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Navigation signals are the basis of satellite navigation systems, which determine the navigation systems' performance (such as tracking accuracy, anti-interference capability etc.), and are determined by some constraints (such as radio frequency resources, construction cost, technology level etc.). The aim of signal design is to select "proper" signals considering these above factors and make tradeoffs between performance and constraints. This paper will summarize the signal structure evolution of present satellite navigation and conclude some principles and issues which will be greatly valuable to our 2nd satellite navigation system. There's four main satellite positioning systems presently, the U.S. Global Positioning System (GPS), the Russian Global Navigation Satellite System (GLONASS), the Europe Galileo satellite navigation system and the Chinese BD-1 satellite positioning system. Among these systems, the signal characteristics of GPS and Galileo are good reference to our 2nd satellite navigation system. The improvement of GPS signals is a main aspect of GPS modernization. The aim of GPS modernization is mainly to modernize its signal structure so as to improve the whole systems' performance. The signal structure of Galileo has been revised for several times, and its present signal is the results of long time signal design. From GPS modernization and Galileo signals status, we can summarize some key specifications of navigation signals and some technologies to support these specifications. Based on this, we'll discuss some problems which should be attentive in our country's 2nd satellite navigation system and give some suggestions. This paper is organized into five sections. The first section will give a brief introduction of present satellite navigation system and their main features. Section two will review the GPS and Galileo signals structure evolution, including GPS modernization, Galileo signals status and the comparison of two systems' signals. In the third section, the relationship between signal parameters and system performance will be analyzed, including RF signal characteristics, modulation type, ranging code (such as code type, length and rate) and navigation data. In section four we will discuss some inspiration and technologies concerning our 2nd satellite navigation system, such as the use of multiple frequency signals (ionospheric delay estimation and carrier ambiguity resolution), pilot tones & dataless channel, binary offset carrier (BOC) modulation, and so on. The last section is conclusion, in which the whole paper will be retrospected and some useful conclusion will be given.
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