[Sasha]

To use triangulation, you need to be in a position where you get signal from both beacons. Since each of them is supposed to be set to 180 degrees mode, facing in exactly opposite directions, I do not see how this is possible.

Using signal strength is not very reliable either - for example, it depends on the battery level of the beacon.

If you just need the distance to the central structure, ultrasonic seems like the easiest choice. If you also need to determine the orientation of the central structure, this can be done using only one beacon. Our strategy is to have the robot circling the structure and stopping when the ir sensor sees the beacon from the right direction