Global Navigation Satellite Systems（ GNSS）,including the United States Global Positioning System（ GPS）,Russian GLONASS,the European Union’s Galileo,and China’s Bei Dou,provide L-band microwave signals with high temporal resolution.The applications of GNSS are extended from positioning / navigation to remote sensing. The versatile refracted GNSS signals sound the atmosphere and ionosphere. The reflected signals,which involve measuring the reflections from the Earth,have recently shown their capacity for Earth observations over ocean or land surfaces. This newly developed technique is called GNSS reflectometry（ GNSS-R）. The structure of GNSS-R observations can be theoretically considered as a bistatic radar. Analyzing the principles and characteristics of GNSS-R,we define GNSS-R observations as two patterns,namely,Double-Antenna Pattern（ DAP） and SingleAntenna Pattern（ SAP）,by using signal-receiving approaches. DAP has two antennas that straightforwardly receive the direct and reflected signals. The main data processing procedure of the DAP is based on the bistatic radar equation. The SAP receives the interference of the direct and reflected signals by using a single antenna. The main data processing procedure of the SAP is based on theories of signal interference. Some special issues related to the DAP and SAP are also illustrated in this study; these issues include the multiple polarization,footprint,and locations of the specular points. An initial frame for GNSS-R applications is also proposed. The results of several ground-based or airborne experiments previously conducted by our group and collaborators are shown to further explain the proposed DAP and SAP patterns. These results include those from both soil moisture and vegetationwater-content retrievals. The next generations of GNSS systems,especially GPS III and Bei Dou II,are expected to demonstrate improved performance and increased capabilities to users worldwide. This study may serve as a reference for China’s future development of GNSS-R.

GNSS-R;BeiD ou;GPS;observation pattern;interference;soil moisture