Virtual Coasting Rule

About the Virtual Coasting Rule

Virtual coasting rules are broken when the following conditions are met:

1. A track has been seen in an initialisation area

2. The same track, or a match-able track can be followed through the coasting areas

3. The same track, or a match-able track is detected in the alarm area

Consider for example this imaginary use case:

Open

We want to know if a boat has left the harbour (labelled pier here) and entered a deep water area. Using normal rules, this will be very difficult in watery conditions as depending on the weather, there will likely be wakes and waves that break up tracking by hiding real targets and also creating false tracks causing false alarms.

Using a virtual coasting rule, we can much more accurately satisfy this requirement as we are now able to:

• follow boats leaving the harbour area even if they are lost for short periods of time to form into new tracks and raise alarms when they enter the deep water area

• ignore false tracks created by wakes or waves

Configuring our Areas

While setting up areas for the Virtual Coasting rule, we should consider the following factors:

• Patches of difficult tracking should be covered by zone areas with a healthy margin around it.

• Discontinuity in areas along the expected path of tracks should be small relative to track speed.

• You can include as many areas as you desire in the same zone.

• The initialisation areas should have excellent tracking and minimal risk of picking up false tracks.

In our example above, we have configured a single area to act as our initialisation area and a number of touching areas to coast the tracks in. When setting up areas, we used the Ctrl key to snap area corners together to join up multiple areas seamlessly.

Configuring our Rule

Open

We then create a new virtual coasting rule and configure the areas we wish to use in the "Zone Areas" tab by ticking the checkbox next to the areas. We will then also need to set up zones for these areas, and for our pier area, we set Zone "1". This is the smallest zone number and will thus act as the initialisation area. The number we use here as the smallest number does not matter, and you may wish to use e.g. 10, 20, 30, 40 instead. We set the first ring of 3 areas as zone 2, and the next ring of 2 areas as zone 3 - so long as they are in the same zone, the Virtual Coasting rule will treat these areas exactly the same and we could thus break up large zones to multiple smaller areas. The alarming zone is the zone with the highest number, in this case being 4.

Configuration parameters:

Apart from the standard configuration parameters that are used for all rules, the Virtual Coasting rule also has the following four additional parameters:

• Max Coasting Time - This is the maximum time we could lose a track before picking it up again for our virtual coasting. After this time, we will discard the original track from our cache.

• Deviation Multiplier - The deviation multiplier controls how sensitive our matching is when we have a new track that could match in track speed and direction. A very eccentric track will have a high deviation value and will be matched to easily. However, regardless of the multiplier or eccentricity of the track, the directional matching is hard limited to at most be the 180 degree arc it was originally moving in.

• Match Zones - Zones difference we can match against in the area zone number. For example if we use zones 10,20,30,40, we would need to set this to 10 or higher. By using different increases in zone numbers and a higher match zone, we can set up patterns of areas were we can skip some zones but require sightings in other zones - e.g. with zone numbers of 1, 2, 4, 6, 7 and a match zone of 3, we will require the track to be seen on zone 4, but can skip zones 2 and 6 to still raise an alarm.

• Match Sightings - Number of sightings for a new track that need to match before it is considered a matching track. This number of sightings must all be matched within the max coasting time. If we increase match sightings to a large number, we need to ensure max coasting is always larger than rotation period x match sightings.

Results:

Performance

Note that while the virtual coasting rule is very powerful and configurable, it is also more expensive than other rules to process.

You should avoid setting up a configuration that processes more than several thousand track sightings per second through all virtual coasting rules on a standard Navtech RPU. If you are using a custom device for your Picadilly server, this will vary depending on the hardware. Limiting the track sightings proceeded can be done either by keeping the number of virtual coasting rules low or avoiding using very busy areas of high track count in the virtual coasting rules.

Rule Combinations

After matching the tracks to an origin zone, the virtual coasting rule effectively becomes an area movement rule for the last zone areas. However, sometimes we may wish to do something more sophisticated, in which case we can combine the virtual coasting rule with other rules by using a History Rule or Compound Rule.

For example, if we wish to follow tracks that approach a fence, and alarm any tracks that cross the fence, we use a history rule to combine the virtual coasting rule (action - follow, no alarm) with a line breach rule for a breach line drawn after the alarm zone.

Related information