Corner reflectors designed for radar are targets that reflect the wave emitted by the satellite back to the antenna. The combined reflections on the 3 mirors redirect the waves in their original direction and it can be demonstrated by simple math. We can decompose the propagation vector of the wave k in the 3-dimentionnal basis make by normal to the 3 mirors, n1, n2 and n3 : k=a1.n1+a2.n2+a3.n3 . The optical law of reflection that we can use in radar says that the reflection on the miror 1 will change the sign of the composante n1 of the vector k: k’=-a1.n1+a2.n2+a3.n3 . After the reflection on the second miror, the composante n2 will change sign: k”=-a1.n1-a2.n2+a3.n3 and after the reflection on the last miror, k”‘=-a1.n1-a2.n2-a3.n3 = -k . In conclusion, after the three reflections the wave that leaves the corner reflector go in the opposite direction that the wave that entered in the corner reflector and thus go back to the antenna.
For our experience on the Eiffel Tower, we put 2 corner reflectors above the restaurant “Jules Vernes”, located at the second level on a platform which we know the altitude. The corner have been on the Eiffel Tower for nearly two month, during the new TerraSAR-X and TanDEM-X acquisitions over the city. We are now waiting for the delivery of the images. Then, when we locate them on the images, they serve as reference for the height measurment that will allow us to have a precise deformation measurment.
The first corner and its view
The second corner and its view.