The invention relates to a gas analysis based on the mobility of ions. When the signal quality is maintained at a stable level, subsequent power control signals including information andor data are transmitted to the terminal. A least squares technique may be employed to minimise the time averaged error between signal reproduced at the intended position of a listener and the desired signal, or it may be applied to the frequency domain. When the counter value equals or exceeds the threshold, the system transmits the data. The shape is designed to avoid discomfort associated with known strips when they are peeled away from the membrane areas immediately below the eye.