds are also about 0.3 m/s. 

Table 4-2: Monthly mean wind speed measured in 100 m height at the Sodar site and the two top mounted 

anemometers at the Mast site. 

Month Availability (%) Sodar [m/s] Mast Ano A [m/s] Mast Ano B [m/s] 

2010 October 33 8.84 8.56 8.56 

2010 November 27 8.67 8.36 8.39 

2010 December 22 8.26 7.66 7.67 

2011 January 38 9.88 9.56 9.61 

2011 February 38 8.51 8.19 8.19 

2011 March 86 9.24 9.01 9.02 

2011 April 52 8.26 7.96 7.94 

Mean of months: 54 8.81 8.47 8.48 

We have also carried out the above comparison for all wind speeds above 2.0 m/s (filtering out 

all measurements when either the anemometers or the sodar recorded wind speeds below 2.0 

m/s). This did not change the outcome, the discrepancy of 0.3 m/s persisted. 

We do not expect a difference in the wind field of this magnitude over the 200 m distance 

between the sodar and mast position, but some variation can be expected between the sites. 

We have preformed a cross-prediction check with the micro scale model WAsP 10 between the 

sodar and mast site. The model predicts that the average wind speeds at the two locations are 

identical. Our qualitative assessment of the sites and the WAsP model both indicate that the 

difference in measured wind speed cannot be ascribed to the terrain variation alone. We 

cannot, however, expect a perfect agreement in the absolute wind speed measured with the 

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the Sodar and the Mast measurements. There are uncertainties in both the calibration of the 

anemometers (estimated to about 1%) and in the calibration of the sodar. Further the sodar 

measures in a large volume over the sodar site while the anemometer measures in a single 

point. We have taken great care to avoid influence by configuration and ice on the 

anemometers, but we cannot rule out that a slight bias is still present in the dataset from the 

Mast. We have here compared state of the art measurements carried out with two different 

techniques, and the result shows that one must accept and expect a discrepancy in the 

absolute wind speed. 

Difference between vector (SODARS) and scalar (LIDARS and anemometers) averaging 

It is often expected that sodar data recorded in modest to very complex terrain will yield a 

lower wind speed than measurements made by an anemometer in the same wind field. One 

reason for this is the difference in the averaging technique between the two instruments 

discussed in the following. Note that correcting for the different averaging techniques in this 

case will make the discrepancy larger. 

The sodar computes the 10-minute wind speed based on the 10-minute average radial wind 

speed sampled in three directions separated by an azimuth angle of 120°, i.e. averaging 

vectors. Common practice with other wind speed measuring techniques, such as LIDAR and 

anemometer, is to calculate the wind speed (scalar) every time a sample is recorded and then 

average this result to form the 10-minute average wind speed. The latter calculation routine is 

referred to as scalar average and the former one (which the AQ500 sodar utilize) is called 

vector averaging. When the same homogenous wind field is measured, it is theoretical 

impossible for the vector average to be larger than the scalar average. They can be equal if the 

wind direction in the averaged interval is constant. If the direction however varies, the scalar 

mean will be larger than the vector mean. This is why it is commonly expected to measure a 

lower wind speed with the sodar compared to other measuring techniques. The discrepancy 

arising from the different averaging is often referred to as DP-error (“data processing-error”) in 

the literature. The DP-error is dependent on the fluctuations in the wind direction, which in 

turn is dependent on the turbulence in the atmosphere. A typical value of 2.5% for the DP-error 

is reported in (Ioannis Antoniou (ed.) 2003). 

In an attempt to reproduce results that are in better agreement with anemometer 

measurements one can employ a “vector-to-scalar” transformation on the SODAR data. The 

vector-to-scalar transformation of the volume measured 10-minute average wind speed, U vector , 

is carried out with the equation, 

, where 

U scalar is the calculated point wind speed corresponding more closely to what is measured by an 

anemometer. The height is denoted by z and the turbulence intensity by TI (10-min standard 

deviation of wind speed/10-min mean wind speed). Note that TI is calculated from the sodar 

measurements and therefore will contain the bias we are trying to correct for. This equation is 

today the best Kjeller Vindteknikk knows for reducing the discrepancies between vector and 

scalar averaged measurements. The equation is based on the findings in the article (Ioannis 

Antoniou (ed.) 2003). The sodar data from the Sodar site that has undergone the above 

transformation yield a 0.6 % higher average wind speed than the sodar data that has not been 

transformed. The transformation from vector averaged to scalar averaged wind speed will 

always yield an increase in the wind speed. Correcting for the different averaging techniques 

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for the campaigns in question can only increase the discrepancy in wind speed measured by the 

Sodar and the Mast. 

The time series with concurrent direction measurements from the Sodar and Mast agrees very 

well at 95 m height. The scatter plot in Figure 4-2 shows the agreement between the direction 

measurements made in 95 m in the mast and sodar. 

Figure 4-2: Scatter plot showing the agreement between the concurrent direction measurements in 95 m height 

for the sodar and in the mast. 

4.3 Comparison of measured turbulence intensity (TI) 

The turbulence conditions at a wind turbine site is normally described through the turbulence 

intensity (TI) defined as the 10-min standard deviation divided with the 10-min average wind 

speed. The mean and 90 th percentile of the TI as a function of wind speed measured by the 

Sodar and the Mast in 100 m height is presented in Figure 4-3 and Table 4-3. Only 

simultaneously recorded measurements are used in the comparison. The figures and table show 

that the TI measured by the sodar and mast between about 2.5 m/s up to about 13.5 m/s are 

very similar. Above this wind speed the TI recorded with the sodar and anemometer starts to 

deviate. The deviation can possibly be explained by the more frequent occurrence of smaller 

sized turbulence structures at higher wind speeds. The temporal and the spatial resolution of 

the sodar and anemometer measurements are very different, and it is plausible that the 

instruments ability to record turbulence varies with the size of the turbulence structures. One 

should note that there are relatively few concurrent measurements available at the higher wind 

speeds. The result shows that at the particular site in question the TI is described very similar 

by the two measuring techniques at moderate wind speeds. 

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Figure 4-3: Turbulence intensity with wind speed recorded in 100 m height with the Sodar (left) and Mast 

(right). 

Table 4-3: The table that shows the measured turbulence intensity in 100 m height measured by the Sodar and 

the Mast. Only concurrent measurements from the mast are used in the calculations. 

Velocity class [m/s] 

Sodar 

# observations TI 90 TI mean 

Mast 

#observations TI 90 TI mean 

[2.5 – 3.5> 207 0.21 0.11 223 0.21 0.12 

[3.5 – 4.5> 375 0.19 0.11 463 0.20 0.11 

[4.5 – 5.5> 501 0.19 0.10 562 0.19 0.10 

[5.5 – 6.5> 702 0.16 0.09 757 0.17 0.09 

[6.5 – 7.5> 928 0.16 0.09 1077 0.16 0.09 

[7.5 – 8.5> 1402 0.15 0.09 1467 0.16 0.09 

[8.5 – 9.5> 1686 0.15 0.09 1660 0.16 0.09 

[9.5 – 10.5> 1547 0.15 0.10 1587 0.15 0.09 

[10.5 – 11.5> 1359 0.15 0.10 1216 0.15 0.09 

[11.5 – 12.5> 849 0.15 0.10 701 0.16 0.10 

[12.5 – 13.5> 456 0.16 0.11 345 0.17 0.11 

[13.5 – 14.5> 254 0.15 0.12 235 0.18 0.13 

[14.5 – 15.5> 158 0.15 0.12 142 0.17 0.13 

[15.5 – 16.5> 92 0.14 0.12 68 0.18 0.14 

[16.5 – 17.5> 33 0.14 0.11 25 0.18 0.14 

[17.5 – 18.5> 7 0.12 0.10 9 0.17 0.15 

[18.5 – 19.5> 1 0.07 0.07 4 0.16 0.15 

4.4 Comparison of measured wind shear 

The velocity profile within a wind farm, i.e. the wind speed change with height, is often 

expressed through the dimensionless wind shear coefficient α. The wind shear coefficient can 

be estimated with measurement data through the following expression, 

 

V 

z 

V 

r 

z d 

 

 

zr 

d 

, 

where z is the height above ground level, z r is the reference height, d is the local zero-plane 

displacement height, V is the horizontal velocity, and V r is the velocity in the reference height. 

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By rearranging the expression above we can express α through the wind speed measured at 

different heights in the mast. We have estimated α using the wind profile measured by the 

anemometers mounted alongside the Mast and by the Sodar. The wind shear coefficients 

calculated between various heights are presented in Table 4-4. 

Table 4-4: Comparison of measured wind shear by the Sodar and in the Mast. The calculated coefficients are 

based on 3487 concurrent 10-min wind speed measurements from the sodar and mast. 

Wind shear coefficient 

measured between: 

50m-70m with sodar 

47.1m-71.6m in mast 





Sodar 0.40 0.39 0.38 

Mast 0.38 0.37 0.35 

The agreement between the wind shear measurements made in the mast and by sodar is good. 

The wind shear measured in the Mast is systematically somewhat lower than the wind shear 

measured by the sodar. This could be attributed to the differences in the roughness and terrain 

around the instruments. In addition, for sites with high wind shear, an overestimation of the 

wind shear measured is expected because of the sodar measurement principle. The sodar 

averages the wind speed over a volume with a vertical extent of about 20 m. As a result of the 

high wind shear the wind speed measured by the sodar over the volume will be lower than a 

point measurement in the middle of the volume. This effect will drop of with decreasing wind 

shear at higher heights, which leads to a higher calculated wind shear coefficient for the sodar 

compared to anemometer measurements in the same heights. An example given by the IEA 

Topical Experts Sodar Recommended Practises Group states that a sodar will overestimate the 

wind shear between 30/50 m with 5 % (Kathleen Moore 2010). This example supposes a volume 

average 20 m in depth (same as AQ500) and a wind shear coefficient (point measurement) of 

0.40. Another factor that can lead to an overestimation of the wind shear measured with the 

sodar is the effect of the DP-error (vector vs. scalar averaging). The effect of the DP-error, that 

leads to a lower measured wind speed, is reduced with decreasing turbulence and thus with 

increasing height. 

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5 Summary and conclusions 

We have in this report assessed the performance of the AQ500 sodar and compared its 

measurements with those made with cup-anemometry in a 100 m tall lattice mast located 200 

m from the sodar. 

We have seen that the correlation between the time series from the Sodar and the 

anemometers in the Mast is excellent. The strong covariance with the mast, and high 

availability of the sodar measurements, makes the sodar suited for identifying ice periods on 

the cup-anemometers in the mast with high accuracy. 

We have found a discrepancy of about 3.5 % in the absolute wind speed measured by the sodar 

and the cup anemometers in the mast. We conclude that a disagreement of this order can be 

expected between sodar and anemometer measurements because of the intrinsic uncertainties 

and differences in the measuring techniques. We have found that the availability of valid wind 

speed measurements is correlated with wind speed. This is important to account for when 

analysing the data from the AQ500 sodar. 

The turbulence intensity (TI) measured with the sodar and in the mast agrees well for wind 

speeds between about 3 – 13 m/s, and we can conclude that the sodar is able to describe the TI 

at these wind speeds at the considered site. For wind speeds above this the sodar might have 

problems resolving the turbulence structures properly. It is uncertain if this actually is the case 

since there are few concurrent measurements from the sodar and mast at higher wind speeds. 

The agreement in the measured wind shear with the two techniques is satisfying, and we 

believe that the observed discrepancies can be explained by differences in the measuring 

techniques. 

We regard the AQ500 sodar, on background of our findings, as a valuable contribution to the 

ongoing measurement campaign in the Mast. The measurements from the instrument will lead 

to a reduction of the uncertainty in the predicted wind climate at the site and give valuable 

information regarding the wind shear at the site. 

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6 References 

Ioannis Antoniou (ed.), Hans E. Jørgensen (ed.) et al. On the Theory of SODAR Measurement. 

Final reporting on WP1, EU WISE project NNE5-2001-297, Risø-R-1410(EN): Risø National 

Laboratory, Roskilde, Denmark, 2003. 

Kathleen Moore, et Al. Recomended Practices for the Use of Sodar in Wind Energy Resource 

Assesment. Expert recomendations (draft), Roskilde: IEA Topical Expert Group, 2010. 

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Appendix A 

Table 0-1: Deployment and system details for the Sodar site 

Deployment details 

Installation Crew : 

Max Gråhed (AQ Systems), Peter Rothmund (KVT) 

Location coordinates and elevation : 

RT90: N XX, E XX. ~497masl 

System orientation (North = 0°) : 334° (measured 330° + magnetic declination 4°) 

System details 

System type 

AQ500C 

System name 

AQS3 

Trailer reg. number, system serial number 

XX 

Sodar/control unit phone number 

XX 

Alarm number 

+47 XX 

SODAR software version 

RE01 

Measurement height 50-200m 

Control unit software version 4.00 

Clock settings (measurements) 

UTC (GMT+0) 

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Table 0-2: Instrumentation overview for the Mast site. 

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KVT/YY/2011/R043 

Appendix B 

Log book for the Sodar site 

Site: XXXX AQS3 Location: Sverige, XXXX Customer: Statkraft Development 

* ok . Limited data Quality - No data High wind or snow is "ok" data, but can be low quality 

Date/Sign. Year Month 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 

Sodar mounted 19.October 2010 by AQ systems / Max Gråhed and KVT / Peter 

2010 October * * * * * * * * . * . * * 

2010 Nevember * * * * * * * . * . . * * * * . * * * * . . * * * * * * * * 

2010 December * * . * * . * . * * . * . * * * . * * * * * . * * * * . * . * 

2011 January * . . * * * . * * . * * . . * * * * * * * * . . . . * * * * * 

2011 February * * * * * * * * * * * * . * . * * . . * * . * * * * * * 

2011 March * * * * * * * * * * * * * * * * * * * * * . . * * * * * . . . 

2011 April * * * * . * . . * * 

Site visits and notes 

19.10.2010 YY Sodar deployed 

16.11.2010 10 nov installert dieselvarmere av Max. 

29.11.2010 AJ Message sms Low fuel level 27.11.2010 

06.12.2010 AJ 1.12.2010 filled up 160 l fuel. 40 l reserve. 

13.12.2010 AJ Bad spectrum and small amount of data. Snow on membrans? 

15.12.2010 AJ Looking good again.See other side logg. It seems that the snow has been melted by the heat produced in the agg unit. 

27.12.2010 AJ Sms Low fuel level 23.12.2010. SMS Fredrik regard. assist. 27.12. Filled up 29.12.2010 with 150 l sodar and app 70 l res. 

05.01.2011 AJ AQ system at location 28.12.2010. Shut down between 19:00 and 24:00. 

26.01.2011 AJ Changed sodar time from 9:30 to 9:35 

22.02.2011 AJ SMS Low Fuel Level yesterday. Informed Fredrik. He will take move out today or Wednesday 

24.02.2011 AJ 23.02: Fredrik filled up 154 liters sodar and 70 l agg. 

11.03.2011 AJ Changed sodar time from 15:33 to 15:39 

22.03.2011 YY Changed sodar time from 13:53 to 13:55 

02.05.2011 AJ Dismounted Sodar 20.04.2011 

19/19

Evaluation of AQ500 sodar performance, Sweden

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