The path to the satellite is mostly not in a vacuum which is why some frequencies are not suitable. For example around 2.4Ghz is an absorption frequency of wate

The path to the satellite is mostly not in a vacuum which is why some frequencies are not suitable. For example around 2.4Ghz is an absorption frequency of water which is why this freq is popular in microwave ovens as well (and thus why they cause interference with WiFi). This is why the 1.6Ghz band is so popular, as it's the least affected. I would imagine 2.5Ghz being not so ideal but Globalstar is a pretty basic repeater system so I imagine they had to separate the input and output frequencies a lot to avoid huge filter systems to avoid it causing interference on itself.