Straight line travel margins of error were reported specifically as 1.9% and it was a direct quote from the researchers.
They were assuming that it was a Round Earth Radar Test, as opposed to a Flat Earth Radar Test. They were comparing the Round Earth coordinate devices (GPS) to the Radar Test values, without knowing which shape of the earth they were on. The distance of a mile would measure differently on a Flat Earth vs using a Round Earth lat/lon coordinate system.
Since the GPS values were giving a wide range of answers we do not know which one is the most accurate, or how accurate they were, if the radar test was done on a Flat Earth.