1. Research Laboratory in Geodesy, National Institute of Cartography (NIC), Yaounde, Cameroon
2. Department of Physics, Faculty of Science, University of Yaounde I, Yaounde, Cameroon
3. Department of Physics, Faculty of Science, University of Douala, Douala, Cameroon
4. Laboratory E3M Faculty of Industrial Engineering, University of Douala, Douala, Cameroon