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There's a quote here from Carl Wiley, the inventor of synthetic aperture radar:

"I had the luck to conceive of the basic idea, which I called Doppler Beam Sharpening (DBS), rather than Synthetic Aperture Radar (SAR). Like all signal processing, there is a dual theory. One is a frequency-domain explanation. This is Doppler Beam Sharpening. If one prefers, one can analyze the system in the time domain instead. This is SAR."

The quote implies that DBS is a type of SAR, or perhaps even that they're the same thing. But I have a very different understanding of each technology, and can't reconcile them.

Consider a fixed array of antennas that form a synthetic aperture. By coherently summing the signals returned to each antenna, you can massively reduce the effective field of view and improve the resolution in the direction parallel to the antenna. The only thing that matters is that a target is "multilooked" by all antennas that form the aperture. Critically, nothing has to be moving relative to each other for this to work, and there is no Doppler shifting of any signals. So how does DBS come into this?

Here's the converse: Consider an orbiting radar altimeter such as CryoSat-2. After emitting a chirp, it receives a returned pulse and splits that return into several different signals based on their various doppler shifts. The various Doppler shifts correspond to strips of the Earth's surface in the along-track direction, with strips further ahead of the satellite experiencing larger positive blue shifts, and strips further behind experiencing larger negative red shifts. By splitting the signal into these strips, you radically improve the spatial resolution in the along-track direction. However, this can be achieved with just one chirp of the satellite, with no synthetic aperture being constructed over multiple chirps, and no multi-looking. So DBS is achieved without the use of a synthetic aperture.

As you can see above, I have a very different understanding of how SAR works compared to how DBS works, and I can't reconcile them. If somebody could clarify (especially using the example of a SAR altimeter with DBS, such as CryoSat-2) that would be very much appreciated.

Robbie Mallett
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  • Quote doesn't make it clear they are the same. It means that this technique (DBS) is an example of SAR. But it might be wrong to conclude that every example of SAR is also DBS. –  Jul 07 '23 at 12:37
  • This is a good point - have changed question in response. – Robbie Mallett Jul 07 '23 at 15:12
  • SAR can be used as a general term, but when in the industry it means a much narrower type of thing. DBS can be achieved without the use of a "synthetic aperture", and thus has less stringent requirements. DBS was a way to get finer angle resolution using Doppler, which is a straight-forward and even trivial calculation. Modern SAR requires a set of dwells to be collected with a known platform motion (synthetic aperture), and on top of that you have various SAR image forming techniques that come into play. – Envidia Aug 19 '23 at 23:46

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