20. Should I use the positional coordinates for the L1 phase center or those for the antenna reference point (ARP)?

The L1 phase center is the theoretical point in space where the L1 carrier phase is received "on average." The actual location where this signal is received, however, varies as a function of the direction of the incoming GPS signal, and hence an averaging process is required.

The antenna reference point (ARP) is a specified physical point on the antenna.

The relative spatial relationship between these two points is determined via a calibration process in a laboratory-type environment. This process involves collecting and processing several hours of GPS data, and it involves several assumptions about antenna characteristics.

The National Geodetic Survey (NGS) has calibrated many antennae and has determined the average spatial relationship between these two points for each of several classes of antennae. See http://www.ngs.noaa.gov/ANTCAL

For sites in the CORS system, NGS provides spatial coordinates for both the L1 phase center and the ARP, as different GPS-processing software packages may use one or the other of these points or possibly even some other reference point.

To find out whether your GPS-processing software uses the L1 phase center or the ARP, you need to contact the company that produced this software.

NGS encourages GPS companies to have their software use the ARP, but our agency can not require this practice.