Simulation objects in VR-Forces are configured with sensors that use the VR-Forces Signature Sensor system to determine what other objects in the simulation they are able to sense.

This system allows VR-Forces to simulate sensors in many different domains, such as visual, radar, infrared, and sonar. The sensors delivered with VR-Forces are organized as systems in system definition files. For a description of how sensor systems work, please see 9.10 - IR Sensor System, 9.11 - Visual Sensor System, and 9.12 - Detecting Objects with Sensors. For details about systems and system definition files, please see "Editing an Entity’s Systems" and "System Definition Files", in VR-Forces Users Guide.

7.1.1 Signature Sensor Concepts

Signature sensors are based on the following concepts:

A sensor domain is a particular class of sensor, such as visual, radar, sonar, infrared, and so on. You can add new sensor domains by editing configuration files.
A sensor signature is a floating point number that represents the ease of being sensed in a particular sensor domain. The higher the signature value, the easier it is to sense a simulation object in that domain.
An absolute signature is the sensor signature of a particular simulation object, in a particular domain, at a particular time, independent of the observer. This is a property of the simulation object being sensed.
An apparent signature is the sensor signature of a particular simulation object, in a particular domain, from a specific viewpoint. This is the absolute signature corrected for the distance and obstructions between the simulation object being sensed and the viewpoint.

The sensor model is implemented through the following parameter blocks in the object parameter database and configuration files:

Target object - Each object that is potentially observable produces an absolute signature value for any sensor domain in which is it detectable. Entries in the object parameter database set the base signature value, and optionally, some rules about how that signature is altered based on the state of the simulation object.
Signature propagation - Each sensor domain has a global propagation model. The global propagation model applies to all simulation objects using sensors in a given sensor domain. These models modify absolute signatures from target objects to correct for the viewer's location, as well as any environmental effects, such as being obscured by terrain, and return the relative signature. These models are registered and configured in the ./data/simulationModelSets/<model_set>/physicalWorldParams.mtl file.
Sensor - The sensor component configured on a simulation object that is doing the sensing takes the relative signature returned from the propagation model and determines probability of detection. Sensor parameters are configured in the descriptor for each sensor in the object parameter database.

7.1.2 Target Object

Every object type in VR-Forces can be configured with sensor signature values for any of the sensor domains. These are configured in the object parameter database entry for the object type, and apply both to local objects and remote objects. For each domain, an object has a base signature, and a set of modifiers. The base signature is a single value that represents the signature for that domain. The modifiers specify multipliers for that base value based on the current state of the object. For example, a simulation object might have a base value of 5.0 for its visual signature, and a modifier of 1.5 when moving. This means the signature of the simulation object will be 7.5 when moving and 5.0 when not moving. Signature modifier rules can be placed in an external file, and referenced from many different object types.

The absolute signature values configured in the target object have no units. When used with the standard propagation model and a standard sensor, the absolute signature is the maximum distance (in kilometers) from which the object can be spotted. However, in general, the absolute signature is not necessarily expressed in units of distance. It is a function of the combination of a particular propagation model and sensor models.

Absolute signatures are specified separately for each sensor domain, and are completely independent of each other.

7.1.3 Signature Propagation

The signature propagation portion of the sensor system models the spatial relationship between the sensor and the target and the effects of the environment on detection. This is modeled by converting the absolute signature calculated by the target object into the relative signature given an observer position.

Signature propagators are configured for each of the sensor domains in the physical world parameter file. Each domain can use a different propagation model, or have its models configured differently.

The VR-Forces standard signature propagator, which is used by default for all sensor domains, returns a relative signature equivalent to the absolute signature * 1000 / range between observer and target. Optionally, the standard propagator can be configured to check for terrain or simulation objects blocking the direct line-of-sight between the target and observer, and return a relative signature of 0 when there is an obstruction.

7.1.4 The Sensor Component

The sensor is the component that is configured on the observing simulation object. The basic version of this sensor is called the signature-object-sensor. It is configured for a specific sensor domain and uses the signature propagator for that domain to determine the relative signatures of any other simulated objects within its maximum range. Sensors are configured with detection probability tables, that map relative signature values to probability of detection. The VR-Forces default configuration is the "standard" sensor, which can detect any object with a relative signature of 1.0 or greater. Higher relative signatures have a greater probability of detection, so they will be seen sooner. More sensitive sensors could specify smaller relative signature values in their detection probability files.

7.1.5 Adding Sensor Domains

Sensor domains are specified using a string name of the domain, such as "visual." To work, the sensor domain must be listed in all three of the sensor system's components - the target object configuration, the propagator configuration, and the sensor itself. By default, VR-Forces has includes sensor domains for "visual", "radar", "infrared", and "sonar". You can add new sensor domains by choosing a new unique sensor domain name and adding it to the configuration files. For example, to add a new acoustic sensor to a simulation object, so that it could hear nearby simulation objects:

For all simulation object types that would potentially be detected by the acoustic sensor, add a new entry to the "sensor-signatures" block in the object parameter database entry with the name "acoustic" and the base signature value. Modifiers could be added as well.
In the physicalWorldParams.mtl configuration file, add a new entry for an acoustic propagator, using "acoustic" as the signature name.
Add a signature-object-sensor to the sensors list in the object parameter database entry for the simulation object that will detect objects this way. It would specify "acoustic" as its sensor-domain.

If the standard propagator model, or the signature rules defined by VR-Forces are not sufficient, new ones can be defined and added though factories.

7.1.6 The Radar Sensor

VR-Forces provides a radar sensor, which is a specialized version of the signature object sensor. It has all the same functionality of the signature object sensor, plus it publishes an emitter system and one or more emitter beams, which are configured though the descriptor for the sensor.

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