earth science

Radio acoustic sounding system

Radio acoustic sounding system is a earth 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 Radio acoustic sounding system rather than just read about it. In short: A radio acoustic sounding system (RASS) is a system for measuring the atmospheric lapse rate using backscattering of radio waves from an acoustic wave front to measure the speed of sound at various heights above the ground. This is possible because the compression and rarefaction of air by an acoustic wave changes the dielectric properties, producing partial reflection of the transmitted radar signal.

Key takeaways

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

Reference excerpt

A radio acoustic sounding system (RASS) is a system for measuring the atmospheric lapse rate using backscattering of radio waves from an acoustic wave front to measure the speed of sound at various heights above the ground. This is possible because the compression and rarefaction of air by an acoustic wave changes the dielectric properties, producing partial reflection of the transmitted radar signal. From the speed of sound, the temperature of the air in the planetary boundary layer can be computed. The maximum altitude range of RASS systems is typically 750 metres (2,460 ft), although observations have been reported up to 1.2 km (3,900 ft) in moist air.

Principle

The principle of operation behind RASS is as follows: Bragg scattering occurs when acoustic energy (i.e., sound) is transmitted into the vertical beam of a radar such that the wavelength of the acoustic signal matches the half-wavelength of the radar. As the frequency of the acoustic signal is varied, strongly enhanced scattering of the radar signal occurs when the Bragg match takes place. When this occurs, the Doppler shift of the radar signal produced by the Bragg scattering can be determined, as well as the atmospheric vertical velocity. Thus, the speed of sound as a function of altitude can be measured, from which virtual temperature (TV) profiles can be calculated with appropriate corrections for vertical air motion. The virtual temperature of an air parcel is the temperature that dry air would have if its pressure and density were equal to those of a sample of moist air. As a rule of thumb, an atmospheric vertical velocity of 1 m/s (3.3 ft/s) can alter a TV observation by 1.6 °C (2.9 °F).

Configurations

RASS can be added to a radar wind profiler or to a sodar system. In the former case, the necessary acoustic subsystems must be added to the radar wind profiler to generate the sound signals and to perform signal processing. When RASS is added to a radar profiler, three or four vertically pointing acoustic sources (equivalent to high quality stereo loud speakers) are placed around the radar wind profiler's antenna, and electronic subsystems are added that include the acoustic power amplifier and the signal generating circuit boards. The acoustic sources are used only to transmit sound into the vertical beam of the radar, and are usually encased in noise suppression enclosures to minimize nuisance effects that may bother nearby neighbors or others in the vicinity of the instrument. When RASS is added to a sodar, the necessary radar subsystems are added to transmit and receive the radar signals and to process the radar reflectivity information. Since the wind data are obtained by the sodar, the radar only needs to sample along the vertical axis. The sodar transducers are used to transmit the acoustic signals that produce the Bragg scattering of the radar signals, which allows the speed of sound to be measured by the radar.

Resolution The vertical resolution of RASS data is determined by the pulse length(s) used by the radar. RASS sampling is usually performed with a 60 to 100 metres (200 to 330 ft) pulse length. Because of atmospheric attenuation of the acoustic signals at the RASS frequencies used by boundary layer radar wind profilers, the altitude range that can be sampled is usually 0.1 to 1.5 kilometres (330 to 4,920 ft), depending on atmospheric conditions (e.g., high wind velocities tend to limit RASS altitude coverage to a few hundred meters because the acoustic signals are blown out of the radar beam).

References

This article incorporates public domain material from Meteorological Monitoring Guidance for Regulatory Modeling Applications (PDF). United States government.

Illustrations

Worked examples

Example 1 — a first encounter with Radio acoustic sounding system

Start with the simplest possible case. Write down what Radio acoustic sounding system claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In earth 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 Radio acoustic sounding system 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 Radio acoustic sounding system 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 Radio acoustic sounding system

In research: Radio acoustic sounding system appears in earth 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 Radio acoustic sounding system 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: Radio acoustic sounding system is common in secondary-school and first-year university syllabi. It links to neighbouring topics Atmospheric sounding, Meteorological instrumentation and equipment, Weather radars, so understanding it makes those chapters shorter.
In everyday life: Look for Radio acoustic sounding system 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.

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How to study Radio acoustic sounding system in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Radio acoustic sounding system 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 Radio acoustic sounding system out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Radio acoustic sounding system in simple terms?

Why does Radio acoustic sounding system matter?

Because it connects several earth 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 Radio acoustic sounding system?

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 Radio acoustic sounding system.