an angle not greater than three minutes of arc. This characteristic makes the color change very conspicuous, a key feature of the PAPI signal. To form the PAPI guidance signal, the color transition boundaries of the four units are fixed at different angles. The lowest angle is used for the unit furthest from the runway, the highest for the unit nearest to the runway. The designated glideslope is midway between the second and third light unit settings. Depending on the position of the aircraft relative to the specified angle of approach, the lights will appear either red or white to the pilot. The pilot will have reached the normal glidepath (usually 3 degrees) when there is an equal number of red and white lights. If an aircraft is beneath the glidepath, red lights will outnumber white; if an aircraft is above the glidepath, more white lights are visible. PAPI systems are readily available from airfield lighting manufacturers worldwide. PAPI is normally operated by air traffic control (ATC). If ATC services are not normally provided at an aerodrome, PAPI along with other airport lights may be activated by the pilot by keying the aircraft microphone with the aircraft's communication radio tuned to the CTAF or dedicated pilot controlled lighting (PCL) frequency. Design A typical engineering design specification for a PAPI light unit is shown below: Optical construction: Preadjusted 2-lamp optical assembly. Anodized aluminium reflectors. Red color filters. Precision-ground lenses. Lamps and reflectors replaceable without recalibration. 2 x 200 W / 6,6 A prefocused halogen lamps, Pk30d base. Average lifetime 1000 hours at rated current. 2008 saw the advent of new PAPI devices manufactured using solid state LED lamps instead of incandescent lamps. The LEDs produce sufficient brightness to satisfy ICAO light intensity and beamspread standards, and average lifetime with the LED based systems is 50,000 hours or more. By using LEDs, the device's power consumption is lowered considerably. The LED systems run internally on DC voltage, so the DC voltage requirements, along with the LEDs' inherently low power consumption, now allow for solar-powered PAPIs, enabling them to function completely independently of a power grid. The PAPI system is co-opted for use by the Final Approach Runway Occupancy Signal (FAROS) system being introduced by several major airports in the United States for the purpose of allowing pilots to resolve a runway incursion without requiring a priori notice of an occupied runway from