Architecture for a Future C-Band/L-band GNSS Mission, Part 2

A Potential Signal Plan and Related User Terminal Aspects

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This column continues an exploration of possible use of the C-band radio frequency for GNSS navigation. Part 2 focuses on C-band signal design in the context of non-interference with other services in nearby RF bands, as well as user equipment design and performance.

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
Based on a thorough trade-off analysis, the Service with Precision and Robustness (SPR-C) and the Public Regulated Service for C-band (PRS-C) have been identified for a future Galileo signal plan in C-band.

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
Compatibility is the fundamental aspect in the design of any navigational signal. Indeed, this criterion was assigned higher priority than other characteristics such as navigation performance . . .

Gaussian MSK (GMSK)
GMSK is a special case of continuous phase frequency-shift keying (CP-FSK) that employs Gaussian filtered frequency pulses to smooth the transitions from one point to the next in the signal status constellation while minimum shift keying (MSK) is obtained directly from the rectangular shape of frequency pulses . . .

Compatibility of GMSK Signals
. . . given the different directivity of the SPR-C and PRS-C antennas, the contribution of each service to the PFD on the ground will strongly depend on the final equivalent isotropic radiated power (EIRP) . . .

Payload Constraints
As we have seen in previous sections, the main constraint of C-band for navigation is the very small amount of bandwidth that is available, together with the very stringent requirements for compatibility with the nearby services. This is particularly difficult for the case of the uplink receiver, which is spectrally located directly on the left of the assigned downlink band. The simplest way to ensure compatibility would be to directly filter the signals after generation, using a steep raised cosine filter, for example . . .

GMSK Performance
We present … the multipath performance of the proposed GMSK signals. In addition, other solutions considered in the definition of the C-band signal and service plan are also presented. We present first the results based on a single static multipath reflection with a signal to multipath ratio (SMR) of -6.5 dB . . .

Overall User Terminal Architectures
The C-band signal was designed to make use of data and pilot channels. Using a pilot channel will provide a longer coherent integration, thereby producing less noisy range information . . .

Signal-In-Space (SIS) Model
. . . the additional half-chip (T_c/2) code delay in the Q-channel comes from the “offset” in offset QPSK (OQPSK) to restrict an instant phase change within ±90 degrees, thereby reducing the spectral leakage of the intended signals as much as possible. If we omit the terms T_c/2 from the preceding equation — that is, with no delay between I and Q — the signal model becomes a generic balanced QPSK . . .

Signal Acquisition
. . . The acquisition system consists of two BPSK acquisition detectors that produce the sum of I²+Q² in a signal branch for the first SS code. This is added to the sum of I²+Q² in the other signal branch for the second SS code in order to drive the noncoherent integration . . .

Signal Tracking
As with the signal acquisition scheme, a code- and carrier-tracking system for OQPSK DSSS signals was proposed consisting of two BPSK tracking blocks . . .

Boundary Condition   
. . . The typical receiver operation C/N₀ region, where the noise jitter line is aligned almost horizontally and produces a reasonable accuracy of tracking results, was obtained at unshadowed environmental conditions where the satellite-receiver link loss only encompasses the free space loss, atmospheric loss, and receiver antenna polarization loss . . .

Conclusions
A C-band signal plan was designed to fulfill the high-level requirements for both identified services, namely the SPR-C and PRS-C. The effort focused on signal modulation schemes to comply with the stringent requirements on spectrum confinement set out to ensure compatibility with other services, according to ITU regulations, with the neighboring bands (e.g., radio-astronomy, uplink receiver, and MLS) as well as to protect the Galileo uplink receiver.

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.

Acknowledgements
Authors Note: It is highly remarked that this column is based upon a C-band GNSS study being conducted within the European Space Agency (ESA) GNSSEvolution Program. Please note that the views expressed in the following reflect solely the opinions of the authors and do not represent those of ESA.

Additional Resources
[1] Ávila-Rodríguez, Á. J. (2008), and S. Wallner, J. H. Won, B. Eissfeller, A. Schmitz-Peiffer, J.-J. Floch, E. Colzi and J.-L. Gerner, “Study on a Galileo Signal and Service Plan for C-band”, Proceedings of ION GNSS 2008, Savannah, Georgia, USA
[2] Irsigler, M. (2004), and G. W. Hein, and A. Schmitz-Peiffer, “Use of C-Band Frequencies for Satellite Navigation: Benefits and Drawbacks,” GPS Solutions, Wiley Periodics Inc., Volume 8, Number 3, 2004
[3] ITU Regulations, www.itu.int/pub/R-REG-RR/en
[4] Schmitz-Peiffer, A. (2008), and D. Felbach, F. Soualle, R. King, S. Paus, A. Fernandez, R. Jorgensen, B. Eissfeller, J. Á. Ávila-Rodríguez, S. Wallner, T. Pany, J. H. Won, M. Anghileri, B. Lankl, and E. Colzi, “Assessment on the Use of C-Band for GNSS within the European GNSS Evolution Programme,” Proceedings of ION GNSS 2008, Savannah, Georgia, USA.
[5] Schmitz-Peiffer, A. (2009), and L. Stopfkuchen, J. J. Floch, A. Fernandez, R. Jorgensen, B. Eissfeller, J. Á. Ávila-Rodríguez, S. Wallner, J. H. Won, M. Anghileri, B. Lankl, T. Schüler, O. Balbach, and E. Colzi, “Architecture for a Future C-band/L-band CNSS Mission - Part 1: C-band Services, Space- and Ground Segment, Overall Performance,” Inside GNSS magazine, May/June 2009.
[6] Won, J. H. (2008), and B. Eissfeller, B. Lankl, A. Schmitz-Peiffer, and E. Colzi, “C-Band User Terminal Concepts and Acquisition Performance Analysis for European GNSS Evolution Programme,” Proceedings of ION GNSS 2008, Savannah, Georgia, USA
[7] Won, J. H. (2008a), and J. Á. Ávila-Rodríguez, S. Wallner, B. Eissfeller, J.-J. Floch, A. Schmitz-Peiffer, and E. Colzi, “C-Band User Terminal Aspect for Bandwidth Efficient Modulation Schemes in European GNSS Evolution Programme,” International Symposium on GPS/GNSS 2008, Tokyo, Japan
[8] Won, J. H. (2008b), and B. Eissfeller, A. Schmitz-Peiffer, and E. Colzi, “C-Band User Terminal Tracking Loop Stability Analysis for European GNSS Evolution Programme,” Proceedings of ION GNSS 2008, Savannah, Georgia, USA
[9] Won, J. H. (2008), and B. Eissfeller, A. Schmitz-Peiffer, and E. Colzi, “C-Band User Terminal RFI Effect Analysis for European GNSS Evolution Programme,” Proceedings of the Fifth ESA NAVITEC-2008, Noordwijk, The Netherlands

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