Teleporter

Use case: Teleporting an object away
1. Bob places the item on the teleporter pad.
2. Bob uses the directory to find the teleporter that is closest 
   to the desired location.  He can do this through the console.
3. Bob selects that teleporter as the destination.
4. Bob uses the console to initiate the teleportation.
5. The teleporter teleports the object.
   5.1.a. The console reports that the destination pad is occupied.
   5.1.b. The console reports that the destination unit is malfunctioning.
   5.1.c. The console reports that the destination unit is missing.
6. The console reports that the item arrived safely.
   6.1.  The console reports that the item was not teleported.

Use case: The position of a teleporter unit changes

Requirements:
- A teleporter unit and pad can be transported in a large backpack.
- Units are identified by 4-digit codes.
- Locations are specified as GPS coordinates.
- Each unit knows its own location and broadcasts it to other units 
  via a satellite.
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Sketch a UML class diagram for the teleporter system based on the above.