deg. There is a thin plastic sheet over the flare opening to seal the antenna against moisture. The
GPRI-II antennas and support is designed to fit in a 125 mm PVC shipping tube.
The antennas are supported on a carbon-fiber truss structure as shown in Figure 23. The carbon tubes
are symmetrically positioned about the waveguide so there is access from -45 degrees to +45 degrees
in elevation.
4.2.3 Mechanics
GPRI-II Antenna Tower
The antenna support tower has a height of 80 cm and a square cross-section of 28 x 28 cm and is
constructed using ITEM aluminum extrusion. The tower is further stiffened on each face through the
use of stainless-steel rods under tension connected to a central stainless-steel ring as shown in Figure
10. These cross-struts stiffen the tower to reduce torsional deformation.
Tripod, Positioner and Tribrach Leveler
The tripod supporting the scanner and antenna tower is a Heavy-Duty Leica tripod. It comes with a 5/8”
threaded screw that is standard and is adjustable to permit repeatable positionig. A small bubble level
embedded in the tripod mounting plate is used to ensure that the tripod itself is level within a few tenths
of a degree. The tripod can be leveled approximately by moving out the legs. The feet of the tripod
have been modified so that there is an aluminum bracket that can be fixed to the ground using a screw
in an anchors (Figure 24). This arrangement permits accurate repositioning of the GPRI-II for later
data acquisitions. A small laser on the tribrach rotational axis can be used to see the rotation axis as
projected on the ground. Additional metal struts have been added at the tripod base to improve the
rigidity and to permit precise repositioning of the tripod on preexisting anchors.
Figure 23: GPRI-II Tower with RF electronics.
A rugged custom tribrach supports the tower and is used to adjust the axis of rotation to be vertical. The
tribrach incorporates a small laser for centering the tower axis of rotation about a benchmark. In
addition a heavy-duty adapter is provided to mount the tribrach on geodetic monuments or other
structures (Figure 25).
