An aircraft ice detection system (10; 100A; 100B; 100C) configured to determine a condition of a cloud, the aircraft ice detection system comprising: a radar transmitter (16; 108) configured to produce quasi-optical radiation having a wavelength of 0.1mm to 3mm; optics (20, 22; 102, 104) configured to direct the quasi-optical radiation from the radar transmitter (16; 108) to the cloud and receive reflected quasi-optical radiation from the cloud; a radar receiver (18; 110; 152) configured to receive the reflected quasi-optical radiation from the optics (20, 22; 102, 104); and a splitter (24; 134) configured to direct the reflected quasi-optical radiation from the optics (20, 22; 102, 104) to the radar receiver (18; 110; 152), wherein the radar transmitter (16; 108) is configured to project the quasi-optical radiation having a transmitted circular polarization, wherein the radar receiver comprises: a first receiver component configured to receive a first polarization component of the reflected quasi-optical radiation, and wherein the first polarization component is opposite the transmitted circular polarization; a second receiver component configured to receive a second polarization component of the reflected quasi-optical radiation, wherein the second polarization component is of the same polarization as the transmitted circular polarization; and a septum duplexer configured to direct the first polarization component to the first receiver component and the second polarization component to the second receiver component. a radar transmitter (16; 108) configured to produce quasi-optical radiation having a wavelength of 0.1mm to 3mm; optics (20, 22; 102, 104) configured to direct the quasi-optical radiation from the radar transmitter (16; 108) to the cloud and receive reflected quasi-optical radiation from the cloud; a radar receiver (18; 110; 152) configured to receive the reflected quasi-optical radiation from the optics (20, 22; 102, 104); and a splitter (24; 134) configured to direct the reflected quasi-optical radiation from the optics (20, 22; 102, 104) to the radar receiver (18; 110; 152), wherein the radar transmitter (16; 108) is configured to project the quasi-optical radiation having a transmitted circular polarization, wherein the radar receiver comprises: a first receiver component configured to receive a first polarization component of the reflected quasi-optical radiation, and wherein the first polarization component is opposite the transmitted circular polarization; a second receiver component configured to receive a second polarization component of the reflected quasi-optical radiation, wherein the second polarization component is of the same polarization as the transmitted circular polarization; and a septum duplexer configured to direct the first polarization component to the first receiver component and the second polarization component to the second receiver component. wherein the radar transmitter (16; 108) is configured to project the quasi-optical radiation having a transmitted circular polarization, wherein the radar receiver comprises: a first receiver component configured to receive a first polarization component of the reflected quasi-optical radiation, and wherein the first polarization component is opposite the transmitted circular polarization; a second receiver component configured to receive a second polarization component of the reflected quasi-optical radiation, wherein the second polarization component is of the same polarization as the transmitted circular polarization; and a septum duplexer configured to direct the first polarization component to the first receiver component and the second polarization component to the second receiver component. a first receiver component configured to receive a first polarization component of the reflected quasi-optical radiation, and wherein the first polarization component is opposite the transmitted circular polarization; a second receiver component configured to receive a second polarization component of the reflected quasi-optical radiation, wherein the second polarization component is of the same polarization as the transmitted circular polarization; and a septum duplexer configured to direct the first polarization component to the first receiver component and the second polarization component to the second receiver component.