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 all together or to one unique controller. In this case our GPS can be used to synchronize the flashes and also the change of color /intensity:

• Synchronisation of the flashes: The GPS synchronize the flashes at 20, 30, 40, 60 flashes per minute (ICAO setting) based on the UTC clock starting at the second 0, (and optionally with a sequence of flashes 1/13 2/13 and 10/13 between 3 levels of lights in the special case of transmission lines)

• for the day/twilight/night change of color and light intensity, the GPS is using the 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:

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.

2\. External analog 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 or inside our OBSTA controller OFC-CTR:

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)

• Dual color medium intensity

• High intensity