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\subsubsection{Laser Sampling Errors}  *** Probably doesn't need to be its own subsubsection ***  \citep{Messerschmidt2010} \citep{Messerschmidt_2010}  showed that the Bruker IFS125HR instruments used at most TCCON sites could be affected by laser sampling errors caused by the presence of a faulty electronics board. The fault caused the rising and falling zero-crossings of the laser signal to be sampled unevenly in time, and therefore in optical path difference. The result is that in the InGaAs region some spectral information from above the Nyquist frequency (7899 $^{-1}$, half the laser wavenumber) is folded into the spectrum below this frequency, and vice versa. This results in "ghosts", which can impact on the retrieval of species of interest. The faulty electronics boards were replaced by an updated model at most TCCON sites during 2011. At Wollongong, this occurred on July 22, 2011. The new model (ECL04) allows the ghosts to be minimised via adjustment of a potentiometer; this is regularly monitored, and has not affected the Wollongong data since the board change.