GJD GJD515, Manual, Page: 4 / 15

3. Operation principles
3.1. Sensor placement
The sensor has a maximum detection range of up to 500
metres. Recommended range for demanding applications
is 300 metres, to ensure detection of small objects such as
humans, even in difficult weather conditions.
There are two ways to create a continuous virtual fence with
the sensors. They can be placed 300 metres apart and have
an overlapping detection area as shown in figure A, or they
can be placed in pairs, with 600 metres between each pair,
as shown in figure B. Sensor placement type A is recom-
mended to get highest security level. With the overlapping
detection areas the sensors covers each other. Sensor
placement type B allows for cheaper installation due to fewer
points needing network connection, but creates a weak point
at the sensor locations.
When placing the Laser-Watch opposite each
other (as shown in B below), make sure the
lasers are crossed and not aimed directly at
each other.
3.2. Laser divergence
The laser has a beam divergence of 2.0x0.2 m Rad. It
spreads more in height than in width. This means that the
laser beam is approximately 1 x 0.1m at a distance of 500
meter.
3.3. Multiple reflections
As the laser beam diverges it is possible for the beam to
hit multiple targets. Each target will cause a reflection to
the sensor. The sensor algorithm analyses the different
reflections and calculates a distance to the object. If it is
more than 5 meters between the objects the sensor will only
use measurements from the far object to calculate object
position. If for example a tree branch partly covers the line of
sight the laser may still be able to detect objects behind it. It
is however strongly recommended to ensure the line of sight
is clear to maintain highest performance.
If it is less than 5 meters between the objects the sensor
cluster the objects and use weighted averaging to calculate
a virtual object position. If each object gets 50% of the laser
beam the calculated distance will be in the middle between
the objects. If the first object gets 75% and the other 25%
the calculated distance will move closer to the first object.
3.4. Alarm zones
Up to 20 alarm zones can be created. Each zone has
individual start- and stop distance, alarm sensitivity and
alarm action.
Using all 20 zones makes it possible to have unique alarms
for each 25m section of the perimeter if the zones are
equally distributed. The sensor is also able to send the exact
intrusion distance for even more accurate location. Most
times it is not optimal to have the zones equally distributed –
it is better to set the zones to match the surrounding area.
For example, if the detection area includes a crossing road
it is very useful to create zones to match the road, as seen
below.
Each zone in the Laser-Watch sensor has an individual start-
and stop distance. This means the zones can overlap in any
way and also that only areas that are interesting will create
alarms. It is also possible to have different alarm sensitivity
for each zone, allowing to fine tune the system.
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