Effects of birefringence within ice sheets on bistatic radar measurements

Ice fabric is known to affect radio wave propagation within polar ice sheets. Strong vertical contrasts in the fabric cause reflection, while anisotropy in the fabric integrated along the propagation path causes birefringence. Bistatic radar measurements (i.e. using a transmitter and receiver separated from each other) have more potential to assess fabric than monostatic measurements, because they allow more general geometric relationships between the radio wave and ice fabric. We worked out the effect of fabric on radio waves propagating obliquely through the polar ice sheets in terms of radar frequency, polarization and geometry of the propagation path. The model is applied to fabrics that approximate those found in the polar ice sheets. When ice fabrics are unknown, the geometry of the propagation path relative to the ice fabric is arbitrary. In this case, 1-km- thick ice with a quite gentle and strongly developed fabric causes significant (> 1 dB) birefringence-origin echo variations at radar frequencies higher than 200 MHz and 20 MHz, respectively. An azimuthal pattern of the echo collected with bistatic radar is 90° periodic, not 45° as found in the vertical propagation (monostatic radar sounding). It suggests that bistatic radar can be used to identify principal axes of the ice fabric.