Technology Nugget—SODAR: TMT's Bat in the Night
  Tony Travouillon

It’s dark. Too dark to see. And yet as it flies through the night hunting for its first prey, a bat makes no mistake and dives with opened jaw right into an unsuspecting moth. If you take a close look at a bat, it is obvious that the animal does not rely on its tiny eyes to see in the dark. In some species, the eyes are even hard to find. What you won’t have problems finding, however, are its ears. Sometimes several time the size of its head, a bat’s ears are clearly its main navigation system. As it flies, a bat emits ultrasounds that reflect off solid surfaces like trees or insects. Listening to sound reflections, the bat can build a map of its surrounding without using its eyes.

This amazing technique led to one of the best examples of science borrowing from nature. In 1906, Lewis Nixon used it to detect icebergs. Like a bat in the night, it is not possible to see very far through water and so the SONAR was born. This term, meaning SOund NAvigation and Ranging, was first coined during World War II when SONARs were the only way to detect submarines.

Thanks to the improvement of the sensitivity of acoustic instruments, the 1970’s saw a new application for the technique. When emitted in air, sound waves reflect off turbulent cells in the atmospheric. These cells are not in thermal equilibrium with their surroundings and it is the temperature difference that scatters the sound waves.

This led to a new instrument called SODAR (SOund Detection And Ranging). Sending a series of acoustic pulses up into the atmosphere, a SODAR can detect the amount of turbulence in the atmosphere as a function of height by listening to the strength of the echoes as a function of time delay (echoes from higher altitudes take a longer time to come back to the SODAR). Wind speed can also be determined across the same range using the frequency shift of the received pulse. Typical SODARs have a range of several hundred meters and depend on the amount of acoustic power emitted to reach longer ranges.

SODARs are classified as remote sensing instruments meaning that they measure turbulence and wind speed at different layers in the atmosphere without being physically there. This has several advantages. The first is that you can measure properties of the air without disturbing it. The second is that it would be impractical to build towers several hundred meters high. These advantages determined a range of useful applications for the instrument including the measurement of wind speed and turbulence profiles at airports where the use of tower would be, to say the least, dangerous. SODARs are also very useful for the testing of astronomical sites.

The atmospheric turbulence sensed by SODARs is the main cause of the loss of resolution in ground-based telescopes. As photons pass through our atmosphere they are refracted in random directions by the turbulent air. Observed through a telescope, stars appear to dance and twinkle because of this effect and make it difficult to resolve small objects like planets. SODARs quantify the amount of turbulence in the atmosphere above a given site. They are a powerful tool helping us to determine the site which will maximize the resolution capability of our telescope.

TMT SODAR equipment is prepared for testing in Chile on Cerro Tololo by Warren Skidmore.