The future Global Navigation Satellite System (GNSS) is proposed to be multi-constellation with multiple civil frequency transmissions. The multi-constellation GNSS is very important as this can provide improved accuracy, reliability, integrity and continuity of service to the users with more visible satellites than any standalone GNSS system. Moreover, by using common RF frequencies, the complexity of the receiver's RF-front end can be considerably reduced. This paper presents the existing constellations, upcoming ones and signal structures of some of the civil navigation signals. It also discusses various accuracy optimization techniques. Some important issues in a multi-constellation scenario, such as compatibility and interoperability, are also discussed.

Global Navigation Satellite System (GNSS) is making advancements much more rapidly than anticipated with increased participation and funding from various countries, to tap the huge potential benefits. Most of the developed countries are coming out with their own satellite-based navigation systems. The present satellite-based navigation system available for the user is Global Positioning System (GPS) which is conceived and developed by the US. The Russian federation, which had the satellite-based navigation system called Global Navigation Satellite System (GLONASS), has announced its plans to modernize the system for global users. The European Union is coming out with its own satellite-based navigation system called GALILEO. The Chinese Government has proposed to have a separate satellite-based navigation system called COMPASS (also known as Beidou-2). Japan has proposed to make full use of GPS for its Quasi-Zenith Satellite System (QZSS). The Government of India has already approved the implementation of the Indian Regional Navigation Satellite System (IRNSS).

The salient feature of civil satellite-based navigation is in multiple frequency transmission, in addition to advanced and more robust signal structures. This is true for all the proposed satellite-based navigation systems. The current civil GPS signal consists of a single frequency transmission designated as L1. The modernized GPS has additional signals such as L2 and L5. Multiple frequencies greatly enhance the performance of satellite-based navigation (Dennis et al., 2003). One of the most commonly cited limitations with GPS is the vulnerability of L1 signal to Radio Frequency Interference (RFI), either intentional or unintentional. Frequency diversity improves this potential limitation of the system, and multiband receiver presents the improved integrity and robustness against jamming attempts. In addition, multiple frequencies provide ionosphere estimation capabilities, removing one of the most significant error sources in the current standalone GPS system. GALILEO and other upcoming systems are designed to provide the capabilities of a multiple civil frequency satellite navigation system.

Telecommunications Journal, Global Navigation Satellite System, GNSS, Global Positioning System, GPS, Quasi-Zenith Satellite System, QZSS, Radio Frequency Interference, RFI, International Atomic Time, TAI, Galileo System Time, GST, Galileo Terrestrial Reference Frame, GTRF, Frequency Division Multiple Frequency, FDMA.