A Rule correspond to a movement of interest that needs monitoring.
A Rule first specifies which vehicles need monitoring and then details which types of movements need reporting. To precise the movements, a Rule uses two Events, a start event that dictates when the monitoring of the movement must start, and an end event when the movement no longer need to be monitored.
Events
An Event occurs when a vehicle makes an action relative to an Area of Interest.
Opera constantly monitor vehicle movements and raises events as they occur.
Actions
Possible vehicle actions relative to areas of interest are:
- Entering (an area of interest)
- Leaving (an area of interest)
- Being stopped inside an area of interest
- Crossing an area of interest.
Actions are determined on the basis of the penultimate and the last known position of a vehicle. For example, when a vehicle is not in an area of interest in the penultimate position, and inside an area of interest in the last position, we assume the vehicle entered the area of interest.
When the position of a vehicle has not changed between reported positions, or when the average reported speed of the vehicle is close to 0, or the relative distance between the two reported positions is small, we assume the vehicle has stopped in the area of interest.
Finally, all areas of interest that are intersected by the line joining the penultimate and last positions, we assume they are/were crossed by the vehicle, assuming the vehicle move onto a straight line between the last two points. In practice, the penultimate and last position must be outside the area of interest and the line joining the penultimate and last position must have at least one intersection point with the area of interest.
For most events, crossing points between vehicle trajectories and areas of interest are recorded and it is possible to determine timing information if necessary.
Areas of Interest
Areas of interest are polygons on the ground of the airport. They can be as large as the entire airport or as small as a parking location.
Areas of interest are named following the same identification mechanism as vehicle, using four attributes. This allow for basic areas of interest grouping, by classes or types.
When writing an Event, it is not necessary to supply a single, unique area of interest. It is possible to supply a collection of areas of interest (AoI). The Event will match any AoI that is in the collection.
Rule
A Rule is a coordination of two events for a vehicle, or category of vehicle.
Name
Each Rule has a name for identification purpose. It usually is a serial number.
If the name of the Rule starts with #, the Rule is ignored (commented out).
Vehicles
When writing a Rule, it is not necessary to supply a single, unique vehicle identifier. It is possible to supply a type and/or a class of vehicles. The Rule will match any vehicle that is in the collection. Examples of vehicle classes are aircrafts, or ground support vehicle. Examples of vehicle types are aircraft models or ground support vehicle function like refueling or baggage handling.
In a Rule, the vehicle is specified as a regular expression that must match the vehicle identifier.
Areas of Interest
In a Rule, the collection of areas of interest is specified as a regular expression that must match the area of interest identifier.
Start and End Events
A Rule is defined by two events of interest, a start event and an end event.
For example:
- Start: When a vehicle exits any runway
- End: When a vehicle enters any ramp
Same Area Of Interest
This boolean flag indicates whether the area of interest of the start and end event must be exactly the same or not.
Rule Timeout
The rule has a timeout that is started when the start event occurs and determine the time before which the end event must arrive.
The timeout is a fine parameter that can be adjusted to generate multiple events when necessary. For exemple, if a vehicle runs back and forth through a area of interest, a long timeout will only capture the first and last traversal of the area of interest in a single promise/resolve pair because the promise will be slow to expire. On the opposite, with a short timeout, each traversal with create a new promise/resolve pair since the previous promise will be expired due to the short timeout.
Rule Monitoring
Each time a new position arrives, Opera determine the vehicle and monitor whether the vehicle triggers some Events.
Event Message
A vehicle making an action relative to an area of interest produces a Message with:
- The vehicle identifier,
- The position of the vehicle,
- The time of the event,
- The action,
- The area of interest that is involved.
If a triggered event is part of a Rule, the Rule is updated as follow:
Promise
When an event matches the start event of a rule, the rule is activated. The rule becomes a promise for a precise vehicle and area of interest.
Promise Timeout
The Rule remains a promise until it times out. The start time of the timeout is reset each time the same start event occurs, for the same vehicle, for the same area of interest, while the promise is not expired.
When a Promise has timed out (is expired), it can no longer be resolved nor reset. It is archived. If a new start event arrives for the same vehicle, for the same area of interest, and an previous promise has expired (and has been archived), a new promise is created.
Resolution
When an event matches the end event of a Promise, the rule is resolved. The result of the rule is archived for later processing.
The Rule must be resolved by the same vehicle that triggered it.
When a rule is resolved, its promise is not removed. The promise remains until it times out.
Resolved Rule Data
When a rule is resolved, the following data is stored:
- Identifier of the rule,
- Event data of the promise,
- Event data of the resolution.
The most valuable data that is retained is the time difference between the promise and the resolve, in other words, the duration of the rule.
Other data is used as follow:
The vehicle that triggered the rule gives information about actors in the activity. Who is doing what, what activities are going on.
The area(s) of interest where the rule occurred gives location information: Where is the activity taking place, what are the areas under stressful situations.
The unique combination of actors and areas of activity allow Opera to build an image of all activities running on the ground of the airport.