How to determine whether an antenna for GPS is circularly polarized by Smith Chart?

In many papers, it is mentioned that circularly polarized characteristic can be ensured when a cusp in observed. But why? And, how to understand the principle of cusp.

I believe they are talking about the actual artwork shape of the antenna. There are "cusp" antennas that are dual polarized. i.e. you have to pry open the antenna and look at it.

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