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Three different systems are typically used to control movement
of the antennas: Equatorial, Azimuth-Elevation, and X-Y axis.
Equatorial systems are found
on many of the early antennas and have the simplest operation.
This design is dependant upon the site’s position relative
to the Earth’s equator. The original antenna on site had
its turning structure set 35o from the horizontal, as Canberra
is 35° south of the equator. Once in position, the antenna
only had to move in one direction to track the spacecraft. This
system requires large counter weights and is difficult to build
for a very large antenna.
The Azimuth-Elevation system
used for the later antennas is a symmetrical design that does
not rely on the antennas location on the Earth. This design requires
two drive systems working simultaneously to move the antenna
while tracking.
For an X-Y axis system of movement
the X-axis of the antenna is aligned with the North-South compass
direction, while the Y-axis is aligned with the East-West. Like
the Azimuth-Elevation antenna, this antenna requires both drive
systems to work simultaneously to track a spacecraft.
Drive systems also vary between antennas. The 34-metre antennas
are driven by electric motors, while 70- and 26-metre antennas
are driven hydraulically.
The 70-metre antenna, due to
its very large size, rotates on an unusual azimuth bearing system,
supported on a film of oil only 0.17mm thick. This is called
a hydrostatic bearing and allows the massive structure to rotate
smoothly.
The antenna’s tipping
structure is counter-balanced with large blocks of lead and steel
to reduce torque on the elevation drive. |
