NOAA became aware of an opportunity to upgrade the control systems on these antennas through communications with the administration at PARI. The PARI staff were looking for spare parts for their two antennas since PARI had decided to upgrade their antennas and use the antennas for radio astronomy. DFM Engineering was contracted by PARI to rebuild the drives and provide modern controls for the antennas to allow them to track slowly and smoothly enough for stellar observations-a function that the original control system and drives could not perform.

The antennas were originally powered using hydraulic motors. At the time the antennas were built, electric servo motors with sufficient power and dynamic speed range did not exist so hydraulic motors provided the power and the variable speed needed for the antennas to track satellites. The variable fluid flow at high pressure was provided using variable displacement hydraulic pumps driven by constant speed AC motors. A hydraulic system allows placing high force, or torque in this case, into a limited space. A hydraulic system normally has a rather low efficiency. It was common for one of these antennas to require over 300 horse power resulting in high electrical connect and usage costs, especially considering that a 3-phase AC induction motor requires a starting current of about 3 times the running current.

The new drive system consists of two 15 hp modern AC servo motors per axis, so there is a total of 60 hp required by the antenna. The servo motors and their controllers are also "soft start", so the starting current is less than the running current. Additional gearing between the motors is necessary to match the electric servo motor speed characteristics to the existing antenna gear reduction units.

The increase in efficiency by using the modern servo motors will result in a considerable reduction in the costs to operate the antennas. At the Fairbanks site, the electrical connect and usage cost savings are estimated to pay for the upgrade in a few years.

The new electrical servo motor drive system also has favorable environmental effects. The noise is highly reduced as the new servo system is very quiet. Also, the hydraulic system components are nearing their useful life. If a hydraulic component has a catastrophic failure, it could very easily result in a massive spill of hydraulic oil requiring a very expensive cleanup.

The new drive system will be controlled by an industrial PC. The PC allows automatic downloading of satellite ephemeris data in three different data formats from web site databases. The software propagates the satellite ephemeris data and converts it to the proper mount angles and velocities to drive the antenna to track the desired satellite. The antennas can track not only geosynchronous satellites (which is very easy), but can also track polar low orbit satellites and satellites that are in very high and elongated orbits. The control system also allows auto tracking where position information from the radio signal is used to provide position corrections to the antenna drives.

The NOAA control system builds on DFM Engineering's experience of driving the antennas at PARI to acquire and track stars. The software will be expanded to allow tracking satellites and incorporates other features to interface with the existing station operations. DFM Engineering was chosen because of our proven capabilities of providing control system retrofits to similar antennas and optical telescopes. The DFM proposal was deemed to be technically superior to the other proposals received and also was priced slightly lower in cost.

The Wallops NOAA Station antenna upgrade will be installed in April, 2003.
The Fairbanks NOAA Station installation will be performed in August, 2003.

Related stories and products:

• DFM Control System Upgrade at PARI

• DFM TCS (Telescope Control System)

Related Sites:

• PARI (Pisgah Astronomical Research Institute)

• NOAA (National Oceanographic and Atmospheric Administration)

• Wallops NOAA Station

• Fairbanks NOAA Station