South Africa - Inkaba.org

High-precision steering and pointing control of a satellite/lunar
laser ranging telescope
G.L. Nickola 1 and W.L. Combrinck 2
1. University of South Africa (UNISA) and Tshwane University of Technology, South Africa,
lnickola@unisa.ac.za
2. Hartebeesthoek Radio Astronomy Observatory (HartRAO) Space Geodesy programme, South Africa,
ludwig@hartrao.ac.za
ABSTRACT
A Satellite Laser Ranging System (SLRS) enhanced for Lunar Laser Ranging (LLR) is being developed as part of
geodetic instrumentation to be located at a new fundamental space geodetic station for South Africa.
The project under discussion here, will involve the development of high-precision steering and pointing control for
the LLR telescope. The role of the pointing system is to safely point the telescope towards the desired target and to
track the target in spite of internal and external disturbances. With the need for high-precision accuracy and
regulation, feedback control is required. The basis of a telescope’s closed-loop system consists of continuously
comparing the actual pointing direction to the desired pointing direction and feeding back any difference to the
drive system for correction.
An emphasis on safety aspects will be the most important design consideration of all. This will include hard-wired
safety interlocks as well as software protection for humans as well as the equipment. The mechanical design of the
telescope will determine some of the control design criteria. Compensation for mechanical behaviour will be
required in the control software.
National Instruments (NI) LabVIEW is a graphical programming language that is ideal for use in developing
engineering applications. LabVIEW programs use virtual instruments which have the look and feel of physical
systems or instruments. A working model using LabVIEW and an NI CompactRIO interface will be developed. NI
9514 motion control modules with encoder feedback will be used for control of the telescope’s two axes (azimuth
and elevation). An NI 9401 8-channel input/output module will be used for other circuitry. Vagon AC-drive units
with 3-phase asynchronous motors will be used to drive the model.
KEYWORDS: lunar laser ranging, telescope control.
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