These "Openings" as they are known, are generally first spotted by amateurs operating SSB (Single Side Band) and CW (Continuous Wave) modes since amateurs using these modes typically are attempting distance contacts (DX) and alert for signal enhancement events. Completion of contacts using these weak signal modes involves the exchange of signal level reports and location by grid square which is known as the Maidenhead Locator System. Two way ducting contacts can have very strong signals and are often made with moderate power, small antennas and other types of modes. Long distance ducting contacts do occur using FM modes as well but for the most part go unnoticed by many FM operators. Another form of VHF propagation is called Sporadic E propagation. This is a phenomenon whereby radio signals are reflected back towards Earth by highly ionized segments of the ionosphere which can facilitate contacts in excess of with very strong signals received by both parties. Unlike some other long distance modes, high power and large antennas are often not required to make contact with distant stations via a sporadic E event. A two-way conversation can take place over a distance of several hundred miles or more, often using low levels of RF power. Sporadic E is a rare and completely random propagation phenomenon lasting anywhere from a matter of minutes to several hours. Satellites are basically repeater stations in orbit. The 2 meter band is also used in conjunction with the 70-centimeter band, or the 10-meter band and various microwave bands via orbiting amateur radio satellites. This is known as cross band repeating. On-board software defines what mode or band is in use at any particular time and this is determined by amateurs at so-called earth stations who control or instruct the satellite behavior. Amateurs know what mode is in use via published internet schedules. For instance, a favorite mode is Mode "B" or "V/U" which simply indicates the uplink and downlink frequencies or bands the satellite is currently using. In this example, V/U means VHF/UHF or VHF uplink with UHF downlink. Most amateur satellites are Low Earth Orbit satellites, or LEO's as they are affectionately known, and generally are about 450 miles high (700 km). At that height amateurs can expect reception distances of up to around . There are a few amateur satellites that have very high elliptical orbits. These satellites can reach altitudes of 30,000 miles (50,000 km) above the earth where an entire hemisphere is visible providing outstanding communications capabilities from any two points on the earth within line of sight of the satellite; distances that are far beyond the reach of the LEO's.