Navigation and Clocks, Time, Relativity Modelling: a Short Survey

In the spirit of the Workshop, a survey is given on some useful math modelling recipes, especially considering the issues of clocks, time, relativity; there are, in fact, questions which are related to satellite radio-assisted naviga- tion but often not detailed in a comprehensive and hopefully simple overview. Thus, focus is here on some fundamental but "less common" subjects, at least not frequently presented in the same work, whilst other arguments that are usually well documented and often debated (e.g.: ionospheric delays), indeed important, are just mentioned.