Acknowledgements

We would like to thank D. Burgess of QMUL for helpful discussions and the encouragement to start this work, J.-P. Simon of the Bureau de Longitude for providing data and information on the VSOP87 system, R.S. Parthasarathy of NSSDC for providing spacecraft position data and a discussion on Helios coordinate systems, K. Ackerson of the Univ. of Iowa, and T. Mukai of ISAS for a discussion of Geotail coordinates, R. Skoug of LANL for a discussion on ACE coordinates. We also would like to thank E. Wright and C. Acton of JPL for information on the SPICE system and the referees for very helpful comments and corrections. This work was partially supported by PPARC grants GR/L 29903 and PPA/Z/R/1999/00606.