GMI Instrument Spin Balance Method,  Optimization, Calibration and Test Laoucet Ayari*, Michael Kubitschek*, Gunnar Ashton*, Steve Johnston*, Dave Debevec*,  David Newell* and Joseph Pellicciotti** Abstract The Global Microwave Imager (GMI) instrument must spin at a constant rate of 32 rpm continuously for  the 3-year mission life. Therefore, GMI must be very precisely balanced about the spin axis and center of gravity (CG) to maintain stable scan pointing and to minimize disturbances imparted to the spacecraft and  attitude control on-orbit. The GMI instrument is part of the core Global Precipitation Measurement (GPM) spacecraft and is used to make calibrated radiometric measurements at multiple microwave frequencies and polarizations. The GPM mission is an international effort managed by the National Aeronautics and Space Administration (NASA) to improve climate, weather, and hydro-meteorological predictions through more accurate and frequent precipitation measurements. Ball Aerospace and Technologies Corporation (BATC) was selected by NASA Goddard Space Flight Center to design, build, and test the GMI instrument. The GMI design has to meet a challenging set of spin balance requirements and had to be brought into simultaneous static and dynamic spin balance after the entire instrument was already assembled and before environmental tests began. The focus of this contribution is on the analytical and test activities undertaken to meet the challenging spin balance requirements of the GMI instrument. Thenovel process of measuring the residual static and dynamic imbalances with a very high level of accuracy and precision is presented together with the prediction of the optimal balance masses and their locations. Introduction The GMI instrument is one of the payload instruments on the GPM core spacecraft and must be spun  continuously at 32 revolutions per minute (rpm) ± 0.1% on-orbit for the 3-year operational life of the instrument and to provide the desired geo-location for the science data. Details pertaining to the GMI design and testing were presented in references [1, 2]. The GMI instrument had to be spun balanced to  less than 0.032 kg-m (2.77 lbm-in) static imbalance and less than 0.055 kg-m 2(188 lb-in2) dynamic  imbalance, total imbalance, for the 123-kg spinning portion of the instrument payload. The design and  development of the spin balance method, analytical solution, test setup, design, calibration, and measured test results are described. The verification and validation of the GMI instrument to demonstrate compliance of performance through environmental testing and spin balance has been successfullycompleted. At the writing of this contribution, GMI has been fully integrated onto the GPM spacecraft, and the GPM spacecraft has been shipped to the launch site in Tanegashima, Japan for launch in February 2014.  The static and dynamic balancing approach consists of spinning GMI on top of an accurately calibrated  dynamometer, designed specifically for GMI to measure the imparted forces due to imbalance. This  device continues a long tradition of building precision balancing machines, vibration input benches and  force measurement instruments for space hardware at Ball Aerospace & Technologies Corporation [5,6]. The dynamometer measures all forces and moments and consists of three force transducers mounted to  an aluminum base plate and a corresponding top plate that functions as the mounting interface for the GMI instrument (Figure 1). The calibration of the dynamometer showed that the total measurement error  of this technique and dynamometer were much smaller, by an order of magnitude, than the imbalance  *Ball Aerospace & Technologies Corporation, Boulder, CO **NASA Goddard Space Flight Center, Greenbelt, MD /g51/g85/g82/g70/g72/g72/g71/g76/g81/g74/g86/g3/g82/g73/g3/g87/g75/g72/g3/g23/g21/g81/g71/g3/g36/g72/g85/g82/g86/g83/g68/g70/g72/g3/g48/g72/g70/g75/g68/g81/g76/g86/g80/g86/g3/g54/g92/g80/g83/g82/g86/g76/g88/g80/g15/g3/g49/g36/g54/g36/g3/g42/g82/g71/g71/g68/g85 /g71/g3/g54/g83/g68/g70/g72/g3/g41/g79/g76/g74/g75/g87/g3/g38/g72/g81/g87/g72/g85/g15/g3/g48/g68/g92/g3/g20/g23/g16/g20/g25/g15/g3/g21/g19/g20/g23/g3 303