Reliability in

TECHNICAL NOTE on GPS for day twilight night change and flashes synchronisation

ICAO, FAA and many Civil Aviation Authorities (EASA, STAC, GDF03, DSANA, CASA, directive AD I-006 F, CAP168 (UK), BL 3-10 (Denmark) etc) defined the thresholds for day, twilight (high intensity only) and night mode for the red, white and dual color obstruction lights using built-in censor or external photocell:

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 et 50 cd/m2;
  • and night mode when illumination decrease to à 50 cd/m2 (or 50 lux)

1 Using GPS instead of photocell or photocensor

In case of an extended obstacle with many flashing lights, it is sometimes not possible to connect the lights to one unique photocell. In this case our GPS can be use for synchronize both :

The GPS is normally used for the synchronization of the flashes (20, 30, 40, 60 flashes per minute) based on the UTC clock starting at the second 0. It can be also use for the synchronisation of the sequence of flashes 1/13 2/13 and 10/13 between 3 levels of lights in the special case of transmission lines

However it can be used also for the day/twilight/night change, the behavior of day/twilight/night change is then based on astronomical twilight definition:


On this drawing, the duration of the twilight of the GPS is based on the civil astronomical twilight (-6° for the sun below horizon), this duration vary a lot depending on the date and localization of the obstacle

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

The table below indicates the change of light output and color depending if a photocensor or the GPS is used: GPS and photocell correspondance

The GPS solution for day/twilight/night change is in used especially for obstruction lighting systems installed on

  • transmission lines
  • red only or dual color medium intensity on wind turbine

Otherwise, this solution is used in our dual color medium intensity and high intensity as a “back-up” in case the controller at the bottom of the tower is not responding or not connected, or in absence of signal from the photocell after 48 hours

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, however the change of color (or on/off) of all lights will be perfectly synchronized without wires or wireless communication in between the lights.

1. External GPS module

For installation with low intensity and red medium intensity all wired together on each obstacle, it is also possible to add a GPS module on each obstacle (instead of using the GPS built-in the flashing light). This GPS module is powered through 24-48Vdc and shares 2 output DC voltage to indicate twilight and night signals to all the lights (similar to the photocell) in addition to the electrical pulse used for the synchronisation of the flashes. This GPS module is usually installed inside the main power cabinet of the obstacle.


DesignationInput voltagePart number
GPS-24/48-I-V224-48 Vdc113746IV2

2. Flashing lights with built-in GPS

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

  • For red medium intensity OFC, the internal GPS allows to synchronise the flashes and on/off during night mode.

  • For white or dual color medium intensity, the internal GPS allows to synchronise the flashes and the change of light intensity and color between day/twilight and night mode

  • For high intensity, the internal GPS allows to synchronise the flashes and the change of light intensity depending on the 3 modes day, twilight and night mode 'or as back-up if a controller is in place at the bottom of the obstacle)

  • Red medium intensity (or L-864)


DesignationInput voltagePart number
OFC-RI-240-G110 -240 Vca113790RI-240-G
OFC-RI-240-RG110 - 240 Vca113790RI-240-RG
OFC-RI-048-G48 Vcc113790RI-048-G
OFC-RI-048-RG48 Vcc113790RI-048-RG

  • Dual color medium intensity


DesignationInput voltagePart number
OFI360-RW-04845-55 Vdc113792A
OFI360-RW-240I110-240 Vac113792-240-G
  • High intensity

high intensity controlled by GPS

DesignationInput voltagePart number
OFH-120-RW-240B110-240 Vac113780B