<Li> incoherent scatter radars such as the EISCAT, Sondre Stromfjord, Millstone Hill, Arecibo, and Jicamarca radars </Li> <Li> coherent scatter radars such as the Super Dual Auroral Radar Network (SuperDARN) radars, </Li> <Li> special receivers to detect how the reflected waves have changed from the transmitted waves . </Li> <P> A variety of experiments, such as HAARP (High Frequency Active Auroral Research Program), involve high power radio transmitters to modify the properties of the ionosphere . These investigations focus on studying the properties and behavior of ionospheric plasma, with particular emphasis on being able to understand and use it to enhance communications and surveillance systems for both civilian and military purposes . HAARP was started in 1993 as a proposed twenty - year experiment, and is currently active near Gakona, Alaska . </P>

The atmospheric layer that reflects radio waves is