GrafNav expects the height at the base station to reflect that of the L1 phase center. To facilitate that, we have added antenna profiles generated from NGS. However, to use the NGS profiles, you must measure the vertical height to the ARP. GrafNav does support the slant height measurement, but only if changes are made to the antenna profile to account for this. The antenna profiles contain the L1/L2 offsets with respect to the ARP. So, if you choose to use the slant antenna measurement, then you will need to modify the antenna profile using the information at: http://www.ngs.noaa.gov/cgi-bin/query_cal_antennas.prl?Model=NOV Basically, you need to modify the L1/L2 height offsets to be with respect to the center of the ground plane instead of the ARP. Otherwise, GrafNav will compute the phase centre to be higher than it is. Also, you would need to enter the horizontal distance from center of the ground plane to the edge. Note that GrafNav uses "Pythagoras Theory" to derive a vertical height from the slant height and horizontal distance.