# Number of spectral bands on radar satellites

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?

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).

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

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.

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).

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).

A satellite can only observe as many bands as it is designed to. Generally, astronomical satellites are rarely designed the same as other astronomical satellites for a multitude of reasons.

The more bands you wish to observe, the more instrumentation (/weight) needed to observe those bands. Also, the fewer bands you wish to observe, the more instrumentation (/weight) can be dedicated to observing that band thus increasing the fidelity/detail/resolution of your observation. So instead of seeing everything close to you, you can instead see things further out more clearly. Weight is very important when talking about putting a satellite in orbit.

So to the first set of questions, the number of spectral bands is entirely dependant on the satellite in question. In this case wikipedia is a better source than anything here.

To the next question: while the constellation was observed in the C-band by that satellite it would be quite a scientific uproar, if that were the only band it was producing as it would radically redefine people's understanding of physics.

For instance, Hubble visible spectrum images of space cannot see anywhere near as far or as much as radio telescopes can.

• This is general information that does not answer the question. "In this case wikipedia is a better source than anything here" No. – Organic Marble Jul 23 '18 at 16:46