@conference{2179,
  author = {Peter Bender and D. Wiese and R. Nerem},
  title = {A possible Dual-GRACE mission with 90 degree and 63 degree inclination orbits},
  year = {2008},
  publisher = {ESA},
  address = {Noordwijk, The Netherlands},
  note = {JILA Pub. 8161},
}