It is now almost two years since the European Union decided to go ahead with plans to launch a satellite-navigation network to rival America’s existing Global Positioning System (GPS). For much of this time, Galileo, the European system, met with strong opposition from America. However, a round of talks last November seems to have ease American concerns. The final details remained to be negotiated in talks, but the outlook for an agreement was good.
The core of the disagreement between the EU and America was whether the signals from the two competing systems might interfere with one another. More specifically, the Americans wanted the ability to jam Galileo without rendering GPS signals ineffective. The agreement reached in November was the first step in this direction. In return for the modification of Galileo’s signals, the Americans agreed to give Europe technical assistance in developing Galileo, and to make sure that the third generation of GPS, to be carried out in 2012, will conform to Galileo’s standards. This will aid the interoperability (互用性)of the two systems, which is a commercial goal of both sides. It will also, in principle, give the Europeans the ability to jam the American signals in the of a cr in which the two sides’ interests differ.
There is a bewildering array Of different sorts of signals involved in each network. GPS currently has two, a civilian channel known as C/A and a military one, Y-channel. Plans for an additional military channel, called M-code, are in the works. Galileo will debut (初次公开) with five different signals: one freely available to all; a commercial service which is more precise; a "safety-of-life" service that can be used for critical applications; a "public regulated service" (PRS) which will be used by the EU’s governments; and a fifth service that combines positioning information with a distress beacon, which could be used by ships at sea. The negotiations in November resolved a conflict between America’s M-code and the European PRS. What remains is to harmonize Galileo’s free signal with the M-code.
Both systems rely on signals precisely timed from atomic clocks carried by the satellites. A user looks at the time on at least four satellites, and triangulates (作三角测量)between them to find his position. Differences in the details of the different signals are what make the fees of applications. Some are more precise than others, and they also have different levels of encryption, to pr unauthorized users from accessing them.
What does the word "encryption" (Line 4, Para. 4) possibly mean