The GPP NULL Antenna, also referred to as ADVNULLANTENNA, represents a specialized antenna calibration concept used in GNSS reference station operations and precise positioning applications. This designation indicates an idealized, isotropic antenna model where all calibration parameters are set to zero, essentially a theoretical antenna with perfectly uniform phase center characteristics in all directions.
In precise GNSS positioning, the antenna phase center, the effective point where the signal is received, does not coincide exactly with a fixed physical point and varies with signal direction and frequency. Real antennas exhibit systematic variations in phase center position depending on satellite elevation and azimuth angles, as well as between different signal frequencies. These phase center variations (PCVs) and phase center offsets (PCOs) must be calibrated and applied in precise applications to avoid millimeter to centimeter-level positioning errors.
The GPP NULL Antenna concept originated with Geo++ GmbH, a German company specializing in GNSS corrections and reference station infrastructure. When absolute antenna calibrations are not available or when users wish to process data without applying antenna corrections, the GPP NULL Antenna designation provides a standardized way to indicate that zero calibration values should be used. This is functionally equivalent to assuming an ideal antenna with no phase center variations.
In practice, using null antenna calibrations is appropriate only in specific circumstances, such as comparing relative positioning between identically equipped stations, testing processing software, or situations where antenna calibration data is unavailable but approximate results are acceptable. For precise geodetic work, surveying control networks, and high-accuracy positioning applications, proper antenna calibrations (absolute or relative) from antenna calibration databases should always be applied. The ANTEX (Antenna Exchange) format maintained by the IGS provides calibration data for most geodetic-grade antennas used in the GNSS community.
