Asian Journal of Control, Vol. 15, No. 2, pp. 1 9, March 2013 Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/asjc.368 UKF BASED IN-FLIGHT CALIBRATION OF MAGNETOMETERS AND RATE GYROS FOR PICO SATELLITE ATTITUDE DETERMINATION Halil Ersin S ¨ oken and Chingiz Hajiyev ABSTRACT In this paper an unscented Kalman filter based procedure for the bias estimation of both the magnetometers and the gyros carried onboard a pico satellite, is proposed. At the initial phase, biases of three orthogonally located magnetometers are estimated as well as the attitude and attitude rates of the satellite. During the initial period after the orbit injection, gyro measurements are accepted as bias free since the precise gyros are working accurately and the accumulated gyro biases are negligible. At the second phase estimated constant magnetometer bias components are taken into account and the algorithm is run for the estimation of the gyro biases that are cumulatively increased by time. As a result, six different bias terms for two different sensors are obtained in two stages, where attitude and attitude rates are estimated regularly. For both estimation phases of the procedure an unscented Kalman filter is used as the estimation algorithm. Key Words: Attitude determination, magnetometer, gyro, bias. I. INTRODUCTION Cooperating magnetometer and rate gyro utiliza- tion in pico satellite missions is a common method for achieving accurate attitude information. By the use of a Kalman filter algorithm measurement inputs of these sensors can be easily integrated in order to estimate Manuscript received December 31, 2009; revised May 10, 2010; accepted December 8, 2010. Halil Ersin S ¨ oken (corresponding author) is with the Depart- ment of Space and Astronautical Science, Graduate University for Advanced Studies (Sokendai), 3-1-1, Yoshinodai, Sagami- hara, Kanagawa, Japan and he was with Aeronautics and Astronautics Faculty, the Istanbul Technical University, 34469 Maslak, Istanbul, Turkey (e-mail: ersin soken@ac.jaxa.jp). Chingiz Hajiyev is with the Aeronautics and Astronautics Faculty, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey, and also with the Institute of Cybernetics of Azerbaijan National Academy of Sciences, Az1141, F.Agayev Str.9, Baku, Azerbaijan (e-mail: cingiz@itu.edu.tr). This work was supported in part by TUBITAK (The Scien- tific and Technological Research Council of Turkey) under Grant 108M523. precisely the attitude parameters of the satellite. However, these sensors are not error free because of the bias that their outputs involve. These bias terms inhibit the filter efficiency, and therefore the attitude data accuracy, and may even bring about filter divergence in the long term. The attitude accuracy requirements demand compensation for the magnetometer and gyro errors such as misalignments and biases [1]. Estimating magnetometer and gyro biases, as well as the attitude of the satellite, is a proposed technique to solve such problems and increase on-board accuracy. However, methods to estimate the biases of these two sensors jointly is not a topic that has been examined much in the literature, and usually bias estimation problems are taken into consideration independently. One method to calibrate magnetometer measure- ments is to follow up an attitude free scheme. In usual cases, magnetometers may be the only operating sensors at the orbit injection phase where attitude data is not available since the spacecraft is spinning rapidly. Besides, magnetometer measurements are not bias free at that stage because of the large magnetic disturbances on the spacecraft such as charging during the launch and the electrical currents within the spacecraft [2].  2011 John Wiley and Sons Asia Pte Ltd and Chinese Automatic Control Society Asian Journal of Control, Vol. 14, No. 3, pp. 707–715, May 2012 Published online 16 March 2011 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/asjc.368