# How to review telemetry data

Telemetry is the data sent by a device to the HERE Tracking API. The data must be accompanied by a valid access token to both authenticate the device and authorize the data storage.

Telemetry can contain GPS position, WLAN, Bluetooth, or cell measurements, along with a `payload` of sensor measurements and a `timestamp` specifying the time when the measurements were taken. The received telemetry is evaluated against the associated [geofences](https://docs.here.com/tracking/docs/geofences), [rules](https://docs.here.com/tracking/docs/rules) and [sensor rules](https://docs.here.com/tracking/docs/sensor-rules). [Events](https://docs.here.com/tracking/docs/events) produced as a result of the rules evaluations are recorded in the Tracking backend.