Step 2: Measure Antenna Height
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P/N: 1004636-01
System Setup
Step 2: Measure Antenna Height
The location of the antenna relative to the point being measured is very important for both surveys in which the
elevation of the point is important and in surveys for horizontal location only. Horizontal surveys are often larger
in area than can reliably fit on a flat plane; therefore, the antenna adjustment must be done in three dimensions
and then projected onto a two dimensional plane.
The receiver calculates the coordinates of the antenna’s phase center. To determine the coordinates of the
station marker, the user must specify the following:
•
Measured height of the antenna above the station marker
•
Method of measuring the antenna height
•
Model of the antenna used
Antennas have two types of measurements:
•
Vertical – measured from the marker to the antenna reference point (ARP) located on the bottom of the
antenna at the base of the mounting threads.
•
Slant – measured from the marker to the lower edge of the antenna slant height measure mark (SHMM).
The point to which all measurements are being referenced is called the Phase Center of the antenna. This is
analogous to the point at which a distance meter measures in a prism. A user must enter the prism offset to
compensate for this point not being at a physical surface of the prism. For a GNSS antenna, the offset is entered
depending on the type of measurement taken.
•
For vertical, the offset is simply added to the measured vertical height to produce a “true” vertical height.
•
For slant height, the vertical height must first be calculated using the radius of the antenna, then the offset
can be added.
The offsets are different because of the difference in location between the slant measuring point and the vertical
measuring point.
1. Measure the antenna height above the control point or marker, either the slant height or the vertical height
(
Figure 31).
2. Record the antenna height, point name, and start time in the field notes.
The height of the antenna and it’s offsets depend on the type of antenna used. Refer to the antenna’s
documentation for details.
Figure 31: Measure Antenna Height – Example
Refer to the antenna’s offset measurements card for measurement and offset details.
Reference Surface for
Oset Measurements
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