As the RPi doesn't have a system clock you will need to make sure you update the time when you start up (i.e. it doesn't have a clock powered by a small battery when there is no power at all to the RPi). While I don't run StellarMate I do run my KStars on a RPi and even at home I have issues with WiFi and the RPi has issues getting a GPS fix until I get its time synced. I just use my mobile phone, set up a WiFi hotspot, and have the RPi connect to it. Once the RPi has connected I get a GPS fix within seconds with my GPS dongle. The reason for all that, having the RPi connect to the phone allows for the time to be synced.

You may not need the GPS fix to get the right location if you're just setting up a location in KStars but you will need the date and time up to date so you can point your OTA in the right direction. As Jasem says, you don't need the tethering once it has got the GPS fix similarly it doesn't need the the tethering once it has the correct date and time.

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