Trackers
Finding models
Models in duration competitions can gain significant heights and fly long distances, especially if it is windy. Trackers can help find a model that has flown out of sight.
Radio beacons
The original trackers were based on wildlife trackers, using omnidirectional radio beacons on the model and a directional antenna to locate the model. They are relatively simple and have better range, but objects between the model and the receiver can make location difficult.
GPS systems
A significant step forward has been devices that not just send out a signal, but can give their position via GPS. The position is given on a handset or phone app, guiding you and allowing you to walk directly to the plane. I use the BMK B1/B2 device.