WORKING PAPERS Architecture for a Future C-band/L-band GNSS Mission Part 2: Signal Considerations and Related User Terminal Aspects JOSE-ANGEL AVILA-RODRIGUEZ This column continues an exploration of possible use of the C-band radio frequency for GNSS JONG-HOON WON navigation. Part 2 focuses on C-band signal design in the context of non-interference with STEFAN WALLNER other services in nearby RF bands, as well as user equipment design and performance. MARCO ANGHILERI BERND EISSFELLER he radio navigation satellite higher free space losses due to the limi- BERTHOLD LANKL service (RNSS) portion of the tations on the higher signal frequency. TORBEN SCHÜLER radio frequency (RF) spectrum An omnidirectional C-band antenna at UNIVERSITY FAF MUNICH T is already overcrowded, and the 5 GHz will be 3.2 times smaller in the OLIVER BALBACH bands suitable for new uses are very linear dimension than an equivalent L1- IFEN GMBH limited. This is especially true for the band antenna. (The latter signal has a ANDREAS SCHMITZ-PEIFFER E1/L1 band occupied today by GPS and 19-centimeter wavelength at 1.575 GHz JEAN-JACQUES FLOCH Galileo. compared to the wavelength of 6 centi- LARS STOPFKUCHEN In addition, Japan’s quasi-zenith sat- meters at 5.015 GHz.) DIRK FELBACH ellite system (QZSS) and potentially also Because of this wavelength-driven EADS ASTRIUM Compass and GLONASS will be trans- design factor, the area of the C-band mitting navigation signals in this fre- antenna will be 10 times smaller than ANTONIO FERNANDEZ quency band. But E1/L1 is not the only that of a standard L-band antenna. As a DEIMOS SPACE case. Even those RF bands that are not result, a C-band antenna receives only ROLF JORGENSEN being used yet will certainly be shared 1/10th the broadcast power of its L-band TICRA by many systems in the near future. counterpart. (For details of relevant ENRICO COLZI Thus, the search for unused frequen- research, see the articles by M. Irsigler et ESA-ESTEC/VEGA IN SPACE cy resources will almost certainly con- alia and A. Schmitz-Peiffer et alia (2008) tinue during the next years. The World in the Additional Resources section near Radio Communications Conference the end of this article.) 2000 (WRC-2000) allocated the por- Another important factor is the AUTHORS NOTE: IT IS HIGHLY tion of C-band between 5010 and 5030 increased signal attenuation of C-band REMARKED THAT THIS COLUMN MHz for RNSS space-to-Earth applica- signals due to foliage, heavy rain, or IS BASED UPON A C-BAND tions. The general idea was to provide indoors, as well as other negative envi- GNSS STUDY BEING CONDUCTED access to a frequency band that is not ronmental effects on signal tracking. On WITHIN THE EUROPEAN SPACE yet overloaded by other signal sources the other hand, C-band exhibits much AGENCY (ESA) GNSS EVOLUTION and, consequently, not so susceptible to smaller ionospheric errors for standard PROGRAM. PLEASE NOTE THAT interfering signals as guided by Inter- single-frequency applications. The hope THE VIEWS EXPRESSED IN THE national Telecommunications Union is that technological progress might bal- FOLLOWING REFLECT SOLELY THE (ITU) regulations. ance some of the disadvantages from a OPINIONS OF THE AUTHORS AND Navigation in C-band presents both long-term point of view, given that an DO NOT REPRESENT THOSE OF ESA. advantages and disadvantages, the actual application of C-band for RNSS is most important drawback being the not foreseen before the year 2020. 52 InsideGNSS JULY/AUGUST 2009 www.insidegnss.com WORKING PAPERS We began our discussion in the pre- SPR-C GMSK PRS-C GMSK BPSK(10) Data vious column (May/June 2009, Inside BOC(5,5) Data GNSS) with an explanation of the scope of the C-band project, service analysis, sat- f(MHz) ellite constellations, ground segment, sat- ellite transmit signal power requirement, SPR-C GMSK payload design, spacecraft accommoda- BPSK(10) Pilot PRS-C GMSK tion, and end-to-end performance. BOC(5,5) Pilot In this column we talk about the C- 4990 5000 5010 5030 band signal design driven to respect the given constraints of other C-band servic- FIGURE 1 C-band GMSK Signal Plan es, and the C-band user terminal equip- ment design and performance analysis in tures by providing additional robustness selected signal plan for the C-band. the context of expected applications. in degraded RF situations. Moreover, the Note that both the SPR-C and PRS- Additional discussion of the naviga- proliferation of GNSSs and lack of high C services provide a data and a pilot tion message structure design and the precision signals that work on a single channel. related added value concerning the tro- frequency have also been important posphere corrections (e.g., the combina- drivers in the C-band study. Compatibility of C-Band tion of navigation data and numerical In order to design C-band signals Signals weather data from meteorological satel- the top-level requirements for both ser- Compatibility is the fundamental aspect lites), together with critical user-terminal vices were analyzed and established in in the design of any navigational sig- technologies needed to prepare C-band terms of geometric dilution of precision nal. Indeed, this criterion was assigned for use in a future GNSS constellation, (GDOP), availability, and continuity risk higher priority than other character- have been added to this digital and on- among other factors, and so on. In addi- istics such as navigation performance. line version of the article. tion to this, the SPR-C requires authen- As briefly mentioned earlier, the signal tication capability to provide robustness plan in the C-band must be compatible C-Band Signals Considered in terms of anti-spoofing while the PRS- with: Based on a thorough trade-off analysis, C needs code-encryption capability to • radio-astronomy (RA) band between the Service with Precision and Robust- provide enhanced anti-spoofing perfor- 4990 and 5000 MHz ness (SPR-C) and the Public Regulated mance. Both service signals should be • microwave landing system (MLS) Service for C-band (PRS-C) have been spectrally decoupled from each other. between 5030 and 5150 MHz identified for a future Galileo signal plan The C-band signal plan was opti- • Galileo uplink receiver (ULR) in C-band. mized for maximum occupied band- between 5000 and 5010 MHz A quick look at this service definition width and spectral separation between We will first describe our assump- reveals the main motivation for both the two provided services. In conse- tions in calculating the potential for services: 1) the SPR-C was to maximize quence, the signals presented next must C-band interference and describe the the possible user communities under be interpreted as an envelope of solu- GMSK signal in greater detail before C-band, following the civil/public dual- tions in the sense that derived alterna- reporting the results of our compatibil- use concept of satellite navigation; 2) tive signals with lower chip-rate and ity analysis. the PRS-C was to provide selected users lower sub-carrier frequencies would Radio-Astronomy. RA compatibility with the access to this service in order to also fulfill the criteria for compatibility is assured according to International fulfill high security requirements (e.g., with nearby C-band services. These are Telecommunication Union (ITU) regu- anti-jamming and anti-spoofing). namely the radio-astronomy service lations if the power flux density (PFD) As discussed in the first part of this (RA), the microwave landing system of the C-band downlink signals is not series, the PRS-C consists of two small (MLS) service (MLS), and the Galileo higher than a threshold value that is a spot beams with approximately 1,500 up-link (UL) service. function of the number of simultaneous kilometers of radius. Moreover, these Figure 1 shows the spectrum of the satellites within the very narrow beam of two spot beams shall provide high selected signal plan for C-band RNSS an RA telescope. geographic flexibility to point at any signals relying on the Gaussian mini- In our analysis we assumed that a required area on earth. mum shift keying (GMSK) modulation. maximum number of 10 C-band satel- In addition, use of C-band shall aim This scheme was found to satisfactorily lites could be seen at any time by any at mitigating problem areas of current L- accomplish the stringent requirements on RA antenna on the ground and that all band signals. In fact, the C-band Service spectrum confinement to ensure compat- the signals coming from these satellites Plan was designed to address the vulner- ibility with adjoining C-band services. have the same power at the surface of the ability of L-band in critical infrastruc- Table 1 summarizes the parameters of the Earth. Given that the antenna beam of www.insidegnss.com JULY/AUGUST 2009 InsideGNSS 53 WORKING PAPERS Frequency Band C-band, 5010 to 5030 MHz downlink signals, including output Service SPR-C PRS-C multiplexer (OMUX) filtering, basically Channel Data Pilot Data Pilot depends on the SSC between the down- Signal type BPSK(10) BPSK(10) BOC(5,5) BOC(5,5) link signals and the uplink signals of the Galileo Uplink receiver as well as on the Modulation GMSK BTc = 0.3 GMSK BTc = 0.3 GMSK BTc = 0.3 GMSK BTc = 0.3 power of the downlink signals as seen by Symbol rate 50 sps N/A -- N/A the uplink receiver. Maximum code length 51,150 k*51,150 -- -- In addition to these considerations, TABLE 1. Parameters of considered C=band signals in order to compute the power of the downlink signals that leaks into the uplink receiver, we need to consider the antenna decoupling between them.