CEMS operate for longer durations but currently suffer several issues: (a) estimates are highly uncertain and do not converge to actual emissions over extended periods , (b) systems have extended 110 periods when estimate accuracy drops due to relative positioning of sensors and current wind direction, and (c) observations may be impossible due to lighting, topography or weather conditions. While CEMS may eventually develop to the point where a combination of such systems would provide data equivalent to the best BU inventories, data indicates such results will take years to develop, and increasing sensor count to avoid periods where weather variability prevents the system from quantifying emissions may create cost issues.