Divagations on the Appropriate Satellite System from GNSS for Aviation

American military satellite GPS system is an element of the global GNSS. It emits two codes: the precise and civil one. As late as in 2000, intentional interference that decreased precision of localization was introduced, allowing navigation accuracy of 20 m. GPS was made available to civil users, however, at their own responsibility. So, if we use GPS for our own goals, we must consider the errors that occur and their consequences. Nonetheless, commercial and operational needs determine the necessity of permanent access to satellite signal with appropriate consistency, accessibility, reliability and precision. Hence, natural and intentional errors are compensated for, using appropriate methods, based on a given activity.