Characterizing Nominal Analog Signal Deformation on GNSS Signals

Satellite-based navigation requires precise knowledge of the structure of the transmitted signals. Accurate knowledge of the shapes of the code correlation peaks is required to ensure no biases are introduced into the position solution. It is often presumed that all incoming ranging codes (e.g., C/A codes) are effectively ideal. However, nominal signal deformations—small distortions of the code chip shapes—can lead to ranging errors. Distortions which are created by satellite filter imperfections in particular, may cause errors that vary significantly with receiver filter characteristics and code tracking loop implementations. These analog signal deformations should be well-understood so that receiver performance meets user requirements. This paper discusses a methodology for characterizing nominal analog distortions. First, it identifies a general approach for specifying the nominal limits for GNSS codes. Second, it leverages actual measured data from code chip (step) responses of several existing GPS SVs to determine practical performance bounds. Next, it develops a generalized filter model to simulate and further analyze these signals. Finally, it provides examples of the potential user range error implications of these specifications to potentially determine whether or not they should be modified. While this methodology is developed using GPS C/A codes, it is proposed as a practical approach for determining useful specifications on future GNSS codes as well.