| A circular auto-correlation
of power vs. time produces this plot. From the autocorrelation we
can extract many more details about our signal. The 1/e width- the
time for the lightning signature to reach 36.79% of its peak value- is
found. If this value is less than 10 microseconds, we look further
in time for a secondary echo.
The local maximum after the 1/e time value is considered the secondary echo - even though it may not be an actual echo. By computing the Signal-to-Noise Ratio (SNR) of this local maximum, we can determine whether or not we will consider it a secondary pulse. The criteria is for SNR to be greater than or equal to 5 dB. SNR is calculated using 10*log(secondary peak/background noise). Background noise is considered the average value of the last 10% of the autocorrelation function). |
