Albeit almost insignificant in comparison, there are records with more than 90% missing values.
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The sentences
It is important to note that the number of active ground stations that had been continuously decreasing since the late 1960s had a relatively recent change point in 2001, reaching a maximum of 42 270 simultaneously active ground stations in 2012, and, regrettably, went back into a decreasing trend ever since (Fig. 1 ).