Architecture for a Future C-Band/L-Band GNSS

Part I: C-Band Services, Space and Ground Segments, Overall Performance

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Within the European Space Agency (ESA) GNSS Evolution program, EADS Astrium has led a team analyzing the potential benefits, performance, and technical requirements for adding a C-band navigation capability to existing L-band services on a second-generation Galileo.

The C-band issue is not new. Between 1998 and 2004, U.S. and European researchers undertook significant work on this subject. One of the factors that led to the decision at that time not to choose C-band for Galileo was that the required satellite payload power would have been difficult to provide.

However, C-band has returned as a candidate for GNSS systems. Among the main reasons for renewed interest in C-band is that its frequency (5010 - 5030 MHz) is rather untouched compared to the L-band, where existing and new navigation satellite systems have proliferated. Moreover, C-band offers a variety of technical characteristics compared to L-band that make it particularly attractive for regulated and safety-critical applications.

The provision of a C-band navigation signal on a future GNSS would make sense if a new set of services can be offered (with markets behind them) and the satellite power requirements and associated link budget deficiencies can be solved.

In this issue, we introduce the first of a two-part column examining the potential for incorporating C-band technology into a GNSS system. The first part discusses prospective C-band services and applications, signal propagation and user equivalent range error (UERE), spacecraft payload design, satellite constellation, and end-to-end performance.

In the July-August issue of Inside GNSS, the second part of the column will focus on C-band signal design for GNSS given the constraints of other C-band services, optimal navigation message design, C-band user equipment design in the context of expected applications, and identification of critical technologies needed to prepare C-band for use in a future GNSS constellation.

The Scope of the C-Band Project
In order to show the benefits of a future C-band navigation in addition to the L-band system used by GNSSs, including the Global Positioning System and Galileo, the C-band analysis included an architecture study that considered likely technology developments through 2020.

The main justification for offering an additional C-band navigation capability would be to provide new GNSS services for new or existing applications. Consequently, a detailed user market analysis was performed in light of present and future market trends and parallel developments in user receiver design. We will discuss the outcomes of these analyses a little later in this article.

For the identified services, we then performed satellite constellation analyses in order to derive the required navigation parameters: number of the satellites available for the user, dilution of precision (DOP), positioning performance, and so forth.

We simulated the performance of various candidate signals in order to identify robust C-band signals that fulfill the C-band user requirements. In analyzing C-band signal propagation, we applied the latest atmospheric models. Appropriate C-band GNSS signals were then designed, considering the spectral constraints imposed by adjoining C-band services, such as radio astronomy (RA) and microwave landing systems (MLS).

On the user equipment side, we investigated appropriate receiver architectures and derived link budgets for various services to verify the design.

C-Band Service Analysis
A detailed market and user receiver analysis has identified two baseline C-band services: A Service with Precision and Robustness (SPR-C), with global coverage, and a Public Regulated Service in C-band (PRS-C) with spot beam coverage over two selectable service areas.

The SPR-C would provide users with additional robustness, protection, and precision for non–security-related critical infrastructures and applications for which vulnerability is a threat. In this regard, C-band offers the following advantages: no spectrum proliferation, smaller signal propagation effects from the ionosphere and unintentional interference, and higher jamming resistance compared to the L-band for same C/N0.

As envisioned by the C-band service analysis, the SPR-C could support professional satellite navigation in situations where L-band signals are degraded and would provide additional value-added elements with the navigation message, such as clock and tropospheric correction data. The service would be protected against spoofing by authentication …

. . .

Satellite Constellation with C-Band
Navigation service requirements such as availability and position dilution of precision (PDOP) have a direct effect on the configuration of the satellite constellation. Consequently, a variety of constellations were analyzed in order to find the best solution.

The C-band study conducted a trade-off analysis of a global and a regional SPR-C service. A regional SPR-C would provide continuous service over three selected industrial areas, covering North America, Europe and Eastern Asia (each area covering a circle of roughly 6,000 kilometers in diameter on the Earth’s surface) …

. . .

C-Band Ground Segment
The Galileo ground segment needs to be modified to consider the additional use of C-band signals. The extended ground segment will provide new navigation message data, including improved tropospheric corrections based on numerical weather data, and the mission planning for PRS-C operations.

The Galileo architecture remains valid for the C-band services. Upgrades of a subset of ground sensor stations (GSSs) will be necessary to include C-band signal tracking capabilities in order to determine the biases associated with the spacecraft, C-band antenna, and RF chain …

. . .

C-Band Transmit Power Requirement
For both C-band services, we calculated link budgets in order to determine the DC power required at the payload level. These will be described later in the end-to-end performance section …

. . .

C-Band Payload Design
The preferred payload architecture would accommodate the C-band payload on a spacecraft in combination with the current Galileo L-band payload.

With this objective in mind, we will now cover the following points:

• general payload architectures to perform the beam forming for PRS-C

• RF front-end technologies, such as frequency up-conversion principles

• possibilities of high-power amplification and effects on power budget and signal distortion

• trade-off between antenna design and signal generator payload

• interference with mission up-link receiver and preservation of ITU regulatory

• power and mass budgets

The design of the most appropriate C-band signals will be described in the July/August issue of Inside GNSS. However, as the signal design is one driver for the payload design, we will briefly introduce it here …

. . .

Navigation Signal Output Filter. Power amplifiers in general exhibit nonlinear distortions in both amplitude (AM/AM) and phase (AM/PM). This causes spectral regrowth of the signal, which leads to adjacent channel interference and becomes an issue due to the stringent requirements of the adjacent bands …

. . .

Payload Architecture for a Global SPR-C Service
The selected payload architecture for the global SPR-C service consists of an NSGU and an FGUU.The signal is amplified by a set of parallel switched TWTAs ...

. . .

Payload Architecture Trade-off
Three different payload architectures were analyzed in order to find the most appropriate one for the PRS-C services: digital beam-forming, RF high power beam-forming, and use of a single reflector antenna with mechanical steering …

Digital Beam-Forming. The proposal would generate the PRS-C service using digital beam-forming for the two single beam PRS-C array antennas with 31 feeds. The major challenge of this approach arises from the complexity of having 31 identical channels running in parallel …

RF High-Power Beam-Forming. In contrast to digital beam-forming, Payload Architecture 2 would implement beam-forming of the PRS-C signals after amplification using ferrite phase shifters …

. . .

Power and Mass Budgets
Several factors drive the requirement for payload power:

• power efficiency of the HPA

• the need to control amplification so as to ensure that signal distortions and OOB emissions are within acceptable limits

• beam-forming network architecture

• number of antenna feeds

• cable and filter losses.

We investigated the payload mass and power budget, taking into account the additional C-band elements and the three different PRS-C payload architectures …

. . .

Spacecraft Accommodation
The C-band/L-band payload architecture design has been taken as an input for the space segment in order to analyze the accommodation on the spacecraft and launcher. The accommodation analysis was performed for the two C-band service concepts displayed in Table 4 using Architecture 1 because it also covers the slightly relaxed power and mass requirements of Architecture 2 ...

. . .

End-to-End Performance
The overall performance of the SPR-C and PRS-C services has been analyzed by calculating link budgets for different receiver classes in simulated application scenarios. The analysis poses a kind of worst-case scenario for the C-band because of the substantial effects of tropospheric water vapor content there.

The link budgets include a section with the constants, service requirements (availability of service derived from the Galileo baseline), payload parameters, the atmospheric environment based on detailed analysis, and receiver requirements.

Additional discussion of the application scenarios, along with detailed tables showing the link budget parameters and values can be found here on the Inside GNSS website.

Conclusions
We performed a detailed system study in order to analyze the advantages and the effect of an additional C-band GNSS navigation system. Two C-band services have been identified that fill niches not covered by the present Galileo L-band signals: a global Service of Precision and Robustness and two independent spot-beam Public Regulated Services.

The constellation analysis shows that a set of 27 Galileo C/L-band satellites provides sufficient navigation performance. The payload accommodation analysis shows that around three kilowatts are required for a combined C/L-band satellite offering the SPR-C and two PRS-C services. The proposed new C/L-band Galileo spacecraft design is based on a EUROSTAR 3000 bus, with two EUROSTARs fitting into an Ariane 5 launcher or one on a Soyuz launcher.

In case only two independent PRS-C services would be offered, an extended Galileo IOV platform with more efficient HPAs would be sufficient. This platform is half the size of the EUROSTAR 3000 solution, which means that four extended Galileo satellites offering L-band and spot-beam C-band services would fit on one Ariane 5 launcher and two on a Soyuz rocket.

For the complete story, including figures, graphs, and images, please download the PDF of the article, above.

Acknowledgments
This paper is based on a C-band study conducted by a team headed by EADS Astrium GmbH within the European Space Agency (ESA) funded GNSS Evolution Program. The views expressed are the opinions of the authors and do not reflect those of ESA or the ESA Galileo team.

Additional Resources
[1] Hein, G.W., and M. Irsigler, J.A. Avila-Rodriguez, S.Wallner, Th.Pany, B.Eissfeller, and Ph.Hartl, “Envisioning a Future GNSS System of Systems, Part 3: A Role for C-Band?” Inside GNSS, May/June 2007
[2] Irsigler, M., and G.W. Hein, and A. Schmitz-Peiffer, “Use of C-Band Frequencies for satellite Navigation: Benefits and Darwbacks,” GPS Solutions, Wiley Periodicals Inc., Volume 8, Number 3, 2004
[3] ITU Regulations, www.itu.int/pub/R-REG-RR/en.
[4] Schmitz-Peiffer, A., and L. Stopfkuchen, F. Soualle, J.-J. Floch, R. King, A. Fernandez, R. Jorgensen, B. Eissfeller, J.A. Avila-Rodriguez, S. Wallner, J.-H. Won, Th. Pany, M.Anghileri, B. Lankl, T. Schueler, and E. Colzi, “Assessment of the Use of C-Band for GNSS within the European GNSS Evolution Programme,” ION 2008, Savannah, Georgia, USA.

Manufacturers

The OD&TS performance assessment of the proposed C-band GSS network was conducted using the GNSS+ software from DEIMOS Space, Madrid, Spain, developed in the frame of an ESA contract.

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