This paper discusses RAMSES, the ONERA airborne radar system, and describes the calibration procedure used in polarimetric interferometric synthetic aperture radar (POLINSAR) mode. The RAMSES system has a single-pass POLINSAR capability in which the radar transmits alternately in H and V polarization and receives simultaneously on four channels (H and V for the master and slave antennas). It relies on a deramp-on-receive procedure, designed to permit very high resolution acquisitions when the sampling frequency can no longer match the transmitted bandwidth. This is at the cost of complexity in the received signals, as the deramp operation introduces a phase ramp in range whose slope is a function of the electronic delays in the channel before deramping, resulting in a different slope for the four receiving channels. This characteristic behavior of the deramp-on-receive mode makes relative phase calibration (essential for polarimetry, interferometry, and POLINSAR applications) considerably more complex.