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\subsection{Instrument stability}  Changes in instrument behaviour with time can vary lead to potential biases in the retrieved amounts of the gases of interest. It is therefore important to assess and maintain the characteristics of the instrument over long time periods. In particular, changes in the instrument lineshape (ILS) can affect TCCON retrievals. Hase et al (check exact ref) \citet{Hase_2013}  have studied the impact of instrument misalignment and resulting ILS changes on X$_{CO_2}$; a change in modulation efficiency of x\% 4\%  at 45cm optical path difference (the maximum OPD for TCCON measurements) results in changes in X$_{CO_2}$ of y. 0.035 \%. To remain within a range of 4\% variability in ME at 45 cm OPD, we aim to restrict the modulation efficiency to values between 0.98 and 1.02 at 45 cm.