| Date: | 1997-06-04 11:00:00 | |
| Sender: | V. Shargorodsky, MCC/Moscow <[Mailed W. Seemueller <seemuell@dgfi.badw-muenchen.de>]> | |
| Subject: | [SLR-Mail] No. 67: Zeya | |
| Author: | V. Shargorodsky, MCC | |
| Content: | ******************************************************************************** SLR Electronic Mail 1997-06-04 11:00:00 UTC Message No. 67 ******************************************************************************** Author: V. Shargorodsky, MCC/Moscow Subject: Zeya Dear Colleagues, Three months has passed since the Zeya satellite has been launched. The initial results are promising, but for reliable evaluations it is necessary to have at least two months of intensive observations of this satellite by the international SLR network. We appeal for help in observational support for the Zeya mission. In the following we present the first results of Zeya satellite observations by the international SLR network. The first laser ranging observations of the Zeya satellite have bean made March 29, 1997, by the Riga SLR station. In April a total of 12 Zeya passes has been observed by Riga, Herstmonceaux, Mendeleevo, Greenbelt, Monument Peak, and Haleakala laser stations. In May a total of 31 passes has been observed, and to the above stations Komsomolsk and Maidanak were added. Table 1. Number of SLR observation sessions -------------------------------------------------------------- Station name and number March Apr May June Total -------------------------------------------------------------- Riga,Latvia 1884 1 5 13 2 21 Herstmoseaux,GB 7840 2 8 10 Mendeleevo,Russia 1870 2 4 6 Monument Peak,USA 7110 2 1 3 Haleakala,USA 7210 1 2 3 Greenbelt,USA 7105 1 1 2 Maidanak,Uzbekistan1864 1 1 Komsomlsk,Russia 1868 1 1 -------------------------------------------------------------- Total 1 13 31 2 47 -------------------------------------------------------------- The ephemerides for the stations of the international SLR network are calculated by the Mission Control Center (MCC). The orbit parameters definition is made on a basis of joint processing of the radio systems and SLR measurements using measurement intervals of two to five days. The obtained results show that errors in definition and prediction of the Zeya spacecraft orbit in lateral direction (along the binormal) are less then 60-100 m at prediction intervals up to 10 days. Errors in the spacecraft motion parameters along its orbit are considerable depending on the interval of prediction and on the atmosphere parameters model at this interval, and may reach several kilometers by intervals of 7-10 days. To reduce these errors using the obtained SLR measurements, on dayly basis TB (Time Bias) cofficients are calculated and sent to the global SLR network stations providing a reduction of discrepances between predicted and real orbit parameters in transversal direction (along the orbit) to 100-300 metres on a dayly interval. Based on the FR analysis, a comparison has been made between calculation and observation data for the effectivity of the retroreflectors mounted on board of the Zeya satellite. A review of calculation data has been made for two versions: a classical value of the velocity aberration, and a partial compensation of this aberration by the Fizeau effect. The amount of obtained ranging data is still not sufficient to make reliable conclusions for practical support of the calculated parameters. But the preliminary analysis show that the return signal strenght by laser ranging is closer to the ”classical” version (without influence of the Fizeau effect). Nevertheless, it may be caused also by poor tracking of the spacecraft, especially during the initial period of the flight, because of its low and instable orbit causing a low accuracy of initial conditions. The analysis of the measurements accuracy shows that it is varying for different stations from several millimeters to 3-4 centimeters. The values of deviations is in good agreement with the stations accuracy. Therefore it seems that the satellite has a really small systematic error in range, not exceeding 0.5 cm. All the equipment installed on board of the spacecraft is operating normally. Begining in early June, operations will be done for calibration of the user,s navigation equipment by means of laser measurements. The Zeya spacecraft orbit analysis shows that at the current moment we have most favourable conditions for laser ranging of satellite by stations of the global SLR network. During June the stations with a latitude of more than 50 deg.N will have everyday visibility zones, when the spacecraft will be seen in reflected sunlight practically in the whole zone. To such stations the following belong: 1868 - Komsomolsk (50 deg.N), 1870 - Mendeleevo (56 deg.N), 1884 - Riga (57 deg.N), 7805 - Metsaehovi (60 deg.N), 7811 - Borowiec (52 deg.N), 7836 - Potsdam (52 deg.N), 7840 - Herstmonceaux (51 deg.N), 8834 - Wetzell (49 deg.N). Many of the global network stations located in the 38 to 50 deg.N belt may also make everyday visual observations of the spacecraft at elevation angles more then 20 deg. when the Zeya spacecraft enters the visibility zone for these stations. To such stations belong: 1864 - Maidanak (39 deg.N), 1893 - Katziveli (44 deg.N), 7105 - Meryland Point (39 deg.N), 7109 - Quincy (40 deg.N), 7237 - Changchun (44 deg.N), 7249 - Beijing (40 deg.N), 7548 - Cagliary (39 deg.N), 7810 - Zimmerwald (47 deg.N), 7835 - Grasse (44 deg.N), 7839 - Graz (47 deg.N), 7939 - Matera (41 deg.N). Stations located between 30 and 38 deg. N (Tokio, Simosato, etc.) may observe the Zeya spacecraft at elevation angles 10 to 20 deg. The successful ranging of the Zeya spacecraft by the 7210 (Haleakala) and 7110 (Monument Peak) stations shows that observations may be done even if there is no visibility of the satellite in reflected sunlight. A more intensive participation of the international SLR network stations in the observations will provide more accurate ephemereides and will create conditions for ranging of the Zeya spacecraft in the automatic mode. Tacking into account that quick and reliable transmission of the ephemerides to the stations is of great importance, the MCC will send IRVS data, DRAG Functions, and TB directly to the stations during the period of intensive measurement campaign. It is evident that the most interesting scientific and application results of the experiment can be obtained only if a sufficient amount of measurement data is available. In particular, if a large amount of measurement data is available, in addition to the positioning equipment calibration, some other possibilities may be studied, including: - evaluation of lower atmosphere fluctuations at heights less than 500 km; - investigation of the higher harmonics of the Earth gravitation field; - building of a high-accuracy model of satellite motion at heights less than 500 km. Currently, in conditions of relatively low solar activity, the above investigations can be made with maximum effectivity. Moreover, during the Zeya spacecraft ranging its visibility zone duration does not exceed two to four minutes at each pass, so the more intensive Zeya observations will not have a considerable influence on the obtained data amount in other programs. Taking this into account, we once more appeal to you for support of the Zeya mission. The results of the experiments with the Zeya spacecraft will be reported at the ION-97 Conference in Kansas City, USA, in September 1997. Shargorodsky [Mailed From: ”W. Seemueller” ******************************************************************************** |