Architecture for a Future C-Band/L-band GNSS Mission, Part 2
A Potential Signal Plan and Related User Terminal Aspects
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
Almost all GNSS navigation signals operate in the crowded L-band portion of the radio frequency spectrum. In the past, C-band spectrum has been considered — and rejected — for GNSS services due to a couple of substantial obstacles, despite some distinct technical advantages. However, continued proliferation of signals in L-band and advances in electronics and spacecraft technologies have prompted a new look at C-band for future GNSS services. This article is the second of a two-part series describing the results of a new European Space Agency–sponsored study on the subject.
The downloadable PDF (above) contains bonus material not available in the print edition.
The radio navigation satellite service (RNSS) portion of the radio frequency (RF) spectrum is already overcrowded, and the bands suitable for new uses are very limited. This is especially true for the E1/L1 band occupied today by GPS and Galileo.
In addition, Japan’s quasi-zenith satellite system (QZSS) and potentially also Compass and GLONASS will be transmitting navigation signals in this frequency band. But E1/L1 is not the only case. Even those RF bands that are not being used yet will certainly be shared by many systems in the near future.
Thus, the search for unused frequency resources will almost certainly continue during the next years. The World Radio Communications Conference 2000 (WRC-2000) allocated the portion of C-band between 5010 and 5030 MHz for RNSS space-to-Earth applications.
The general idea was to provide access to a frequency band that is not yet overloaded by other signal sources and, consequently, not so susceptible to interfering signals as guided by International Telecommunications Union (ITU) regulations.
Navigation in C-band presents both advantages and disadvantages, the most important drawback being the higher free space losses due to the limitations on the higher signal frequency. An omnidirectional C-band antenna at 5 GHz will be 3.2 times smaller in the linear dimension than an equivalent L1-band antenna. (The latter signal has a 19-centimeter wavelength at 1.575 GHz compared to the wavelength of 6 centimeters at 5.015 GHz.)
Because of this wavelength-driven design factor, the area of the C-band antenna will be 10 times smaller than that of a standard L-band antenna. As a result, a C-band antenna receives only 1/10th the broadcast power of its L-band counterpart. (For details of relevant research, see the articles by M. Irsigler et alia and A. Schmitz-Peiffer et alia (2008) in the Additional Resources section near the end of this article.)
Another important factor is the increased signal attenuation of C-band signals due to foliage, heavy rain, or indoors, as well as other negative environmental effects on signal tracking. On the other hand, C-band exhibits much smaller ionospheric errors for standard single-frequency applications.
The hope is that technological progress might balance some of the disadvantages from a long-term point of view, given that an actual application of C-band for RNSS is not foreseen before the year 2020.
We began our discussion in the previous column (May/June 2009, Inside GNSS) with an explanation of the scope of the C-band project, service analysis, satellite constellations, ground segment, satellite transmit signal power requirement, payload design, spacecraft accommodation, and end-to-end performance.
In this column we talk about the C-band signal design driven to respect the given constraints of other C-band services, and the C-band user terminal equipment design and performance analysis in the context of expected applications.
Additional discussion of the navigation message structure design and the related added value concerning the troposphere corrections (e.g., the combination of navigation data and numerical weather data from meteorological satellites), together with critical user-terminal technologies needed to prepare C-band for use in a future GNSS constellation, can be found here on the Inside GNSS website.
C-Band Signals Considered
A quick look at this service definition reveals the main motivation for both services: 1) the SPR-C was to maximize the possible user communities under C-band, following the civil/public dual-use concept of satellite navigation; 2) the PRS-C was to provide selected users with the access to this service in order to fulfill high security requirements (e.g., anti-jamming and anti-spoofing).
As discussed in the first part of this series, the PRS-C consists of two small spot beams with approximately 1,500 kilometers of radius. Moreover, these two spot beams shall provide high geographic flexibility to point at any required area on earth.
In addition, use of C-band shall aim at mitigating problem areas of current L-band signals. In fact, the C-band Service Plan was designed to address the vulnerability of L-band in critical infrastructures by providing additional robustness in degraded RF situations. Moreover, the proliferation of GNSSs and lack of high precision signals that work on a single frequency have also been important drivers in the C-band study.
In order to design C-band signals the top-level requirements for both services were analyzed and established in terms of geometric dilution of precision (GDOP), availability, and continuity risk among other factors, and so on.
In addition to this, the SPR-C requires authentication capability to provide robustness in terms of anti-spoofing while the PRS-C needs code-encryption capability to provide enhanced anti-spoofing performance. Both service signals should be spectrally decoupled from each other.
The C-band signal plan was optimized for maximum occupied bandwidth and spectral separation between the two provided services.
In consequence, the signals presented next must be interpreted as an envelope of solutions in the sense that derived alternative signals with lower chip-rate and lower sub-carrier frequencies would also fulfill the criteria for compatibility with nearby C-band services. These are namely the radio-astronomy service (RA), the microwave landing system (MLS) service (MLS), and the Galileo up-link (UL) service.
. . .
Compatibility of C-Band Signals
Gaussian MSK (GMSK)
Compatibility of GMSK Signals
Overall User Terminal Architectures
Signal-In-Space (SIS) Model
As a result, GMSK (with BT=0.3) modulated both on I and Q channels was selected. Based on an extensive signal performance analysis together with user terminal aspects, this modulation scheme was further optimized for maximum bandwidth occupation and spectral separation between the two identified services.
Detailed signal parameters such as chip rate, chip length, and so on were designed to satisfy the requirement that C-band navigation services shall be competitive with current or planned L-band services.
For the complete story, including figures, graphs, and images, please download the PDF of the article, above.
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