There's one question so far tagged ultrasonic. I am far more used to those sensors being referred to as sonars. I'm wondering if my perception is the most common or if people do use ultrasonic/ultrasound more often, and whether we should replace the tag or establish them as synonyms.
"Ultrasonic" seems like an odd tag to have -- like having separate tags for "LED" and "Red LED". It's still sonar; the only reason it's ultrasonic is because nobody wants to listen to the sound a rangefinder makes.
In the AUV industry, sonar is a broad term for a lot of sophisticated (but very separate) technologies. The simplest active sonar devices measure range only, but others measure the speed of currents / speed over ground (DVLs) or provide imaging capabilities (sidescan, DIDSON). There is also a large use of passive sonar, which can't really be called ultrasonic because it doesn't produce sound.
Update 11/29/12: I think the best thing to do might be to break these out by the function of the sensor, in a "how-what" format. For example:
- acoustic rangefinder (could also say "sonic")
- laser rangefinder
- acoustic imaging
- laser imaging (lidar)
- doppler velocimeter
Ultrasonic sensors use high frequency sound waves, and calculate the time interval between generating and receiving the echo. They are used to detect the presence of a target, or measure the distance to the target.
Sonar stands for SOund Navigation And Ranging. The acoustic frequency used does not have to be ultrasonic (above human hearing range), and can include all sound frequencies, including infrasonic frequencies (below human hearing range). It includes active and passive sonar.
Sonar systems may involve multiple sound transmitters, and sensors, and may involve more complex data processing.