Trans. JSASS Aerospace Tech. Japan Vol. 10, No. ists28, pp. To_4_1-To_4_6, 2012 Topics Attitude Operation Results of Solar Sail Demonstrator IKAROS 1) 1) 2) 2) 2) By Takanao SAIKI , Yuichi TSUDA , Ryu FUNASE , Yuya MIMASU , Yoji SHIRASAWA and IKAROS Demonstration Team 1)The Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Japan 2)Space Exploration Center, Japan Aerospace Exploration Agency, Sagamihara, Japan (Received June 27th, 2011) This paper shows the attitude operation results of Japanese interplanetary solar sail demonstration spacecraft IKAROS. IKAROS was launched on 21 May 2010(JST) aboard an H-IIA rocket, together with the AKATSUKI Venus climate orbiter. As IKAROS is the secondary payload, the development cost and period were restricted and the onboard attitude system is very simple. This paper introduces the attitude determination and control system. And as IKAROS is spin type spacecraft and it has the large membrane, the attitude control is not easy and it is very important to determine the long-term attitude plan in advance. This paper also shows the outline of the IKAROS attitude operation plan and its operation results. Key Words: Solar Sail, Attitude Operation Result, IKAROS 1. Introduction control torque. Therefore, it is important to determine the long-term attitude control plan in advance and operate IKAROS is a solar power sail technology demonstration according to the plan. In this paper, we show the attitude spacecraft developed by Japan Aerospace Exploration Agency. operation plan and results of IKAROS. The spacecraft was launched on May 21st , 2010 together with the AKATSUKI Venus climate orbiter as the secondary payload. The concept of solar power sail has been proposed for years Full success in JAXA. The solar power sail is the hybrid propulsion system. (a half year) It combines the photon propulsion and the ion engine Minimum success (several weeks) propulsion by using the large power generated by flexible 5)Navigation & solar cells on the sail membrane. IKAROS is the precursor Orbit control mission to demonstrate the key technologies for the future 4)Acceleration using solar power sail missions. The main missions of IKAROS are Solar sail. 1) deployment of the large sail membrane in the interplanetary 1)Launched by H‐IIA. 2)Attitude control 3)Sail deployment. space, 2) power generation by the thin film solar cells on the Spin separation. Power generation. before deployment. sail, 3) confirming the acceleration by the solar radiation pressure and 4) demonstration of the navigation and guidance Fig. 1. Nominal operation sequence of IKAROS mission. Sail deployment was achieved on Jun 9th , 2010. of the solar sail type spacecraft. Fig. 1 shows the outline of the IKAROS mission The successful deployment of the solar sail was achieved on Jun 9th and the acceleration by the solar radiation pressure was confirmed immediately following the sail deployment. And the power generation of the thin file solar cells was also verified. Fig. 2 is the picture captured by the deployable camera (DCAM) on June 14th, 2010. As the IKAROS spacecraft was the secondary payload, the development cost and period were restricted. Consequently, the spacecraft system is simple. Especially, the onboard attitude determination is incomplete. The attitude determination of IKAROS is possible only by collaborating with the ground systems. This paper shows the attitude Fig. 2. IKAROS solar sailing in the interplanetary space. This picture determination and control system of IKAROS. was captured by the deployable camara (DCAM) on June 14th, 2010. As IKAROS is the spin type spacecraft, the spin axis reorientation is not so easy because it requires the large [テキストを入力] Copyright© 2012 by the Japan Society for Aeronautical and Space Sciences and ISTS. All rights reserved. To_4_1 Trans. JSASS Aerospace Tech. Japan Vol. 10, No. ists28 (2012) 2. Attitude Determination and Control and System XMGA SSAS XLGA1 2.1. Attitude determination system of IKAROS IKAROS is a spin-stabilized spacecraft equipped with a large flexible structure. The typical spin type spacecrafts in the deep space are equipped with star sensors for the attitude determination. However, IKAROS does not have star sensors because the development cost and time are strictly restricted. Then the alternative attitude determination method is chosen. The attitude determination of IKAROS is realized by the Sun/Earth angle measurement. The sun angle is measured by a Spin Sun Aspect Sensor (SSAS). This sensor can observe the spin period and the angle between the spin axis and sun direction. The earth angle is measured by Doppler modulation RCS XLGA2 of the downlink RF. Fig. 3 shows the locations of ADCS Fig. 3. Locations of SSAS, antennas(XLGA1, XLGA2, MGA) and RCS. components; the SSAS, RCS and the antennas. IKAROS has Antennas are intentionally located offset from the spin axis. three antennas (XLGA1, XLGA2 and XMGA), and all of them are intentionally located offset from the spin axis. As a result, it can be possible to measure the Doppler shift of the spin motion. Fig. 4 shows the Doppler data of IKAROS. The spin period is detected by analyzing the period of the data. The earth angle can be estimated from the amplitude of the wave. The O-C (Observation – Calculation) value of the two-way Doppler data is written as = Ω+θ VAsin( t ) , (1) Fig. 4. Doppler modulation of downlink RF. Spin period and earth angle can be estimated from this data. where Ω is the spin period and A is the amplitude of the spin modulation. Then the earth angle can be estimated as −1 Sun pulse Spin rate η = sin (Ar / (2ANT )) , (2) period Sun sensor where rANT is the distance between the spin axis and the antenna. By combining this earth angle and the sun anble β , Sun angle Sun angle history Spin axis direction 2 the direction of the spin axis can be determined as follows; Spin axis direction 1 Earth angle 1−−xy cosβη cos Integrated Doppler ˆ ˆ ˆ ˆˆ Est. of amplitude A=+ xS yG ± ()SG×. (3) correction ˆˆ2 high-freq. 1(−⋅GS ) component Curve fitting Est. of phase Elimination of arbitrary Doppler Sˆ and Gˆ represent the direction of the sun and the earth data (1Hz) DC Polarization correction LOS Velocity viewed from the spacecraft, respectively. And x and y are component written as cosβ−⋅GSˆˆ cos ηη cos−⋅GSˆˆ cos β Fig. 5. Attitude determination system of IKAROS. Spin axis direction xy==, . (4) can be estimated by combining the date of sun sensor and Doppler data. −⋅ˆˆ22−⋅ˆˆ 1(GS ) 1(GS ) Eq. (3) corresponds to calculating the intersection of two circles on the celestial sphere. The centers of two circles are the sun and the earth, respectively and the radius are β and η . As Eq. (3) shows, two solutions can be found generally. In the IKAROS operations, the true solution can be chosen by using the phase information that can be obtained by handling the time information precisely. Fig. 5 shows the attitude determination system of IKAROS. The sun angle data is extracted from HK data. The Doppler data is given by the ground equipment and the ground-based attitude determination software process the data in quasi real-time. Fig. 6 shows the snapshot of IKAROS attitude determination QL. The spin axis direction is shown on the celestial sphere. Two solutions are found but the true one is chosen automatically by Fig. 6. Snapshot of IKAROS attitude determination QL. The spin axis processing the phase data. direction is shown on the celestial sphere. [テキストを入力] To_4_2 T. SAIKI et al.: Attitude Operation Results of Solar Sail Demonstrator IKAROS 2.2. Attitude control system of IKAROS 3. Long-term Attitude Control Plan of IKAROS The attitude control of IKAROS is done primarily by RCS. IKAROS is equipped with a newly developed Gas-Liquid Here shows the long-term attitude plan of IKAROS. Fig. 9 Phase-Equilibrium Thruster. This is a kind of the cold gas indicates the antenna coverage of IKAROS. XLGA1 is thruster, but the fuel is stored in liquid phase. attached on the top of IKAROS and XLGA2 is on the bottom Hydrofluorocarbon(HFC-134a) is used as fuel. The fuel is in panel. Both antennas have wide beams. But there is the liquid phase in the tank and it is in gas phase at the injection. possibility that the reflected radio wave cause harmful effects As HFC-134a is innoxious and it is not a high pressure to the communication, the antennas are purposely designed system, it is easy to handle and suitable for the secondary not to cover the sail directions (invisible zone). Consequently, payload. As the fuel loses heat when liquid phase is changed the link condition becomes bad when switching the antenna to gas phase, the temperature of the fuel tank and pipes are and the attitude should be controlled to avoid this invisible controlled to keep the phase transition and the control torque zone of antenna. is given intermittently not continuously. And it is important Fig. 10 shows the orbit of IKAROS viewed from the for this propulsion system to separate the gas from the liquid. Sun-Earth fixed frame. As this figure shows, during initial For this purpose, a porous metal is placed in the tank to keep three months after launch, XLGA1 is used for the the liquid in it. IKAROS has eight thrusters for spin up/down communication between IKAROS and ground stations. After and spin axis reorientation. that, the antenna is switched to XLGA2. The attitude control logics of IKAROS are quite simple. The conventional rhumb line control (RLC) and the active Spin axis nutation control (ANC) method are supported. In addition to these conventional logics, IKAROS is equipped with the XLGA1 extended control logics, Flex-RLC/Flex-ANC attitude control ± logics.