5 Radars usedFor the Nederlands they used Radar ID 6234 Den Helder and Radar ID 6260 De Bilt and for Belgium Radar ID 6410 Jabbeke, Radar ID 6451 Zaventem and Radar ID 6477 Wideumont (1).
Principles of a radar on a globeA big problem for radars on a globe is that they can only register what is above their line of sight. For-instance if the radar is approximately 50 m high, their line of sight crosses the horizon at (6371050^2-6371000^2)^91/2) = 25,2 km. From that point on the radar cannot see what is below their line of sight, behind the horizon. In that case for more information they will need to use other radars as well. A high grid of radars would be required.
Radar on a flat earthIf a radar is located on a flat earth, it would work the best. Beams from the radar are send out and come back with the required information on weather, planes and in this case birds. As the radar beam goes in a straight line and comes back it can give much information from ground-level to several km high. A low grid of radars is needed.
Radar Den Helder and bird tracking near Eemshaven