A medical system comprises a probe comprising a terminal distal end. The system further comprises a first coil in the probe and comprises a guide instrument including a terminal distal end and defining a lumen sized to guide the probe. The probe can be inserted through the lumen to reach a worksite. At the worksite, the terminal distal end of the probe is configured to reach at least the terminal distal end of the guide instrument. The system further comprises a sensor embedded in a wall of the guide instrument and comprises processing hardware configured to receive a first induced signal from the first coil and to receive from the sensor an indication of a pointing direction of the guide instrument. The processing hardware is configured to use the first induced signal and the indication of the pointing direction to determine a roll angle of the probe.