Spotlight synthetic aperture radar

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1. derstanderFeb 2025
   > But with SAR you're not beam forming. You're illuminating everything - the whole ground below you. And you get a return from everywhere all at once. Two equidistant reflectors will return signals simulatenously. If your flight path is between these two points, and the distance is always equal, how can you differentiate them?

   There are a couple conceptual ways to think about SAR. One is, in fact, as beamforming. Each position of the radar along the synthetic aperture is one element in an enormous array that's the length of the synthetic aperture itself: that's your receive array.

   Regarding your question about scatterers that are equidistant along the entire synthetic aperture length: typically, SAR systems don't use isotropic antennas. And they're generally side-looking. So you would see the scatterer to one side of the radar, but not the equidistant scatterer on the other side.

   If you had an isotropic antenna that saw to each side of the synthetic aperture, then the resulting image would be a coherent combination of both sides. Relevant search terms would be iso-range and iso-Doppler lines. Scatterers along the same iso-range and iso-Doppler lines over the length of the synthetic aperture are not distinguishable.

   As to your question earlier in the chain, my preferred SAR book is Carrara et al. Spotlight Synthetic Aperture Radar: Signal Processing Algorithms. Given the title, it is of course geared toward spotlight (where you steer the beam to a particular point) rather than strip map or swath (where your beam is pointed at a fixed angle and dragged as you move along). It has decent coverage of the more computationally efficient Fourier-based image formation algorithms but does not really treat algorithms like the back projection that Henrik uses (I also think back projection is easier to grasp conceptually, particularly for those without a lot of background in Fourier transforms). But my book preference might just be because that's what I first learned with.

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2. JachDec 2014
   The lecture notes on the MIT site these projects are inspired from has some useful PDFs: http://ocw.mit.edu/resources/res-ll-003-build-a-small-radar-...

   When I was building my own version, Kai Chang's RF and Microwave Wireless Systems was a really great resource for the basics. Beyond that, you'll want to have a good foundation in signal processing to make use of the data from the device. The author doesn't mention it, but the radar he has is capable of creating SAR images. I'd love to see the results of a 6 GHz radar -- Dr. Greg Charvat (one of the MIT guys) has a demo on his website of a 10 GHz version that's pretty impressive. His thesis can help with some signal processing details, but the book Spotlight Synthetic Aperture Radar: Signal Processing Algorithms by Carrara, Goodman, and Majewski is even better.

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