2 Operation Details
2.5
Antenna Effects
To achieve the highest positioning precision, it is essential to take antenna effects into ac-
count.
PC
ARP
Marker
Figure 2-3:
Antenna mount.
The GNSS measurements (pseudoranges and carrier phases observables) refer to a theoret-
ical point in space called the phase center (noted PC in Figure 2-3). The position of this point
is dependent on the elevation of the satellite and on the frequency band. It varies with time
and it is different for the different GNSS frequency bands. The phase center variation can
reach a few centimeters.
If no correction is applied, the computed position refers to an average phase center with no
easy link with the antenna physical element. This average phase center fluctuates with time
and cannot be used for accurate millimeter-level positioning.
For high-precision positioning, the GNSS measurements need to be corrected in such a way
that they all refer to a common and stable point in space. That point is referred to as the
antenna reference point (ARP). For convenience, it is usually selected at the center of the
bottom surface of the antenna. PC to ARP calibration tables are available on Internet for a
large number of geodetic-grade antennas. For example, the National Geodetic Survey (NGS)
publishes calibration tables that can be downloaded from the following URL:
https://www.ngs.noaa.gov/ANTCAL/
.
The antenna naming convention in such table is the one adopted by the IGS Central Bureau.
The receiver has a similar table in its non-volatile memory. This table can be upgraded follow-
ing the standard upgrade procedure as described in section 1.32 (the upgrade file is named
ant_info.suf
).
2.5.1
Antenna Effects in Rover Mode
If the user specifies the type of his/her antenna using the
command,
the receiver compensates for the phase center variation in all rover positioning modes. If the
antenna is not specified, or the antenna type is not present in the built-in antenna calibration
66
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