Detection of coherent reflections with GPS bipath interferometry
  Results from a GPS reflectometry experiment with a 12 channel ground-based GPS receiver above two lakes in the Bavarian Alps are presented. The receiver measures in open-loop mode the coarse/aquisition code correlation function of the direct and the reflected signal of one GPS satellite simultaneously. The interference between the coherently reflected signal and a model signal, which is phase-locked to the direct signal, causes variations in the amplitude of the in-phase and quad-phase components of the correlation sums. From these amplitude variations the relative altimetric height is determined within a precision of 2 cm. 
Achim Helm, Georg Beyerle and Markus Nitschke (GeoForschungsZentrum   Potsdam, Dept. Geodesy and Remote Sensing, Germany)