A sounding rocket, sometimes called a research rocket, is an instrument-carrying rocket designed to take measurements and perform scientific experiments during its sub-orbital flight.

The rockets are used to carry instruments from 48 to 145 km (30 to 90 miles) above the surface of the Earth, the altitude generally between weather balloons and satellites; the maximum altitude for balloons is about 40 km (25 miles) and the minimum for satellites is approximately 121 km (75 miles). Certain sounding rockets have an apogee (maximum point of altitude) between 1,000 and 1,500 km (620 and 930 miles), such as the Black Brant X and XII, which is the maximum apogee of their class. Sounding rockets often use military surplus rocket motors.

America's National Aeronautics and Space Administration (NASA) routinely flies the Terrier Mk 70 boosted Improved Orion, lifting 270–450-kg (600–1,000-pound) payloads into the exo-atmospheric (beyond the atmosphere) region between 97 and 201 km (60 and 125 miles).

The origin of the term comes from nautical vocabulary to sound, which is to throw a weighted line from a ship into the water to measure the water's depth. Sounding in the rocket context is equivalent to taking a measurement'.

More Info: en.wikipedia.org