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Orbit Determination of Mars Explorer NOZOMI for All the Mission Period Orbit Determination of Mars Explorer NOZOMI for All the Mission Period Makoto Yoshikawa, Takaji Kato, Tsutomu Ichikawa, Hiroshi Yamakawa Japan Aerospace Exploration Agency (JAXA), Institute of Space and Astronautical Science (ISAS) 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, JAPAN Phone/FAX: +81-42-759-8341, E-mail: [email protected] Takafumi Ohnishi, Shiro Ishibashi Fujitsu Limited 3-9-1 Nakase, Mihama-ku, Chiba-shi, Chiba 261-8588, JAPAN ABSTRACT Japanese first Mars explorer NOZOMI, which was launched in July 1998, suffered several problems during the operation period of more than five years. It reached near Mars at the end of 2003, but it was not put into the orbit around Mars. Although NOZOMI was not able to execute its main mission, it provided us a lot of good experiences from the point of the orbit determination of spacecraft. The related issues are the spin modulation, the solar radiation pressure, the small force related to the attitude change, the solar conjunction, and the orbit determination by the side lobe of the high gain antenna without the telemetry. We tried to solve these issues by the conventional way using range and Doppler data. We also tried the orbit determination by using the Delta-VLBI method (VLBI: Very Long Baseline Interferometry). In addition to this, we tried optical observations of NOZOMI at the earth swingbys. ¤NC?RWK;B-6_° j.$1.k¾y¶ l¢ ^½ÖÀ ¢Ê ¢Y²t{nbÅj ­ Ö ¡p×ɵ ·Ó ÁÍ._° j.$1/ à z-«47% #. ÃÑ5.ZÒi f-('.Ï®-¦u!%.$1/ Ã.5-/_.s-¯ !% _ak¾-¼Â/, 7,&%.$1/NC?RW.ÍÕ.θ8¯ "6*/),&% k¾y¶.e¹4/©.Ç-ÐÒ,w|8`-2%4 !(7%k¾y¶-gv!%*!(/ AJWP@QU?RW ªÔØX }-ÆÈ^¨» DUOET¿),q]).H:=:W9WDG .>:FVL-36k¾y¶,+o47674.Ï®-'(/ Õ4.U W@*FCMS8Ë)¬\!%0% !1*!( ¥¬ ³ ´h£ºÄdx-36e§8&% 4- ±mA:W<I:-/ ~cÌ[r- 36e§21% 1. Introduction end of 2003 or at the beginning of 2004 (Figure 3). In addition to this problem, NOZOMI The Japanese Mars explorer NOZOMI (Figure suffered several difficulties, such as the loss of 1) was launched in July 1998. It was planed to S-band downlink, the solar conjunction, and the arrive at Mars in October 1999 (Figure 2). loss of the telemetry. Particularly the damage However, when NOZOMI left form the earth to related to the power supply unit, which Mars, a problem occurred and the orbit plan occurred at the end of April 2002, was very was changed so that it would reach Mars at the serious. We were not able to get the telemetry such critical condition, we managed to get the solution of the orbit, and we were able to carry out two earth swingbys successfully as mentioned above. There are several other issues related the orbit determination of NOZOMI and they are the spin modulation, the solar radiation pressure, the small force related to the attitude change, and the solar conjunction. We tried to solve Fig. 1: NOZOMI spacecraft these issues by the conventional way using range and Doppler data. However, we also tried data after that and the radio link was the new method, that is the orbit determination completely lost for about two months. However, by using the Delta-VLBI method (VLBI: Very we were able to control NOZOMI by using Long Baseline Interferometry). In addition to very special method and we carried out two this, we tried optical observations of NOZOMI earth swingbys in December 2002 and June when it approached the earth at the earth 2003 successfully. NOZOMI was put into the swingbys. orbit that goes near Mars in the middle of December of 2003. But we were not able to Up to now, we have carried out many studied solve the trouble related the power supply unit about the orbit determination of and NOZOMI did not become the orbiter of NOZOMI1,2,3,4,5,6. In this paper, we summarize Mars. This trouble of the power supply unit all the issues related to the orbit determination occurred just after strong solar flare, but the of NOZOMI especially from the point of what direct relationship between the trouble and the we have done. The quantitative results such as solar flare is unknown. the accuracy of orbit determination will be reported in separate papers. Although NOZOMI was not able to execute its main mission, from the point of the orbit determination of spacecraft it provided us a lot 2. Orbit Determination in Normal Period of good experiences. The most difficult work was, of course, the orbit determination for the In this section, we summarize the orbit period without the telemetry. Fortunately it was determination in the normal period. The words possible to get the tracking data (range and "normal period" mean the period when we Doppler) in this period. But most of this period, could get the telemetry and when the high gain the high gain antenna did not point to the earth antenna could be pointed toward the earth. because of a constraint of the attitude of the These analyses are also discussed in previous spacecraft. Therefore the quality of the tracking papers2,3,4. The orbit determination in the data was not good, and for some period it was critical period is discussed in the next section. impossible to get the tracking data at all. Under Fig. 2: Original orbit plan Fig. 3: New orbit plan 2.1 Software orientation maneuver. Such acceleration was We have been developing software for orbit not expected before the launch. After the determination of spacecraft from more than 20 trouble of S-band downlink, the re-orientation years ago. This software is called ISSOP (ISAS maneuvers are executed much more frequently, Orbit Determination Program)7,8. It was first because X-band, which has narrower beam size, used for the mission of SAKIGAKE and was used for downlink. This acceleration acts SUISEI, which were launched in 1985 to the as perturbations of several 10-12 km/s2 comet Halley. And then it was used for HITEN, acceleration. It corresponds to several percent which was launched 1990 for experiments of of solar radiation pressure, which has a swingbys. As for NOZOMI, we tried to modify significant influence on the orbit determination ISSOP in several points to carry out much results. We tried to estimate this acceleration, accurate orbit determination. And also we are but the estimated values seemed to be not modifying this software in order to include accurate enough. Such estimation of small VLBI data. forces remained as one of the future problems. 2.2 Spin modulation 2.5 Solar conjunction Since NOZOMI was rotating about 7 rpm, At the beginning of the year 2001, the angle of there were spin effects in the observed data of Sun-Earth-NOZOMI became very small. This range and Doppler. In order to carry out the meant that NOZOMI was observed very close accurate orbit determination, we should remove to the sun. When such conjunction occurs, we the effects of spin. We obtained a function that cannot establish the link between spacecraft simulated the effects of spin, and eliminated and the ground stations. Actually for NOZOMI them. For example, Doppler data of LGA-B the communication was stopped for about three shows spin modulation with the amplitude of weeks around the conjunction. In addition to about 20 cm/s, and after the elimination of spin this, the range and Doppler data were affected modulation, the amplitude becomes about 1 by noise. Especially the noise level of Doppler mm/s. After the elimination of spin effect, the became very large near the conjunction. As for accuracy of orbit determination became a little the range, the effect of conjunction was small. better. The accuracy of orbit determination became worse around the conjunction. 2.3 Solar radiation pressure As for the solar radiation pressure, we modified our previous model largely. In our new model, 3. Orbit Determination in Critical Period the geometry and material properties of the spacecraft are taken into consideration. The After the loss of telemetry from NOZOMI, we spacecraft is treated as a combination of a could not get range and Doppler data stably. spinning box and two flat plates, representing Figure 4 shows O-C of Doppler just around the the spacecraft body, solar panels, and HGA telemetry loss. Until 27 April 2002, the dish respectively. The surface properties are Doppler data were normal. But the data shows area, reflectance, diffused reflectance, strange behaviors after this. This is because the transparency, and direction against the spin axis. beam of HGA was straying off from the Daily change of the spacecraft orientation is direction of the earth. Since the heater also also taken into account. By these modifications, stopped, the fuel on board was frozen. the accuracy of our orbit determination became Therefore it was impossible to control the better. However, we should still improve our attitude and orbit of NOZOMI. In order to model to achieve much better accuracy. operate NOZOMI without telemetry, we established a special method by using an 2.4 Small force caused by re-orientation autonomous function. And we used the sunlight maneuvers to melt a part of the fuel. Thus we succeeded to When re-orientation maneuvers were carried control of attitude and orbit of NOZOMI out, we found that there are variations about 1 without telemetry and heater. mm/s in the Doppler. This means that there exists acceleration in the orbital motion at re- However, a big problem remained. That was Fig. 4: O-C of Doppler data from April 15, 2002 to May 7, 2002 Fig.
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