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SAR 101: Why do we need Microwave Sensing?

Definition:

Microwave Sensing : A sort of process where we use the electromagnetic wavelengths with microwave energy to create images of the Earth.

Remote Sensing: The science of observing from a distance. In this case, satellites are observing the earth from space.

Synthetic Aperture Radar (SAR) : ??

Introduction

This is an informal writing and personal notes as I read through 'The Essential of SAR' By Thomas P. Ager. I started scribbling some notes in the paper back but for some reasons decided to make them digital. I am sensing this is the best way to stay consistent with reading.

Why do we need Microwave sensing

  • Earth presents major obstacles for satellites: cloud, darkness, with the polar region having ones that lasts for months, atmospheric gases (Water Vapor, CO2), atmospheric particles(smog, dust).

How

In remote sensing, what to consider is how different wavelength of the electromagnetic spectrum interact with the Earth’s atmosphere.

  • Optical and thermal sensors only work in certain “windows”, where the atmosphere does not absorb too much.
  • The main usable window is between 0.4 and 14 µm. Sentinel 2 uses range between 0.44–2.2 µm and Landsat uses 0.4–12 µm, both are placed within clear atmospheric windows.
  • Outside this range, absorption by water vapor and other gases is so strong that optical and thermal sensing becomes impossible, even if there are no clouds.
  • This is why we need to use longer wavelengths (microwaves), which can pass through clouds, darkness, and most atmospheric gases.
  • Microwave's range in the spectrum is between 1mm to 30 centimeter and SAR missions cover ~2.5 cm (X-band) to ~100 cm (P-band).

Wavelenghts affect resolution but HOW??

This can only make sense when we try to understand how camera works and specific terms like focal length, field of view. I am not sure how deep I want to go yet.

But for now, the angular resolution is roughly equal to the wavelength divided by the size of the lens(antenna for SAR).

θ≈λD\theta \approx \frac{\lambda}{D}

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  • Short bands have higher resolution
  • X band have the best combination for space borne sensors, they penetrate the atmospher at 99% under normal condition.