| Date: | 1997-05-09 12:00:00 | |
| Sender: | Natalia Parkhomenko <[Mailed Andy Sinclair <ats@ast.cam.ac.uk>]> | |
| Subject: | [SLR-Mail] No. 59: Request for ZEYA tracking | |
| Author: | Natalia Parkhomenko | |
| Content: | ******************************************************************************** SLR Electronic Mail 1997-05-09 12:00:00 UTC Message No. 59 ******************************************************************************** Author: Natalia Parkhomenko Subject: Request for ZEYA tracking Subject: Appeal to Global SLR Network (via A.T. Sinclair) Dear collegues, Up to now six SLR stations of the world SLR society have done successful ranging sessions of the ZEYA satellite. We are very grateful to everyone who sent us observation data. In addition to previously given information on the ZEYA satellite we are now sending you additional information on the refletion model of the ZEYA retroreflector system. The SLR measurements are made for solving of space geodesy problem and radio navigation systems calibration, as well as for testing of technical solutions adopted in the precision WESTPAC satellite design which is to be launched in accordance with the WPLTN program. To achieve minimum target error of ranging which is determined by the retroreflector system design and the accuracy in their alignment relative to the ZEYA satellite mass center, for any direction of the incoming laser beam only one retroreflector is used. This is made by limiting the field of view (FOV) of each retroreflector by means of cylindricals bafflers, so that each single retroreflector has a FOV of 35 deg. The number of retroreflectors we managed to place on board of ZEYA is 20, which is three times less than on the WESTPAC satellite. All retroreflectors are positioned at equal distances from the space craft mass center and so that all perpendiculars to the retroreflector input aperture are passing through the satellite´s center. The systematic displacement is here 419 mm. As follows from the radio frequency measurements analysis, the ZEYA satellite is now on a sun - synchronous orbit with an apogee 499 km, perygee of 471 km, and inclination of 97.27 deg. The spacecraft is rotating with a rate of 65 rpm around its axis directed towards the nadir point, when the satellite is approximetily at 78 deg. N latitude and 24 deg. E longitude. As a result the total FOV all ZEYA retroreflectors is covering a little bit more than one third of all possible directions of the incoming beam, and the dead zones are positioned so that the spacecraft´s rotation causes no more than two or three gaps in return signals, in average no more then two gaps per three radiated laser pulses. Because of the spacecraft design features, there is some irregularity in the retroreflector positions. This irregularity may possibly cause some reduction or even temporary loss of return signals. The satellite´s rotation axis precession may be a cause of irregular return signal strenghts by different passes. Such information is for us of great value. To avoid long intervals of return signal abcence because of stroboscopic effects, the best pulse repetition rate by ranging should be 5 to 10 times more than the ZEYA spacecraft rotation rate, and so should be 5...10 Hz. Besides the ivestigation of the possibility of achieving the highest range measurement accuracy, the destination of retroreflector on board of ZEYA is also to continue investigations of the possible Fizeau effect influence on the total velocity aberration angle. Therefore on board of the the ZEYA satellite retroreflectors are placed with narrow, nearly diffraction-limited reflection patterns, having a FWHM angular width of approx. 5 arc sec at 532nm at normal incidence. The retroreflectors are made of fuzed silica with a refraction index of 1.461; the reflecting surfaces of each cube corner are aluminium-coated; the cube corner prism height is 18.9mm. The effective aperture diameter of the the retroreflector, determinied by the inner diameter of the cylindrical baffle, is 26.9 mm. In case of the Fizeau effect influence, these retroreflectors must partly compensate the velocity aberation; the remaining velocity aberration shold be 1.7 to 3.4 arc sec, and the narrow (5 arc sec) reflection pattern should help to get sufficiently stronger return signals. If there is no influence of the Fizeau effect, the signal reflection towards the ground station will be possible only for slant incidence angles at the retroreflector, when its reflection pattern is broadened along the satellite´s flight direction; then the return signal strenght will be an order of magnitude less than if the Fizeau effect compensation is present. We are very lager to know the details of the satellite observation condition in different practical situations of ZEYA ranging, especially information on the passes where data on every part of the observation zone are present. Since the begining of May the ZEYA will be visible in the reflected sunlight during nighttime for the SLR stations located in the northern hemisphere. Dear collegues, regarding the above any your messages, including session features, details of signal strenght measurements, etc., would be very useful and could help to achive better accuracy and effectivity of retroreflectors for future satellites. We ask you: 1. To take part in ZEYA observations during May and June of this year. 2. To send us data in the FULL RATE format. 3. To report the parameters of SLR stations making ZEYA observations, necessary for signal energy calculations (for comparision with measurment data). 4 To report any data on the return signal levels, absolute and/or compared to other satellites. 5. To report any features (including negative onse) of the measurement sessions. With best regards V.Shargorodsky [Mailed From: Andy Sinclair [Mailed From: ”Natalia N. Parkhomenko” ******************************************************************************** |