The Open Sky Detector is a legacy alternative (not recommended anymore) capable of determining whether the smartphone is located "under roof" or "under open sky". This is usually useful if, for whatever reason, you want to compute Outdoor Positions when the user is within the building's canvas in outdoor open spaces ("under open sky"). 

The Open Sky detector determines this by using  SNR (Signal-To-Noise-Relationship) of the GPS signals it receives. A high SNR usually means that the user is outdoors, while a low SNR usually means that the user is indoors.  

This mode is not recommended anymore since configuring an appropriate SNR threshold can be tricky and time consumming. This happens because the SNR readings vary a lot depending on the specific building and smartphone model that you use: therefore, achieving an SNR threshold may not be a feasible task in some cases.