A Low Probability of intercept radar, is a kind of stealth radar, designed to prevent detection by enemy passive sensors (e.g Radar Warning Receivers), which locate aircraft through their radar emissions. The LPI radar becomes less observable through a wide bandwidth and frequency variability (for example the APG-77 of the F-22 changes frequency over 1,000 times a second). The radar also uses just as much energy as it is needed to track the enemy and thus, the emissions, which can be detected, are less. LPI radars are used by military aircraft, missile systems and ships.