It is an acronym for the High Frequency Active Auroral Research Program, an initiative to study the effect on the performance of communication, navigation and surveillance systems by simulated heating of the ionosphere. It was started in 1993 and is expected to continue till 2013. It is funded by US defence agencies. A powerful HF signal is transmitted to heat a limited area of the ionosphere for a specified time. Scientists then record the effects of temporarily altered conditions through advanced instruments. Some organizations claim that the transmission antennae may be used as weapons.