In the IRIS forums at DIYDronres, Ardupilot, and the IRIS Facebook group I have seen reports from various users who have complained about low HDOP (Horizontal Dilution of precision) reported by their IRIS when trying to arm in Loiter or Auto mode (Both are GPS assisted modes). In these cases the HDOP value determined and reported by Arducopter is higher than 2.0 after powering up IRIS. This value is higher than the default value of 2.0 for the GPS_HDOP_GOOD paramter value. As a result the pre-arm checks fail and IRIS is not able to arm in a GPS-assisted flight mode. …