LAS MkII Scintillometer

  • Stand-alone operation
  • Range 100 m to 4.5 km
  • Built-in data logger
  • Built-in data processing
  • GPS

 

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LAS MkII Scintillometer

The LAS MkII Scintillometer provides continuous measurements of sensible heat fluxes over path lengths from 100 m up to 4.5 km. The measurement process is very straight forward and yields fast results. Since limited power is required, in most cases a simple solar-charged battery is sufficient.

The LAS MkII receiver has internal digital processing unit automatically computes all relevant parameters, such as Cn2 and the sensible heat flux. Real-time data is available on the display so you will know the status of your experiment in seconds. The built-in data logger stores several months of measurements and results.

A digital interface allows remote real-time display and full control over the instrument operational settings. Analogue outputs are also available enabling connection of the instrument to virtually any data acquisition system. This allows for easy integration into new or existing measurement networks.

Carrying instruments around for field deployment is already labour intensive enough, without having to think about laptops, interface cables and software. The LAS MkII can be installed and configured using its built-in display and key-pad without the need for any extra equipment.

The scintillometer on its own measures changes in refractive index of the atmosphere, Cn2, but with the optional meteorological sensor kit it can calculate sensible heat flux. This kit consists of wind speed, temperature and pressure sensors and plugs directly into the LAS MkII receiver.

The EVATION® software suite is included as standard to view real-time data numerically and graphically and provides advanced data post-processing.

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Specifications

Path length 100 m to 1 km (10 cm aperture), 250 m to 4.5 km (aperture 15 cm)
Dimensions 400 x 300 x 240 mm. (LxHxD) 8.5 kg
Wavelength 850 nm
Scintillation bandwidth 10e-17 to 10e-11
Power requirements LAS MkII 12 VDC, 6 W (54 W max. with heaters on)
Data processing Internal processing of Cn2, sensible heat flux and other parameters. Built-in data logger. GPS time.
Control and display Integrated display and key-pad or remote via digital interface
Interfaces Digital RS-232/422, analog output 0 to 2.4 V
Software EVATION® (instrument control and data analysis suite)

FAQ

  • How can I calculate the Cn2 Saturation limit for my LAS MkI Scintillometer?
    • A scintillometer measures the path weighted structure parameter of air, Cn2, using an optical transmitter and receiver.

      In certain cases of relatively high Cn2 values the signal can become saturated depending on the diameter of the lens, wavelength and path length.

      The so called saturation limit for Cn2 can be derived using the following formula (Ochs and Hill 1982)

      Cn2 < 0.18.D5/3.L-8/3.λ2/6

      Where:

      D

      the diameter of the scintillometer

      [0.15m or 0.3 m]

      L

      the path length

      [m]

       

      λ

      the emitted wavelength

      [880 nm]

      In the calculation tool below, you can calculate the saturation limit for the LAS and X-LAS scintillometer as a function of Path length.

      Path length [m]
      Saturation Limit LAS (10cm) in m-2/3

       

      Saturation Limit LAS (15cm) in m-2/3

       

      Saturation Limit X-LAS (30 cm) in m-2/3

       

       

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  • How does WINLAS correct for an incorrect potentiometer setting?
    • In case the path length has been set incorrectly during the installation of an LAS scintillometer, WINLAS can correct the Cn2 data.

      In order to do so please use the following procedure.

      Step 1: 

      After entering the relevant info in the parameters section, enter the path length setting set with the potentiometer of the LAS receiver in meters and enter the correct path length in the input field below.

       

       Select OK

       Step 2:

       Select ‘Run…’  in the WINLAS file menu.

       Result:

      WINLAS will now process the Scintillometer Cn2 data using a correction algorithm for the actual path length.

       Theoretical background

       WINLAS corrects the path length in the following way:

      Using the following equation to derive the intensity fluctuation data from the recorded Cn2 values calculated by the LAS using the incorrect path length setting.

                                      (Wang et al., 1978)

       

      Where 

      D

      Aperture diameter    ~ 15cm



      L    

      Initial path length

      σlnI

      Variance of log intensity

       The equation is re-written to yield the variance of the intensity fluctuations:

                      

      And finally re-calculate Cn2 with the correct path length:

                      

      End of FAQ.

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  • Can I use an LAS Scintillometer over open water?
    • If a body of water takes up for a major portion of the measurement path, using a scintillometer is not recommended.

      A scintillometer is an instrument that can measure the ‘amount’ of scintillations by emitting a beam of light over a horizontal path. The scintillations ‘seen’ by the instrument can be expressed as the structure parameter of the refractive index of air (Cn2), which is a representation of the ‘turbulent strength’ of the atmosphere.

      The scintillations are mostly a result of temperature and water vapour fluxes from the earth surface. Scintillometers use an infra red light source and are designed to be primarily sensitive to scintillations from temperature fluxes.

      Over most surfaces over land, the temperature related scintillations are most dominant and the those from water vapour are less significant. This means that Scintillometer will yield reliable Cn2 values. In addition, if the so called Bowen ratio is known, additional correction can be applied to improve the measurement even further.

      However, over open water the scintillations from water vapour are dominant over temperature fluctuations. As a result, a scintillometer operating at infra-red wavelengths will underestimate values for Cn2. Although in principle the measurement could also be corrected using the fore mentioned Bowen ratio, this ratio is in many cases not accurately known over open water.

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  • How do I set the path length using the Potentiometer on the LAS MkI receiver?
    • Once the LAS has been installed and properly aligned the Path Length dial knob at the receiver control panel must be set for the correct distance between the transmitter and the receiver. The Path Length dial knob has 10 turns maximum with a vernier counter and a locking mechanism.

      These graduations are NOT in units of distance! The precise path length must first be converted to a dial knob setting (Pot) using the following relationship for the LAS. The equations below can be used to find the correct Potentiometer setting as a function of pathlength for the LAS and X-LAS.

      In addition you can use the calculation tool below to calculate the correct potentiometer setting for the (X)LAS.

      LAS:

                  

      X-LAS:

                 

       

      Path length [m]
      Potentiometer Setting [LAS]

       

      Potentiometer Setting [X-LAS]

       

       

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