978-1-4673-1813-6/13/$31.00 ©2013 IEEE 1 Design and Characterization of an Antenna Pointing Mechanism for On-orbit Servicing Missions Ralf Purschke Institute of Astronautics Technische Universität München Boltzmannstrasse 15 85748 Garching, Germany +49 89 289 16029 r.purschke@tum.de Alexander Hoehn Institute of Astronautics Technische Universität München Boltzmannstrasse 15 85748 Garching, Germany +49 89 289 16012 a.hoehn@tum.de Abstract— The goal of this work was to (1) define parameters to characterize a pointing mechanism, (2) design a setup to test these parameters and, (3) verify the test methods by comparing the results to the theoretically calculated or independently verified numbers. The verification of the test results was conducted with an in-house built Antenna Pointing Mechanism for on-orbit servicing applications. The test setup was developed to find a method to measure the behavior of a pointing mechanism. This was realized by mounting a Laser pointer on the antenna interface of the mechanism and pointing it towards a two-dimensional Position Sensitive Detector, providing means to resolve small motions, and to derive velocity and acceleration of the mechanism. The results show good correlation for characteristic parameters such as pointing velocity and acceleration, repeatability, resolution and pointing accuracy of the mechanism. In future work this test method will be qualified for and used to compare the performance of the mechanism at different environmental conditions such as vacuum, temperature and microgravity. TABLE OF CONTENTS 1. INTRODUCTION ................................................. 1 2. MECHANISM PARAMETERS .............................. 1 3. MECHANISM DESIGN ........................................ 2 4. TEST .................................................................. 3 5. RESULTS............................................................ 5 6. DISCUSSION ....................................................... 7 7. CONCLUSION AND OUTLOOK ........................... 7 REFERENCES ........................................................ 7 1. INTRODUCTION Pointing mechanisms have a wide range of applications on board spacecraft. They deploy and turn solar panels, point antennas, move cameras or orient instruments. But they have in common the requirement to operate in the harsh environment of space and to fulfill certain performance requirements. To make sure the mechanism works in space and fulfills its requirements, the mechanism has to be tested and certified, usually in thermal-vacuum chambers, to validate the developed models and to ensure proper operations. But to systematically study the behavior of a mechanism and to evaluate the influence of the space environment on the performance of a mechanism, a specific method must be developed. A first step towards development of a method to systematically characterize the performance of a space mechanism is established in this current work. Chapter 2 defines parameters which are commonly used to describe the performance of a mechanism. These performance parameters include, for example, pointing velocity, resolution and power consumption. A method to measure these parameters was developed and is described in Chapter 4. The basic strategy of the method is to mount a Laser pointer on top of the mechanism and point it towards a Position Sensitive Detector (PSD). With this arrangement the motion can be recorded. An Antenna Pointing Mechanism (APM), built in- house for a Ka-band antenna development project for on- orbit servicing missions was used as a test case. The characteristics of the mechanism are described in Chapter 3. The test method was validated using that APM by conducting different tests and determining a subset of the performance parameters. These tests are described also in Chapter 4, and the results are presented in Chapter 5. 2. MECHANISM PARAMETERS To characterize the mechanism and to compare the results with later tests in vacuum, microgravity and varying temperatures, parameters were identified which help to describe the characteristics of a mechanism. The chosen parameters are the result of comparing requirements and specifications of different Antenna Pointing Mechanisms for different missions found in literature. Takada et al. [1] designed a pointing mechanism for a K-band antenna and described the importance of electrical power, maximum slew rate, step size and holding torque as performance parameters during thermal vacuum tests. Furthermore their test results show that the thermal vacuum environment has an impact on the consumed power, the maximum slew rate and the step size. A detailed and systematic evaluation of these parameters was therefore deemed absolutely necessary. Design drivers described by Shmulevitz and Halsband [2] include pointing accuracy and average power consumption which were tested in functional tests. Serrano et al. [3] identified parameters for the Ka-band APM for ESA’s Envisat Polar Platform that describe the behavior of that mechanism. They included: pointing accuracy, pointing range, pointing speed and acceleration. Herald and Wai [4]