Is Collector for ArcGIS capable of using an external GNSS receiver connected to an iPad to obtain more accurate results?
I have successfully connected a Garmin GLO via bluetooth to an iPad. It worked great.
Yes, there are several products available. I use the Bad Elf GNSS Surveyor with 1 m accuracy. There are others that can serve your needs out there a google search can get you on the right track.
Retrieving data ...