A method of using a radio frequency sensor for determining an estimate of soot load in a diesel particulate filter, the method comprising: receiving a first mean attenuation value derived from attenuation values for each of a first plurality of radio frequencies within a first band of radio frequencies detected by the radio frequency sensor; receiving first standard deviation data for the mean attenuation value relating to the first plurality of radio frequencies within the first band of radio frequencies; determining whether the first standard deviation data exceeds a standard deviation threshold; (a) in the event that the first standard deviation data does not exceed the standard deviation threshold: using the first mean attenuation value to infer a value for soot load in the diesel particulate filter; (b) in the event that the first standard deviation data exceeds the standard deviation threshold: receiving a second mean attenuation value derived from attenuation values for each of a second plurality of radio frequencies within a second band of radio frequencies; receiving second standard deviation data relating to the mean attenuation value for a second plurality of radio frequencies within the second band of radio frequencies detected by the radio frequency sensor; and in the event that the second standard deviation data does not exceed the standard deviation threshold: using the second mean attenuation value to infer the value for soot load in the diesel particulate filter. receiving a first mean attenuation value derived from attenuation values for each of a first plurality of radio frequencies within a first band of radio frequencies detected by the radio frequency sensor; receiving first standard deviation data for the mean attenuation value relating to the first plurality of radio frequencies within the first band of radio frequencies; determining whether the first standard deviation data exceeds a standard deviation threshold; (a) in the event that the first standard deviation data does not exceed the standard deviation threshold: using the first mean attenuation value to infer a value for soot load in the diesel particulate filter; (b) in the event that the first standard deviation data exceeds the standard deviation threshold: receiving a second mean attenuation value derived from attenuation values for each of a second plurality of radio frequencies within a second band of radio frequencies; receiving second standard deviation data relating to the mean attenuation value for a second plurality of radio frequencies within the second band of radio frequencies detected by the radio frequency sensor; and in the event that the second standard deviation data does not exceed the standard deviation threshold: using the second mean attenuation value to infer the value for soot load in the diesel particulate filter. (a) in the event that the first standard deviation data does not exceed the standard deviation threshold: using the first mean attenuation value to infer a value for soot load in the diesel particulate filter; (b) in the event that the first standard deviation data exceeds the standard deviation threshold: receiving a second mean attenuation value derived from attenuation values for each of a second plurality of radio frequencies within a second band of radio frequencies; receiving second standard deviation data relating to the mean attenuation value for a second plurality of radio frequencies within the second band of radio frequencies detected by the radio frequency sensor; and in the event that the second standard deviation data does not exceed the standard deviation threshold: using the second mean attenuation value to infer the value for soot load in the diesel particulate filter. receiving a second mean attenuation value derived from attenuation values for each of a second plurality of radio frequencies within a second band of radio frequencies; receiving second standard deviation data relating to the mean attenuation value for a second plurality of radio frequencies within the second band of radio frequencies detected by the radio frequency sensor; and in the event that the second standard deviation data does not exceed the standard deviation threshold: using the second mean attenuation value to infer the value for soot load in the diesel particulate filter.