The IGS (International GPS Service) early recognized the importance of clarification of antenna types and receivers for a rigorous processing of data from networks with mixed antenna and receiver types. The clarification attemps are commonly accepted, used and referred to by other GPS users. Even real-time applications are using or intend to use the IGS tables (e.g. Radio Technical Commission for Maritime Services RTCM). Hence, the rcvr_ant.tab is generally used to define unique receiver, antenna and radome names.
However, the extented use from post-processing to real-time applications may make some enhancements necessary. This also makes it requisite for IGS to consider the requirements of real-time applications.
Beside the naming convention, the correction of phase center variations (PCV) is essential for mixed antenna use. The problem of PCV is not only restricted to large (global) network, even engineering applications with inclined antenna are faced to it. Thus, the PCV definition and correction do have an impact on regional reference station networks, real-time and precise engineering applications.
There are currently two international and freely accessible sources of PCV, IGS and NGS (National Geodetic Service), which use similar, but not identical ASCII-formats:
The current progress in antenna calibrations and the discussions on PCV, especially in the german speaking areas (documented by two antenna workshops 1999 in Bonn and 2000 in Hannover) show that there is a vital demand for a unification and enhancement of the currently used PCV formats.
Our investigations of the international formats IGS and NGS indicated, that the numerical Offsets and PCV-values do have the same format, but the descriptive line for an antenna type is somehow different. However, it is a convention, which works quite well, but some improvements and extensions are required in order to define a more precise standard for a PCV exchange format.
From this, we suggest to define a proper format and standard, which can account for the existing PCV and should be extended by some basic information. To be discussed:
The enhancements are needed as more individual calibrations are used to correct PCV, absolute and relative PCV are used, and azimuthal PCV sometimes show even larger magnitude than the elevation dependent PCV. There will also be relative and absolute PCV corrections, which must enable all users for post-processing and real-time application to use and to combine both calibration types.
An easy way for the use of relative and absolute PCV with the currently existing relative PCV tables is the definition of the Nullantenna and the antenna type Nullantenna relative to the IGS/NGS reference antenna.
Another important aspect is the antenna reference point (ARP) for PCV. The ARP must be known exactly to relate offsets and PCV correctly to an antenna. Generally, the ARP is designated to the lowest not removable level (i.e. bottom) of the physical antenna. However, the ARP is not a unique height reference for all antenna types. Therefore, an ASCII file (antenna.gra) with the dimension of individual antenna types and the ARP (defined by IGS) is in use. In addition, NGS has established a data base of digital images of all calibrated antennas at NGS, which is also a good reference for an ARP definition. Nevertheless, the ARP has also to be specified in a more strict and unambiguous way.
With the above proposal it is intended to start
a discussion on a more precise standard of antenna PCV exchange format.