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Limits of Compatibility: Combining Galileo PRS and GPS M-Code

Although Galileo operates wholly under civil control, it does include encrypted signals, including those of the Public Regulated Service or PRS, which are broadcast near the new GPS military M-code signals at the L1 frequency. Galileo’s design calls for PRS use by public safety organizations such as police and fire departments and customs agencies. Because of its design, PRS could also be used for military applications; however, the European Union (EU) has not approved such use and several EU members have gone on record opposing it. Nonetheless, in light of a continuing interest in combined use of M-code and PRS, this article examines some of the technical issues surrounding the subject.

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An agreement signed in June 2004 between the European Union and the United States regarding the promotion, provision, and common use of GPS and Galileo has opened a new world of possibilities in satellite navigation.

Simulation studies of the combined use of Galileo and GPS civil signals have demonstrated that users may expect a clear enhancement of performance in terms of positioning accuracy and navigation solution (See the Additional Resources section at the end of this article for further details about these studies). The compatibility and interoperability that the Galileo signal structure will offer with respect to GPS is especially relevant in the E2-L1-E1 band.

After lengthy negotiations, the United States and the EU agreed on the design of the Open Service (OS) signals to be transmitted by Galileo and the future GPS on L1. If we take a more detailed look into the different waveforms, however, we see that not only the Galileo Open Service and the GPS C/A code have a common center frequency on L1 but also the Galileo Public Regulated Service (PRS) and the GPS military M-code.

Because common center frequencies are certainly the main prerequisite for interoperability, the combined processing of PRS and military signals from Galileo and GPS raises the possibility of offering a better positioning and navigation solution. Thus, in this article we want to go one step further to the analysis made in our previous work — cited as [1] and [2] in the Additional Resources section at the end of this article — and assess the performance of a combined Galileo PRS and GPS M-code receiver.

From a political and military point of view, the question of a combined Galileo PRS and GPS M-code service has clearly not been addressed yet and probably it will require time-consuming and lengthy discussions in the future, if the negotiations ever take place. Nonetheless, from a purely technical point of view it makes sense to evaluate the pros and cons as well as the performance that such a service could offer some day, and the time is certainly right for doing that now.

Therefore, this article first evaluates the performance of the two single services separately using identical assumptions. In order to do so, a refined methodology is proposed to estimate the different sources of error that contribute to the User Equivalent Range Error (UERE), particularly the ranging error caused by reflected signals or multipath. Afterwards the same analysis is carried out for a combined processing of Galileo PRS and GPS M-Code signals for a joint position, velocity, and time solution.

Introduction
Interoperability between civil GPS and Galileo was from the very beginning one of the most important drivers in the design of the Galileo signal structure. For that reason common center frequencies of Galileo and GPS signals in E5A (L5) and L1 were chosen.

Although the signal structure for the Galileo OS was specified in this agreement, it still allows some flexibility in the modulation scheme used. Therefore, the EU is still working to optimize the L1 OS signal, which may result in even better performance.

One major point during the negotiations was the necessary coexistence of the Galileo Public Regulated Service (PRS) and Open Service (OS) with the GPS C/A and M-code, in particular on L1 where the necessary separation between the different services played an outstanding role. Thus, the final frequency and signal structure resulted also in the same L1 center frequency for the Galileo PRS and GPS M-code.

Our previous work evaluated the accuracy of a combined Galileo OS and GPS C/A code service. This article will present the positioning accuracy of a combined Galileo PRS and GPS M-code service from a purely technical point-of-view. No doubt that military and political considerations and decisions would be necessary to realize such a combined service in reality.

However, this paper aims to show not only a benefit to use of the interoperability between the two satellite navigation systems for a combined civil service, but also in combining the Galileo PRS and the GPS M-code.

As will be shown, the main source of error is due to the ionosphere. Once this is eliminated by means of differential corrections from satellite-based augmentation systems (SBAS) signals or aiding information provided by assisted-GNSS (A-GNSS) capabilities, for example, multipath remains as the main problem. But one of our main drivers in this article is also to show the potential accuracy that a combined PRS/M-code (military) receiver could offer some day in the future.

In order to accomplish this, we consider realistic and worst-case scenarios. We will first present the main sources of error that contribute to the error budget, and estimate their values using methodology established in previous work (see items [1], [2] and [4] in the Additional Resources section). For a more realistic computation of these values, we introduce a refined methodology.

In the first part of the article we are studying the atmospheric and clock errors, as well as the necessary corrections. The ionosphere-free linear combination for Galileo L1A/E6A and GPS M-code L1/L2 signals will be an important focus of analysis.

Another important source of error is the thermal noise. We analyze the code and phase tracking errors due to thermal noise by means of the Cramer-Rao lower bound of the tracking error variance. A typical value will be obtained by assuming a received C/N0 under normal conditions (46.5 dB-Hz), while for the worst case we will consider a degradation of 15 dB (31.5 dB-Hz). This represents a considerable attenuation and corresponds to a typical worst case scenario we can find in the real world.

Last but not least, the main unavoidable source of error, namely the multipath error, is analysed exhaustively under realistic and worst case conditions. Given the substantial effect and characteristics of multipath, special care will be put on estimating its contribution on the total user equivalent range error (UERE).

In our analysis we will assume a narrow correlator with a spacing of d = 0.1 chip. In line with the results presented in our previous work cited earlier, a more realistic view of the multipath will be given by employing a model that accounts for the statistical distributions of the amplitudes and geometric path delays of the multipath signals in urban and suburban areas.

Then, the total UERE of Galileo PRS and of GPS M-code will be calculated. In a final step, we will take into account the geometry of the constellation of both satellite navigation systems to obtain the desired positioning accuracy for different scenarios and configurations.

As a conclusion, this article will present the theoretically expected positioning and navigation performance of Galileo PRS and GPS M-code, each alone, as well as the one of a combined service.

. . .

GNSS Error Budgets
Systematic errors and random noise affect the code and carrier observations needed for positioning. We can classify these error sources into three groups:

1.    Satellite errors:
a.    Clock bias
b.    Orbital errors
2.    Signal propagation:
a.    Ionospheric refraction
b.    Tropospheric refraction
c.    Multipath
3.
    Receiver errors:
a.    Clock bias
b.    Ranging error (thermal noise)

According to this classification, the so-called UERE will be estimated for both Galileo and GPS in a typical and worst case scenario.

. . .

Multipath Error
Multipath error is the most important unavoidable source of error contributing to the UERE, because it is very difficult to model. As we saw, the ionospheric error indeed presents worse values in a general case, but an appropriate receiver would be able to eliminate it or at least reduce its contribution with corrections coming from SBAS or A-GPS. In this article, we focus on the rural/suburban channel.

. . .

Galileo and GPS Accuracy
Using the values obtained and analyzed graphically in the preceding sections, we will next calculate the error budget as defined in the introduction for Galileo PRS and GPS M-code alone and for a combined service, in the presence of ionospheric error and with corrected values.

. . .

Conclusions
We have analysed in depth the different contributions to the error budget that the Galileo PRS and the military GPS M-code are expected to show in worst case and typical environments. Additionally we have calculated the positioning accuracy for both services, alone and operated together.

In summary, the following conclusions can be drawn:

•    Galileo PRS and GPS M-code alone perform more or less the same in a typical rural/suburban scenario.

•    As it has been shown, the combined processing of Galileo PRS and GPS M-code signals will bring an excellent performance for the potential user. Horizontal accuracies of even 0.26 meter could be achieved, which would represent a reduction of the positioning error of about 64 percent compared to use of GPS
M-code or Galileo PRS alone.

•    This article has also shown that the main source of error comes from the ionosphere for a single frequency receiver. Then, when the iono-free linear combination is applied and the ionospheric error is eliminated, the main source of error that still remains is the multipath.

Because the form and amplitude of the multipath envelopes is characteristic of every modulation, the modulation has a clear and direct impact on the error budget and, therefore, on the whole performance of the system. Many efforts have been undertaken in the past to optimize the Galileo signal modulations in order to achieve the potential of being better than the current GPS signals. Our results here prove that this work was more than justified.

•    The multipath error estimations were made only for the rural/suburban environment. Future work must be carried out using other types of potential scenarios.

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

Glossary and Additional Resources

Author Profiles

Günter W. Hein is Full Professor and Director of the Institute of Geodesy and Navigation at the University FAF Munich. He is responsible for research and teaching in the fields of high-precision GNSS positioning and navigation, physical geodesy, and satellite methods. Hein has been working in the field of GPS since 1984 and is author of numerous papers on kinematic positioning and navigation as well as sensor integration. He is a member of the Galileo Signal Task Force.

José-Ángel Ávila-Rodríguez is research associate at the Institute of Geodesy and Navigation at the University of the Federal Armed Forces Munich. He is responsible for research activities on GNSS signals, including BOC, BCS, and CBCS modulations. Ávila-Rodríguez is involved in the GALILEO program, in which he supports the European Space Agency, the European Commission, and the Galileo Joint Undertaking, through the GALILEO Signal Task Force. He studied at the Technical Universities of Madrid, Spain, and Vienna, Austria, and has an M.S. in electrical engineering. His major areas of interest include the Galileo signal structure, GNSS receiver design and performance, and Galileo codes.

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