Sponsorship: JPL Subcontract 961461. Nadir and off-nadir measurements of electromagnetic (EM) bias measurements are presented and compared with an off-nadir bias model. Measurements of the bias were made during the BYU Off-Nadir Experiment (Y-ONE) in the months of March and April, 2003. Using radar measurements of the surface and backscattered power, the EM bias was computed at angles from nadir to 17º degrees. Simultaneous surface measurements from a laser rangefinder provide accurate measurements of the long wave surface parameters. An off-nadir bias model incorporating the effects of hydrodynamic modulation of short waves and tilt modulation of long waves is developed and compared to measured off-nadir bias values from the experiment. The magnitude of the measured and predicted EM bias decreases with incidence angle so that the average bias vanishes at approximately 15º for C-band measurements and at 17º for the Ku-band values.