Dual-frequency bistatic radar experiments were conducted with Mars Express at a rate of one to two per month during 2005. Each observation provided power measurements of orthogonally polarized surface echoes at one specular point; the ratio of these components yielded values of the dielectric constant in the range 2.0 < $varepsilon$ < 4.0. Doppler sorting of X-band (wavelength λ = 3.6 cm) echoes was used to achieve one-dimensional surface resolutions of about 20 km. Although much weaker, simultaneous S-band (13-cm) echoes yielded dielectric constants that are 10--50% higher than 3.6 cm echoes, consistent with deeper surface penetration.