There are two forms of 0TH radar, surface-wave and skywave. Surface-wave systems are relatively inexpensive and have found applications in sea sensing (see next chapter), for the defence of localized areas against low-flying missiles and to some extent for monitoring ship traffic. Skywave radars are used to monitor very large areas of land and sea to search for air targets, ballistic missiles during launch phase and some types of surface target. They also have remote sensing capabilities, especially storm tracking and ocean wave monitoring. These radars are large, powerful, expensive and require sophisticated frequency management systems in order to operate via the ever-changing ionosphere.