science

Sodar

Sodar is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Sodar rather than just read about it. In short: Sodar, an acronym of sonic detection and ranging, is a meteorological instrument used as a wind profiler based on the scattering of sound waves by atmospheric turbulence. Sodar equipment is used to measure wind speed at various heights above the ground, and the thermodynamic structure of the lower layer of the atmosphere.

Key takeaways

Sodar belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Sodar to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Sodar from memory before moving on to harder problems.

Reference excerpt

Sodar, an acronym of sonic detection and ranging, is a meteorological instrument used as a wind profiler based on the scattering of sound waves by atmospheric turbulence. Sodar equipment is used to measure wind speed at various heights above the ground, and the thermodynamic structure of the lower layer of the atmosphere. Sodar systems are in fact nothing more than sonar systems used in the air rather than in water. More specifically, since they operate using the Doppler effect with a multi-beam configuration to determine wind speed, they are the exact in-air equivalent to a subclass of sonar systems known as acoustic Doppler current profilers (ADCP). Other names used for sodar systems include sounder, echosounder and acoustic radar.

Doppler sodar Commercial sodars operated to collect upper-air wind measurements consist of antennas that transmit and receive acoustic signals. A mono-static system uses the same antenna for transmitting and receiving, while a bi-static system uses separate antennas. The difference between the two antenna systems determines whether atmospheric scattering is by temperature fluctuations (in mono-static systems), or by both temperature and wind velocity fluctuations (in bi-static systems). Mono-static antenna systems can be divided into two categories: those using multiple axis individual antennas, and those using a single phased array antenna. The multiple-axis systems generally use three individual antennas aimed in specific directions to steer the acoustic beam. Using three independent (i.e. non-collinear) axes is enough to retrieve the three components of the wind speed, although using more axes would add redundancy and increase robustness to noise when estimating the wind speed, using a least-squares approach. One antenna is generally aimed vertically, and the other two are tilted slightly from the vertical at an orthogonal angle. Each of the individual antennas may use a single transducer focused into a parabolic reflector to form a parabolic loudspeaker, or an array of speaker drivers and horns (transducers) all transmitting in-phase to form a single beam. Both the tilt angle from the vertical and the azimuth angle of each antenna are fixed when the system is set up. Phased-array antenna systems use a single array of speaker drivers and horns (transducers), and the beams are electronically steered by phasing the transducers appropriately. To set up a phased-array antenna, the pointing direction of the array is either level or oriented as specified by the manufacturer.

The horizontal components of the wind velocity are calculated from the radially measured Doppler shifts and the specified tilt angle from the vertical. The tilt angle, or zenith angle, is generally 15 to 30 degrees, and the horizontal beams are typically oriented at right angles to one another. Since the Doppler shift of the radial components along the tilted beams includes the influence of both the horizontal and vertical components of the wind, a correction for the vertical velocity is needed in systems with zenith angles less than 20 degrees. Also, if the system is located in a region where vertical velocities may be greater than about 0.2 m/s, corrections for the vertical velocity are needed, regardless of the beam's zenith angle. The vertical range of sodars is approximately 0.2 to 2 kilometers (km) and is a function of frequency, power output, atmospheric stability, turbulence, and, most importantly, the noise environment in which a sodar is operated. Operating frequencies range from less than 1000 Hz to over 4000 Hz, with power levels up to several hundred watts. Due to the attenuation characteristics of the atmosphere, high power, lower frequency sodars will generally produce greater height coverage. Some sodars can be operated in different modes to better match vertical resolution and range to the application. This is accomplished through a relaxation between pulse length and maximum altitude.

Sodar applications

Traditionally used in atmospheric research, sodars are now being applied as an alternative to traditional wind monitoring for the development of wind power projects. Sodars used for wind power applications are typically focused on a measurement range from 50m to 200m above ground level, corresponding to the size of modern wind turbines. Some sodar products, such as the Fulcrum3D FS1 Sodar, REMTECH PA-XS Sodar and the AQ510 Sodar, have been specifically developed for this market. Compact-beam sodars are more accurate in complex terrain where the wind vector can change across the measurement area of the sodar. By providing a more compact beam angle, these sodars reduce the effect of any change in the wind vector. This provides a more accurate estimate of wind flow and therefore energy production of a wind turbine. Compact beam sodars also reduce the effect of fixed echos and allow a more compact unit design. Multiple-axis sodars provide the capability for the simultaneous firing of all three sound beams, unlike single-axis sodars which must fire each sound beam sequentially. Simultaneous firing can provide three times the number of sample points in any given period, resulting in a higher signal to noise ratio (SNR), higher data availability and greater accuracy. Sodars designed for the wind energy industry also differ in important aspects such as the traceability of data as some manufacturers do not return full signal and noise spectrum data from the sodar unit, but rather, only return processed wind speed data. This means the raw data cannot be re-analysed or reprocessed.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Sodar

Start with the simplest possible case. Write down what Sodar claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Sodar before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Sodar ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Sodar

In research: Sodar appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Sodar in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Sodar is common in secondary-school and first-year university syllabi. It links to neighbouring topics Meteorological instrumentation and equipment, so understanding it makes those chapters shorter.
In everyday life: Look for Sodar outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Sodar in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Sodar means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Sodar out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Sodar in simple terms?

Why does Sodar matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Sodar?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Sodar.