Determining the attitude of MSG-2 during the Launch & Early Orbit Phase

Paper number

IAC-08.C1.7.6

Author

Ms. Libe Jauregui, Logica, Germany

Coauthor

Mr. Spencer Ziegler, Logica, Germany

Coauthor

Mr. Alain Schuetz, European Space Agency (ESA), Germany

Year

2008

Abstract

The MSG-2 satellite was launched at 22:33:00.000 UTC on 15 December 2005 from Kourou with an Ariane 5 rocket. This satellite is the second in the Meteosat Second Generation (MSG) series of spin-stabilised geostationary satellites operated by EUMETSAT. MSG-2 is currently located at 0 degrees longitude.  The launch and early orbit phase (LEOP) was performed on behalf of EUMETSAT by the European Space Operations Centre (ESOC) and was a complete success. However, during the launch of MSG-1 many unexpected issues were encountered that drastically affected the confidence in determining the satellite’s attitude and ultimately caused an extension to the originally foreseen timeline. Although MSG-1 was successfully delivered into the required orbit and handover attitude ESOC’s Flight Dynamics Division had to considerably revise its attitude determination algorithms to account for the unforeseen spacecraft dynamics. MSG-2 was therefore an opportunity to test the improvements made to the flight dynamics’ operational software and see whether the better understanding of the spacecraft’s dynamics would allow the operational timeline to be maintained. The aims of this paper are therefore to discuss the improvements made to the flight dynamics software with regards to attitude determination, report on the attitude determination process adopted during the LEOP and present the substantial post-flight analysis of
the telemetry that has recently been concluded.

      As liquid fuel is consumed during the perigee raising manoeuvres the satellite’s spin axis diverges further from the geometrical axis of the cylindrically shaped satellite. This is initiated by the instrument’s baffle covers that are not symmetrically mounted about the geometrical centre. An increase in the tilt angle directly affects the liquid fuel distribution within the tanks about the spin axis, which is solved for in the attitude determination process. The liquid propellant distribution leads to interesting and non-intuitive spin-up and spin-down dynamics during orbital manoeuvres, which are presented in the paper.

      Extensive post-flight analysis involved:
      
     -Replacing the infrared model with data from actual MSG-1 infrared images of the Earth corresponding to the appropriate time: this showed little improvement in the determined attitude due to these images being only from the 0 degree longitude vantage point and not      corresponding to the part of the Earth seen by the Earth sensors during the first orbit revolutions.

      -Analysing the change in bias during the orbit manoeuvres as measured by the accelerometers to determine the increase in the spin axis’ tilt angle. We present in the paper the algorithm for estimating the tilt angle and the comparisons to the theoretical predictions, as well as the tilt angle estimated through the attitude determination process.

      -Following the slew manoeuvre to position the spin axis perpendicular to the orbital plane the baffle covers are released thereby removing any asymmetry about the geometrical axis. As a consequence the tilt angle disappears and the spin axis converges to the geometrical axis. Through the obtained telemetry we attempt to measure the final tilt angle, which is an estimate since a steady state is not obtained prior to the cover release avoidance manoeuvres.

      -Following orbit circularisation at the GEO altitude the final step is to obtain 24 hours of data to allow the final attitude to be determined prior to handing the satellite over to EUMETSAT. Our post analysis investigated several strategies to reduce this time period in order to reduce the overall LEOP timeline.

      -In GEO the spin axis is seen to wobble, which we show is due to the heating cycle causing a sloshing resonance and disappears when the thermal cycle is altered.

      -Finally, our post flight analysis demonstrates that our attitude determination based on the Earth and sun sensors better than 0.3 degrees during the perigee raising phase and around  0.1 degrees in GEO.

      The post analysis was recently completed within ESOC in order to conduct some tests of several software upgrades. The results have therefore not been presented at a previous conference.

Abstract document

IAC-08.C1.7.6.pdf

Manuscript document

IAC-08.C1.7.6.pdf (🔒 authorized access only).

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