The Geo++® GNPCVDB allows the access to phase center offsets and variations (PCV) type corrections of GNSS antennas calibrated by the Automated Absolute Field Calibration. The PCV type means are computed from all calibrations conducted for the particular antenna type at Geo++.
GNPCVDB can be used as a html applet, which allows access to the database within a browser.
Open the GNPCVDB database.
Access to general information of the database is free. For the download of actual PCV-files a 3-year-licence must be aquired. Buy your licence at firstname.lastname@example.org.
Automated Absolute Field Calibration
The Automated Absolute Field Calibration of GNSS antennas has been described in several international publications. Look at the internet publication AOAD/M_T Choke Ring GPS Antenna Absolute Phase Center Variations, Results of Absolute PCV Field Calibrations at IfE and Geo++ ® or in the publi of Geo++® for details.
For a quick introduction refer to PCV Calibration of GPS Antennas – General Overview and the Absolute PCV Field Calibration – Basics and Procedure. or Method of Absolute Antenna Calibration.
The Automated Absolute Field Calibration gives very precisely the absolute antenna phase variation of one individual antenna construction (i.e. with radome, additional ground plane, attached radio link antenna, mount, etc.). A complete model for the antenna phase variation are elevation- and azimuth dependent phase variations, commonly with mean offsets referring to the antenna reference point (ARP).
There is very often the question for absolute type means. Generally, individual calibration are the best representation of an antenna for high precise applications, but depending on the actual application or other constrains a type mean is also useful or the only way to correct for this kind of error source. This is served by the GNPCVDB database.
The type means of GNPCVDB are computed from a rigorous adjustment of individual calibrations using the complete variance-covariance information obtained from the Automated Absolute Field Calibration.
The GNPCVDB contains the type means of antennas calibrated with the Automated Absolute Field Calibration at Geo++. To give an overview of the available antenna types refer to the table of currently available antenna types. There are some type names, which are not part of the official IGS naming convention. These names are reported to the IGS, but it takes time to accept and incorporate them into the offical naming (ASCII file rcvr_ant.tab). The type names defined by Geo++ are indicated in GNPCVDB. A list is available as an ASCII file gpp_rcvr_ant.tab, which lists all proposed and declined names.
The history of GNPCVDB is given in history.txt.
The access to the database is via the internet on a GNPCVDB server. The GNPCVDB server gives general information
- antenna reference point (ARP)
- north direction definition
- graphs of antenna phase variations (L1, L2, L0)
- standard deviation of type mean (L1, L2)
- digital photos of antenna (top, side, sub)
- and more
to everybody. Refer to the public GNPCVDB area. But the actual numerical phase variation corrections (in different formats, absolute and conversions to relative phase variations) are only accessible to licensed GNPCVDB users.
Full Access to GNPCVDB
Full access to GNPCVDB including download capability is possible for a small charge:
|GNPCVDB Antenna PCV database access for 3 years||125 Euro|
The price does not include any delivery or taxes. You will get an account with user name and password after you have signed the terms and conditions of the GNPCVDB licence. The license terms and conditions are available in german and english as writeable pdf version. Additional a small size pdf also in german and english.
Need and Models for PCV Corrections
It is well known that a physical GPS antenna does not have a “phase center”, because a “phase center” would mean in theory that pure spherical wave fronts are transmitted from this center. Since the wave fronts are not spherical there is no pure phase center.
A first approximation is a “mean phase center”. However, such a mean phase center needs a clear definition and it is generally not sufficient to describe the complete phase variations. Therefore, a phase center offsets and phase center variations (PCV) are used to describe the variations. Actually, the term PCV is misleading, because it suggests a varying center with spherical wave fronts. An expression like “antenna phase variations” (APV) is more precise. A complete model for the antenna phase variation is obtained by phase center offset and elevation- and azimuth dependent PCV. It is important, that the antenna phase variation always comprised of a unique and consistently determined set of mean phase center offsets and phase variations.
Currently three levels of PCV corrections are commonly used, which do differ significantly in the accuracy to represent an individual antenna:
- (mean phase center offset,) elevation- and azimuth dependent PCV
- (mean phase center offset and) elevation dependent PCV
- mean phase center offsets
The mean phase center offsets is not essentially required as the effect can also be described by the elevation- and azimuth dependent phase variations. A there are no exact definitions for the correction parameters, the actual numbers are generally not comparable between different calibration procedures. The definition for the mean phase center in the Automated Absolute Field Calibration is, that the square sum (integral) of the antenna phase variations over the (half) antenna hemisphere is minimized. Only calibration procedures, which use the same definition and are site independent can yield comparable results.
It is recommended to use absolute PCV to avoid systematic errors and to use elevation- (and azimuth) dependent PCV to have sufficient accuracy. The use of solely offsets, however, is not adequate for geodetic applications. It is necessary to use offsets and PCV (absolute phase variations), while a complete correction consisting of offsets, elevation- and azimuth dependent PCV is the best way.
Format of PCV Corrections
Currently, there exists only a few PCV correction formats, which allow to describe antenna PCV by a complete model of elevation- and azimuth dependent variations. Refer to the updated description of the Geo++ PCV Antenna File Format.
Refer to Standard and Extension of Antenna PCV Exchange Formats for a discussion on commonly used internal format for PCV corrections.
GNPCVDB provides complete information of the antenna types using the Geo++ PCV Antenna File Format consisting of
- mean offset
- elevation- and azimuth dependent phase variations.
To allow the use in software without the capability to handle a complete model, also pure elevation dependent corrections are provided, which do have the drawback of a possible significant loss of accuracy. The pure elevation dependent corrections are also available in the commonly used IGS/NGS format.
Conversion of Absolute to Relative PCV Corrections
For the conversion of absolute PCV corrections to relative PCV corrections it is essential to know the phase variations of the reference antenna used in relative calibration procedures. One individual antenna of such an reference antenna type has been calibrated extensively by Geo++ and IfE. The results are documented in AOAD/M_T Choke Ring GPS Antenna Absolute Phase Center Variations. Refer especially to Absolute PCV Calibration of AOAD/M_T – Details, Accuracy, Precision.
There exist additional problems with the conversion between absolute and relative PCV level, and vis versa, due to missing definitions. Today, it must therefore recommended to restricts the use of PCV corrections to one consistent source, level if a transition to another level is available. It is not advisable to mix calibration results from different procedure or even sites.
GNPCVDB contains also the Geo++ NULLANTENNA. The Nullantenna is a convenient way to use relative and absolute PCV with currently existing relative PCV tables. Thus, a GPS user who wants to stay with relative PCV corrections can restore the missing absolute information by correcting for the Nullantenna type. Refer to Definition of “Nullantenna” and Antenna Type “Nullantenna”. There exists currently no exact definition and exact use of the relative reference antenna AOAD/M_T. Therefore, slightly different numerical values for the Nullantenna are possible, which differ between 0 and 0.6 mm in the PCV corrections for L1 and L2. Refer to the publication list of Geo++® for more details on this topic.
GNPCVDB Application Fields
Refer to Discussion of Absolute PCV – Effects, Large Networks, Engineering Applications. For high precise applications, an individual calibration is highly recommended. Individual calibrations are done routinely by Geo++/GeoService using the Automated Absolute Field Calibration. However, very often the individual antennas are not accessible for an individual calibration or the accuracy requirements make the use of type mean or pure elevation dependent type means sufficient.
Type means of phase variation corrections from GNPCVDB can be used for a wide field of applications, for both:
- post processing
- real time RTK systems