The Galileo Code and Others
Working Papers explore the technical and scientific themes that underpin GNSS programs and applications. This regular column is coordinated by Prof. Dr.-Ing. Günter Hein. Contact Prof. Hein at Guenter.Hein@unibw-muenchen.de
GNSS signals might fairly be characterized as an enigma wrapped inside a conundrum. More than any others, two factors give the signals this quality: spread spectrum techniques and their code structure. The first hides the signals in a “cellar” below the thermal noise floor of the RF spectrum; the second disperses them into a long and apparently random sequence of digits. The advent of Europe’s Galileo system and introduction of new GPS signals stimulated a re-examination of the subject of codes, buttressed by advances in electronics that allowed new approaches to implementing codes in a GNSS receiver. This column explores the growing categories of codes, their production, and the qualities that make them suitable for use in GNSS systems. Along the way, we take a brief excursion to discover the surprising genesis of spread spectrum radio in the collaboration of a glamorous actress and an avant-garde pianist.
Codes are a fundamental element in any code division multiple access (CDMA) system such as GPS and Galileo, because these codes are the tool that enables a GNSS receiver to distinguish one satellite from another. In spite of their great importance, no great innovations have been made in the world of satellite navigation in this area — not since GPS used the Gold codes for the first time in its L1 C/A signal introduced nearly 30 years ago.
With GIOVE-A, however, the first Galileo test satellite is now in space. And, together with new signals and new technologies, new code concepts developed in recent years will appear in Galileo transmissions. Galileo will broadcast for the first time so-called random codes, which are codes optimized in a highly multidimensional space to make them look as random as possible.
But Galileo is not alone in bringing new concepts into the world of GNSS. Modernized GPS signals also use new structures of codes based on so-called Legendre sequences, which will be applied for the very first time in navigation.
Given the great importance that codes play in any GNSS system that relies on CDMA and more generally, on spread spectrum (SS) communications, SS techniques will be an important focus of this paper. This column, therefore, will begin by discussing various techniques that rely on codes and the history behind them. Then we will concentrate on the many possibilities that exist to generate pseudorandom codes, giving special attention to those code structures that GPS and Galileo will be implementing in the near future.
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Spread Spectrum Communications
Today, spread spectrum radio has become one of the most important modulation techniques, covering completely different applications ranging from 3G mobile telecommunications, W-LAN, and Bluetooth to satellite positioning systems such as GPS and Galileo.
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Not So Simple
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Not So Random Noise
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PRNGs: A Brief History
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Pseudorandom Noise Codes
Because codes comprise a very wide field, only some of the most popular sequences are listed here, and only the first six families will be described in detail in this article. Of special interest, then, to CDMA-based systems are the following PRN codes:
• Maximal length sequences or m-sequences
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As we have seen in this paper, optimizing codes is not an easy task. In fact, the desirable properties a family should present depend highly on the application the code is intended to serve. What use we will make of GNSS in the future is something that only time will tell, and thus the goodness of the codes for every certain application will definitely not be uniform.
For the complete article, including figures and graphs, please download the PDF at the top of the page.
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