A method of operating an aerosol generating apparatus is disclosed. The method comprises: sensing, at a first sensor of a sensor arrangement, a first indicia of an article comprising aerosolisable medium; sensing, at a second sensor of the sensor arrangement spaced a predetermined distance from the first sensor, a second indicia of the article; determining a distance between the first indicia and the second indicia; and operating the aerosol generating apparatus based at least on the distance between the first indicia and the second indicia.