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Surveillance and Communications
Surveillance in this simulation may be handled by either of two RFS radar agent models – SimpleRadar and ATCRadar.  The main difference between the two is that SimpleRadar is updated in such a manner that data on all aircraft are provided simultaneously (omnidirectionally), while ATCRadar incorporates a “sweep”, completing one full rotation of the “beam” in a given number of seconds.  For purposes of this demonstration, we are using SimpleRadar.
The following is excerpted from the Georgia Tech documentations on the radar agents (Kalaver, 2001A; Kalaver, 2001B):
The primary purpose of the Simple Radar Object is to record certain parameters of the aircraft.  Thus, the channel interface is to allow the Simple Radar to create a list of aircraft that need to be tracked.  The Simple Radar returns the parameters of the radar contact at the last time step executed by the Simple Radar Object.
When an aircraft is registered with the Simple Radar Object using the communications interface, a radar contact object is created.  This radar contact is added to a dynamic array that is maintained by the Simple Radar Object.  All the registered aircraft are placed in the pre-dependency list of the Simple Radar Object.  The parameters of all the radar contacts are updated at every time step.
The time step can be treated as the scan rate of the radar.  Thus, the radar can be modeled as emitting an omnidirectional pulse at every time step and updating all the radar contacts.  Similarly, if the simple radar is in the dependency list of another object, say a controller, the time step of the radar can be forced by the dependency and update the contacts when the controller updates.  Thus, the time step can be set to a very high value and the 'effective' time step can be driven by the dependency of another object.
The primary purpose of the ATC Radar Object is to model the sweep of a radar beam and record the positions of aircraft when the beam illuminates them.  Thus, the channel interface is to allow the aircraft to be registered with the ATC Radar, which creates a list of radar contacts based on registered aircraft that need to be tracked.  The last known position of the radar contact can be obtained from radar.
The ATC Radar models the bearing of the beam as a function of time using the initial bearing and a constant angular velocity for the beam direction.  Thus, the bearing of the beam can be obtained at any given time.  Furthermore, the time for the beam to reach a given bearing can also be calculated.  This can be used to estimate the time to contact an aircraft at an arbitrary bearing.  When an aircraft is registered with the ATC Radar using the communications interface, the radar object creates a radar contact object.  The current bearing of the aircraft is used to estimate the next time that the beam will intercept the contact.  Since the velocity of the contact is not used to correct the time of next contact, the time of contact is an estimate.  The time step of the ATC Radar Object is based on next contact time of an aircraft registered with the radar.  The time to contact the next aircraft is set as the required time step.  This aircraft is also placed in the pre-dependent object list.  At the next time step, the radar contact is updated.  The pre-dependent object list is then updated with the next aircraft to be swept by the beam.  When an aircraft is removed from the ATC Radar, it is also removed from the list of radar contacts.
Communications are handled by ATC channel agents (ATCChannel), one for each sector, representing that sector's radio frequency.  Following is excerpted from the Georgia Tech documentation on the communication channel agent (Kalaver, 2001C):
The ATC Channel is able to operate in two modes.  In the default mode, messages are sent immediately.  Thus, the message is transmitted to all the objects that are registered with the channel when the postMessage() function is called.  The message data is not stored locally, which is especially important when transmitting text by using character pointers.  In the second mode, a time delay can be forced on the Channel.  Thus, the message is stored in the channel and only transmitted after a specified time.  Furthermore, other objects can not send messages to the channel until the stored message is sent.  The time delay can be used to simulate the transmission length of the message.  To activate this mode, the data packet used for the message must include a field called "MESSAGE_TIME" which specifies the absolute time at which the message can be sent to the objects on the channel.
　
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