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I have learned that optical earth observing satellites have spectral bands often in groups (e.g. panchromatic, multi-spectral, SWIR, etc.).

What about radar satellites such as RADARSAT-1? It seems it only has a C band. Does it mean the constellation RADARSAT-1 and RADARSAT-2 has just one spectral band?

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Not just one spectral band, but a single frequency.

Classical radars use a tuned microwave cavity to produce their radar emissions. These operate at a single frequency. The radar receiver operates in a narrow band around this frequency (to allow for Doppler shift). "C band" roughly indicates at which frequency the radar operates (C band is anything between 4–8 GHz).

There are radars with more frequency agility, but the same principle applies: the receiving end is tuned to whatever the transmitter produces.

Radar is not really an equivalent to optical observation: radar is an active system, sending out a pulse and then processing the return to derive information from that.
A radio receiver is a better equivalent. In radio astronomy, it's common to have receivers that can process a wide range of frequencies. The receiver has to scan through that range one frequency at a time. By recording the input at each frequency you can do spectroscopy etc. in the same way you would for an optical image.

There aren't many radio astronomy satellites (I know of just one, the Russian Spektr-R).

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  • $\begingroup$ Technically, it's one "central frequency". Because satellites' active radars have very short impulses, the impulses have some bandwidth. $\endgroup$ – Heopps Jul 23 '18 at 21:02
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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).

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