The new millennium brought an asymptotic interest in space technology. Besides existing operational global positioning systems i.e. GPS and GLONASS, another two have already emerged: GALILEO and Compass/Beidou. Alternatives to GPS imposed the GNSS concept, which has crystallized in a short time. The true reasoning of this excessive need for positioning information is beyond the scope of this paper; instead, an analysis of orbital behaviour of GPS, GLONASS and GALILEO is conducted. GNSS orbits have similar characteristics permitting us to encompass them under the term Medium Earth Orbits (MEO). However, small differences between mean orbital elements of the three systems produce a significantly different reaction of their satellites' orbits to the natural perturbing factors. This study analyzes the three space segment characteristics, introduces the analytical method used to evaluate variations of orbital elements under the most significant perturbing influences and offers a broad comparative image of the dynamic behaviour of the three GNSS constellations.

Records of the low field magnetic susceptibility at three sites in the Chinese Loess Plateau, each with sediments covering the last 2·5 Ma, were averaged and combined into a single stacked file whose dating is independent of astronomic chronology. The stack reveals orbital frequencies, which are in general agreement with the astronomically tuned oxygen isotope deep sea chronology in the upper part of the Brunhes epoch, but disagree prior to 0·5 Ma ago.