Apple Seeds Sixth Beta of iOS 11.3 to Developers [Update: Public Beta Available]

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For what it's worth try "Reset all settings". i.e. General --> Reset --> Reset All settings. Worked for me to 'fix' GPS issues introduced after update to 11.2.6 on an iPhone 6S. Here's the relevant Apple Discussion thread where I found that particular fix.
https://discussions.apple.com/message/33200392?ac_cid=tw123456#33200392

Thanks! For me the issue is intermittent. I went for several weeks without it, but this week it's back with a vengeance. I might try the full reset later, but for now I'll just plod along, hoping that a future OS updated makes a difference.