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Envisioning a Future: GNSS System of Systems, Part 3

A Role for C-Band?

“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.

 

The radio frequency spectrum is finite. Crowding in the L-band occupied — or planned for use — by the world’s global navigation satellite systems is only going to get worse, as more systems and more signals come on line. Several gigahertz up the RF spectrum from L-band, however, lies a wide swath of bandwidth that is comparatively untapped: the C-band. In fact, early in its development the Galileo program received an allocation of C-band from the World Radiocommunications Conference. Although Galileo system designers decided quite early not to use the allocation for a variety of practical reasons, C-band remains an enticing subject because of certain characteristics that seem to complement or compensate for technical limitations of L-band, particularly the need for better indoor positioning capability. This column examines C-band as a candidate for a future GNSS signal or signals and evaluates its advantages and disadvantages compared with L-band signals.

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The radionavigation satellite service (RNSS) portion of the RF spectrum is overcrowded, especially on L1 where GPS, Galileo, Compass overlap portions of one another’s signal frequencies and GLONASS signals occupy more than 11 MHz of nearby bandwidth. Indeed, even those bands that have not been used so far will certainly be shared by many systems in the near future. Therefore, the search of alternative frequency resources is something that must inevitably occur with a high probability in the coming years.

During the World Radio Conference 2000 (WRC-2000), the Galileo program obtained authorization to use C-band frequencies. At the time, a dedicated portion of the C-band had been assigned for radionavigation, but technical complexities made it impossible for the first generation of Galileo to make use of it. Phase noise problems, increased signal transmit power requirements, and signal attenuation issues — to name only a few — knocked down all the proposed solutions. We will refer to these aspects in detail in the following discussion.

As happens with any kind of technology, however, many ideas that have been abandoned in the past due to excessive technical challenges or demanding drawbacks often become objects of interest some years or decades later. As technology evolves, constraints alter, and the environment of possibilities changes.

Against this background, the question emerges as to whether the use of C-band frequencies could represent a real alternative for a future GNSS. In this column, we will try to shed some light on this interesting possibility. Before that, let us first look at what we understand about C-band and how the regulatory RF spectrum situation affecting its use varies in different countries.

(For the rest of this story, please download the complete article using the PDF link above.)

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