Mathematically, "single frequency" implies an impossible transmitter that never turns on or off (Fourier-wise). but really there are all kinds of radar that uses a wide spectral range.
Chirped radar, e.g. SHARAD at Mars and ICEYE at Earth is a standard radar technique. According to this ICEYE's radar chirp is spread to about 300 MHz around it's 9.65 GHz center frequency.
A quick internet search will show that "dual frequency radar" (also) is standard for measuring water content of planetary atmospheres e.g. for Earth (1, 2) and Mars (1, 2).
Dual (or more) frequencies allows the ability to identify things like water who's highly polar molecule results in a high dispersion (change in index of refraction with frequency). This even allows one to distinguish types of ice (H2O vs CO2) and the phase of the water (liquid, solid, gas).
Dual frequency also allows for measurements of the roughness of surface, and sub-surface interfaces via Raleigh scattering's $\lambda^{-4}$ dependence.
@Heopps's comment points out that the Europa Clipper spacecrtaft's REASON instrument will also use dual frequency radar, at the much lower frequencies of 9 MHz (HF) and 60 MHz (VHF).
For more about this implementation of dual-frequency radar for water characterization, see Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON).
However, all of that having been said, RADARSAT-1 and likely RADARSAT-2's synthetic aperture radar does seem to have only a single band at 5.3 GHz (5.6 cm).