Schematic illustration of the different scenarios for grouping of VAs. Top: Situation when the Earth is between two subsequent VAs (difference in M has opposite signs). In this case we chose j+1-th VA as our primary representative and j-th as a secondary one. Bottom: Situation when all the subsequent VAs are on one side from the Earth (difference in M are all positive). In this situation we chose j+2-th as our primary representative. Note that j+7-th VA is in another group but it will also be a representative of its group.