We present a new forward modelling routine, which can calculate the secondary magnetic field for all times - Also during the transmitter turn-off. This is done by convolving the step response with the time derivative of the system response. The calculations can be compared to the SkyTEM primary field corrected data to obtain a better sensitivity to the near surface, which are probed by the earliest gates in the measured signal from 0 to 10 μs. We describe the implementation of the new routine and compare it to results with the previous routine and to data measured at the Danish test site. The agreement is very good even though the system is described in two completely different ways.