Definitions and comparison
The standard definition of rise time calculation (T10-90%) is acceptable for a standard exponentially rising waveform, but for current waveforms measured in instrumented towers, with oscillatory early time behavior, this measure becomes dependent on the characteristics of the oscillation. An alternative definition of rise time called “derivative rise time” used in UWB antennas is given by tr=max(di/dt) / Ipeak , having the advantage that a direct measure of the current derivative is available.
The two techniques are compared by plotting along its results on the Säntis dataset. Furthermore the data of maximum current derivative and current peak are included (color, size respectively) and its effect can be filtered and seen (click on the labels).
 E. G. Farr, C. E. Baum, and C. J. Buchenauer, “Impulse radiating antenna, part II”, Ultra-Wideband, Short-Pulse Electromagnetics 2, pp.159 – 170 , 1995. :Plenum
Another useful view of the two techniques is shown on the following plot where, additionally the maximum current has been mapped as a third variable (color). Notice the difference on the dispersion of the two measures.