Abstract
We present the activities carried out to calibrate and characterise the performance of the elements of attitude control and measurement on board the Herschel spacecraft. The main calibration parameters and the evolution of the indicators of the pointing performance are described, from the initial values derived from the observations carried out in the performance verification phase to those attained in the last year and half of mission, an absolute pointing error around or even below 1 arcsec, a spatial relative pointing error of some 1 arcsec and a pointing stability below 0.2 arsec. The actions carried out at the ground segment to improve the spacecraft pointing measurements are outlined. On-going and future developments towards a final refinement of the Herschel astrometry are also summarised. A brief description of the different components of the attitude control and measurement system (both in the space and in the ground segments) is also given for reference. We stress the importance of the cooperation between the different actors (scientists, flight dynamics and systems engineers, attitude control and measurement hardware designers, star-tracker manufacturers, etc.) to attain the final level of performance.
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Notes
Quaternions are a single example of a more general class of hypercomplex numbers. By analogy with the complex numbers, a quaternion can also be written as a linear combination of a real and three imaginary components. It is easy to represent the concatenation of rotations in quaternion format, constituting a very convenient and compact means of representing S/C attitudes. It is outside the scope of this paper to give even an introduction to quaternions. A good introduction to this topic is given in [28].
The actual packet rate was 1 Hz but many ACMS parameters were super-commutated (four samples per packet).
Beware that APE ‡ is not the proper APE, that is defined in temporal terms. A justification for the use of this proxy is given in [26], where it is shown that it introduces an additional error of typically only 0.1 arcsec.
The PDE was not explicitly measured within PVP, but the long-stare pointing tests carried out indicated that the requirement was met, provided that the S/C was kept out of “warm” attitudes (see definition below).
The term “cold” refers to those S/C attitudes that keep the service module away from heating by solar radiation, in contrast to “warm” attitudes. Originally, only those with SAA ≥ 110 ∘ were considered as problematic due to the risk of heating some components of the STR support structure.
A cycle is a series of ODs where all the instruments are used in sequence. Typically comprises 14 ODs.
As demonstrated by measurements derived from CCD dumps performed at the initial and final STR2 CCD reference temperature.
The STR is roughly pointing in the opposite direction to the telescope boresight.
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Acknowledgements
We acknowledge the anonymous referee for his/her valuable comments and suggestions. The Herschel spacecraft was designed, built, tested, and launched under a contract to ESA managed by the Herschel/Planck Project team by an industrial consortium under the overall responsibility of the prime contractor Thales Alenia Space (Cannes), and including Astrium (Friedrichshafen) responsible for the payload module and for system testing at spacecraft level, Thales Alenia Space (Turin) responsible for the service module, and Astrium (Toulouse) responsible for the telescope, with in excess of a hundred subcontractors.
PACS has been developed by a consortium of institutes led by MPE (Germany) and including UVIE (Austria); KU Leuven, CSL, IMEC (Belgium); CEA, LAM (France); MPIA (Germany); INAF-IFSI/OAA/OAP/OAT, LENS, SISSA (Italy); IAC (Spain). This development has been supported by the funding agencies BMVIT (Austria), ESA-PRODEX (Belgium), CEA/CNES (France), DLR (Germany), ASI/INAF (Italy), and CICYT/MCYT (Spain).
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Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
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Sánchez-Portal, M., Marston, A., Altieri, B. et al. The pointing system of the Herschel space observatory. Exp Astron 37, 453–479 (2014). https://doi.org/10.1007/s10686-014-9396-z
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DOI: https://doi.org/10.1007/s10686-014-9396-z