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have you tried harrys gps app? years ago I had the intercomp version which was not cheap. those things are a pain in the ***
I tied Harry's a few years back and could only get it to register laps about half the time. it had issues with the start/finish line. maybe it has gotten better since then.
It's dependent on your hardware, lots of people use Harry's with great success. Add a 10hz gps unit and you can get really accurate laps and lots of useful data.