Method of estimating soot using a radio frequency sensor

A method of calibrating a soot load estimating function for a diesel particulate filter uses radio frequency attenuation measurement and temperature measurements. The method comprises identifying a minimum mean attenuation value associated with a standard deviation that exceeds a standard deviation threshold and using this minimum mean attenuation value as a reference value. The method further comprises using a data library that contains gradient values for each of a range of possible temperature values to obtain a first gradient value, the first gradient value corresponding to the first temperature value, wherein each gradient value relates to the gradient of a linear approximation between mean attenuation and soot load at the corresponding temperature. The method involves using the reference value and the first gradient value to determine an axis intercept value for use as an offset value and adopting the offset value as a temperature-independent calibration value for the diesel particulate filter.