In episode 9 of Pro Running News we discuss the topic of GPS and answer the questions: How are marathon courses measured? How are marathon courses run? How does GPS work? Show Notes: Dissecting GNSS systems  https://blog.bliley.com/the-differences-between-the-5-gnss-satellite-network-constellations https://www.gps-repeaters.com/blog/gnss-vs-gps-whats-the-difference/  Comparing GPS to Other GNSS https://www.educba.com/gnss-vs-gps/ Why GPS is sometimes inaccurate https://www.gps-repeaters.com/blog/four-reasons-why-gps-is-sometimes-inaccurate/  Info on the GPS system https://www.gps.gov/systems/gps/performance/accuracy/  DC Rainmaker Breakdown of Marathon Course Errors https://www.dcrainmaker.com/2009/03/racing-line-understanding-how-courses.html  Breakdown of Marathon Course Measurement https://www.justrunlah.com/2014/08/10/how-are-long-distance-race-courses-and-marathons-measured/