An electronic toll collection transponder containing an orientation sensor for measuring the orientation of the transponder is described. The transponder measures its orientation and stores orientation data in memory. The transponder may report the stored orientation data as part of a response signal sent to the ETC system in reply to a trigger or polling signal. The transponder may assess whether it is correctly oriented based on a comparison of the orientation data to ranges or thresholds and may output an indicator of incorrect orientation or may disable communications with the ETC system during incorrect orientation.