What is meant by "doppler extent"?
Contrary to popular belief, there is no "secret decoder ring" that gives a single, precise definition for every word or phrase you might find in a technical paper. Just like anyone else, you're going to have to read the paper and decide whether


it's a standard phrase used in a standard way
a standard phrase used in a non-standard way
a phrase the author made up


Sometimes the author will give a clear definition, and sometimes you need to infer a definition from context. Sometimes you need to see whether you can find the phrase in other papers, and then decide whether the two papers are using it in the same way.