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* Lamps and reflectors replaceable without recalibration.
Each light unit consists of one or more light sources, red filters and lenses. A color filter may not be necessary with colored LED lights. Each light unit emits a high-intensity beam. The lower segment of the beam is red, and the upper part is white. The transition between the two colours must take place over 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.▼
A PAPI installation consists of a bar of four units. Units should be frangible but not susceptible to jet blast. The inner edge of the PAPI installation should be situated 15 meters from the runway edge, and not closer than 14m to any runway or taxiway. The units should be spaced 9m apart. An abbreviated system, A-PAPI, can be used for some categories of aircraft operations. It consists of two units with the inner unit located 10m from the runway edge.<ref name="ICAO"/>
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The light characteristics of all light units are identical. In good visibility conditions the guidance information can be used at ranges up to {{convert|5|mi|km}} by day and night. At night the light bars can be seen at ranges of at least {{convert|20|mi|km}}.
▲Each light unit consists of one or more light sources, red filters and lenses. Each light unit emits a high-intensity beam. The lower segment of the beam is red, and the upper part is white. The transition between the two colours must take place over 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.
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 [[pilot-controlled lighting|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.
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