Low Power Distance Measuring Equipment (LPDME) is a component of instrument landing capability for precision and nonprecision approaches that measures the distance from the aircraft to the runway. It generally has a range of 40 miles from the airport and aids not only in the approach but also in allowing pilots to remain position oriented around the airport. This orientation is especially valuable in high terrain locations and helps airplanes to avoid obstacles or restricted areas.
LPDME equipment in aircraft send a pulse signal to the ground based LPDME, which responds with an answer pulse signal. The ground based LPDME is a transponder in that it answers a querying pulse. The receiver in the aircraft measures the time delay between the sent and received pulses and calculates the slant range distance. There is no azimuth information, only distance. LPDME and DME (high power) differ only in the amplification of the signal from the ground-based unit. DME is used often with VOR for enroute navigation, which has a range of approximately 200 miles. The distance information from a VORTAC or TACAN unit is DME.
The FAA is procuring LPDME from Thales, ATM on a requirements contract signed in 2002. It is a COTS (Commercial Off �the-Shelf) unit called the 415 SE. The FAA primarily uses LPDME for ILS in lieu of Marker Beacons. Besides the advantage of helping pilots remain position oriented during the entire approach, LPDME is often cheaper than using Marker Beacons because of the real estate requirements. Marker Beacons must be at a set distance and be located along the line of the ILS approach. A new Low Power and High Power DME competitive contract will be awarded in 2010.