TECHNICAL NOTE: on photocell and GPS for day/twilight/night change

ICAO, FAA and many Civil Aviation Authorities have defined the thresholds for day twilight and night mode for the red, white and dual color obstruction lights :

As per ICAO, the thresholds are defined as below :

• day mode is defined when ambient illumination is above 500 cd/m2 (or 500 lux);
• twilight mode is defined when ambient illumination is between 500 cd/m² and 50 cd/m2;
• and night mode when illumination decrease to à 50 cd/m2 (or 50 lux)

The FAA defines illumination of 646-376,8 foot-candles for twilight and 53,8-21,5 foot-candles for night mode.

1 - PHOTOCELL FOR DAY/TWILIGHT/NIGHT MODE

Our ambient light sensor (photocell) should be mounted upright, away from artificial light (eg., floodlights), and in a location that will enable its sensor window to have an unobstructed view of the polar sky (eg., pointed north in the northern hemisphere). The ambient light sensor is normally installed at the bottom of the obstacle For dual color medium intensity or high intensity system, the ambient light sensor is powered through 48VDC and emit 2 logical signals in 48Vdc for twilight and night (By default day is the absence from both signal). It can be connected on the power supply of the light or on the controller located at the bottom of the obstacle

• For red only medium intensity or low intensity, it can be used to power directly at night only the red obstruction lights in the limit of 2A maximum using the “night signal”
• Normally dual color medium intensity used the “night signal” only to change both red/white and intensity step from day to night and vis versa (using the twilight signal is also possible and provide less light disturbance because the change of light intensity from 20 000 candelas to 2000 candelas come earlier in the day)
• High intensity are using the 2 signals to change the intensity steps day/twilight and twilight/night

1-1 External ambient light sensor (photocell)

1-2 Obstruction lights with built-in photocensor

Our medium intensity and high intensity can be provided with a built-in photocensor on the base of the lamp For our low intensity NAVILITE-48Vdc series, our 48Vdc command box with transparent cover do have a photocensor directly fixed on the printed board circuit.

• Part number of red medium intensity with photocensor

• 48 Vdc command box with photocensor used with NAVILITE-48V

2 - GPS TO CHANGE DAY/TWILIGHT/NIGHT MODE

The GPS is normally used for the synchronization of the flashes (20, 30, 40, 60 fl per minute) based on the UTC clock (second 0 or 1) together with a photocell or photocensor Although it is not a censor, our GPS can also synchronize the day/twilight/night change of the medium/high intensity lights.

This solution is in used especially for dual color medium intensity installed on

• transmission lines
• dual color medium intensity on wind turbine

This solution is also in place in our high intensity as a “back-up” in case the controller at the bottom of the broadcast tower is not responding or not connected or does not send the information from the photocell (during installation for example)

Of course this is not a censor and in case of drop of illuminance during the day or twilight time, the light will not change its mode. The behavior is based on astronomical twilight :

On this drawing, the duration of the twilight of the GPS is the civil astronomical twilight (between 0° and -6° for the sun below horizon), this duration vary depending on the year and depending on the latitude and longitude

Under normal luminous conditions, this behavior is close to the thresholds defined by ICAO and FAA :

• sunrise/sunset : 500 lux for ICAO, 646-35 lux for FAA
• civil twilight : 50 lux for ICAO, 54-21 lux FAA

2-1 External GPS module

This GPS is powered through 48Vdc and sends 2 output twilight and night signals (similar to the photocell) in addition to the top sync for the flashes. It can be inside an external power supply or in the flash-head (for dual color medium intensity).

2-2 Flashing lights with built-in GPS

All our new model of flashing light do have built-in GPS inside their flash-head:

• Red medium intensity (or L-864)

• Dual color medium intensity