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MBOC vs. BOC(1,1): Multipath Comparison Based on GIOVE-B Data

Second of three stories about Galileo's new signals in space

In 2004 the United States and the European Union signed a ground-breaking agreement to provide common, interoperable signals for civil users of GPS and Galileo: a binary offset carrier (BOC) modulation with a 1.023 MHz sub-carrier frequency and a code rate of 1.023 mega-chips per second — BOC (1,1) — centered at 1575.42 MHz. Subsequent discussions by a US/EU technical working group produced what  was considered — not without dissent — to be an even better signal structure: a multiplex BOC. With the GIOVE-B satellite now in orbit and broadcasting the new signals, a team of researchers have evaluated the actual improvement in multipath mitigation by MBOC relative to BOC(1,1).

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The GIOVE-B satellite — formally, the second Galileo In-Orbit Validation Element — was launched on April 27, 2008 and began transmission of ranging signals on May 07, 2008.

Unlike its predecessor GIOVE-A, GIOVE-B is meant to be a real prototype of future Galileo satellites: the signal generation and clocks are very close to what will be used by the fully operational capability (FOC) Galileo system.

GIOVE-B is transmitting all the foreseen Galileo signals in all frequency bands: L1BC, L1A, E5a, E5b, E6A, and E6BC.

Although the GIOVE-B navigation payload can transmit in all the Galileo frequency bands simultaneously, at this early phase of the testing the all-frequency transmission has not yet occurred.

One of the main points of attention with GIOVE-B signals is the performance of the new L1 multiplex binary offset carrier (MBOC) modulation, an improved alternative to the BOC(1,1) waveform as the Open Service signal on L1. BOC(1,1) was proposed in the original Galileo signal plan and has been transmitted by GIOVE-A, while MBOC was discussed as a possible replacement.

Analyses of the GIOVE-A data have confirmed that the multipath performance of the L1BC-BOC(1,1) was the worst among all the Galileo modulations. (This subject is addressed in the following papers listed in the Additional Resources section below: A. Simsky et alia [2006], A. Simsky et alia [2007], and M. Hollreiser et alia.)

Due to the obvious importance of the L1 signal as the basis for all the single- and multi-frequency positioning techniques, the green light was given to the implementation and testing of the L1-MBOC on GIOVE-B.

The articles by G. Hein et alia, T. Stansell et alia, and G. Gibbons et alia listed in Additional Resources describe and discuss the L1-MBOC signal.

(Links to these stories are available below.)

An important part of the argument in favor of the MBOC modulation arose from the expectation that it would improve multipath suppression by adding a higher-frequency BOC(6,1) modulation on top of BOC(1,1), either by a method of algebraic addition (composite BOC or CBOC) or by time multiplexing (TMBOC).

The discussion in T. Stansell et alia and G. Gibbons et alia revolves around the cost/benefit analysis of the replacement of BOC(1,1) with MBOC. Now, when the actual data is available from observations of the new Galileo signals in space, the real value of the benefits of the MBOC signal as well as implementation costs can be appreciated.

In this article, we present the first results for the actual multipath of MBOC and evaluate the improvement relative to BOC(1,1) using the data collected at our antenna site in Leuven, Belgium. The pseudorange and phase measurements were logged on a GNSS receiver connected to a wide-band antenna.

GIOVE-B can transmit three versions of the L1BC signal: BOC(1,1), CBOC, and TMBOC. At the time of this writing only BOC(1,1) and CBOC have been observed. Although our experimental results are based only on the CBOC data, they are also applicable to TMBOC because the multipath performance of both versions of MBOC is expected to be practically identical.

. . .

Conclusions
Field experience with GIOVE-B signals has confirmed the advantage of the MBOC modulation compared to BOC(1,1) in the matter of multipath mitigation. At the Leuven antenna site, the average multipath errors of MBOC were about 20–25 percent lower than with BOC(1,1).

Moreover, the multipath level of MBOC appears to be about the same order of magnitude as with most other Galileo signals. In this article only static data have been discussed.

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

Acknowledgments

Authors are thankful to Frank Boon of Septentrio and Marco Falcone of ESA/ESTEC for useful discussions.

Additional Resources
[1]  Gibbons, G., and P. Fenton, L. Garin, R. Hatch, T. Kawazoe, R. Keegan, J. Knight, S. Kohli, D. Rowitch, L. Sheynblat, A. Stratton, J. Studenny, G. Turetzky, and L. Weill, “BOC or MBOC? The Common GPS/Galileo Civil Signal Design: A Manufacturers Dialog, Part 2,” Inside GNSS, September 2006, pp. 28-43
[2]  Hein, G. W., and J.-A. Avila-Rodriguez, S. Wallner, J. W. Betz, C. Hegarty, J. J. Rushanan, A. L. Kraady, A. R. Pratt, S Lenahan, J. Owen, J.-L. Issler, and T. Stansell, “MBOC: The New Optimized Spreading Modulation. Recommended for Galileo L1-OS and GPS-L1C,” Inside GNSS, May/June 2006, pp 57-65
[3]  Hollreiser, M., and M. Crisci, J-M. Sleewaegen, J. Giraud, A. Simsky, D. Mertens, T. Burger, and M. Falcone, “Galileo Signal Experimentation,” GPS World, May 2007, pp. 37-44
[4]  Simsky, A., and J-M. Sleewaegen, M. Hollreiser, and M. Crisci (2006), “Performance Assessment of Galileo Ranging Signals Transmitted by GSTB-V2 Satellites,” Proceedings of ION GNSS 2006, September 25–28, 2006, Fort Worth, Texas, USA
[5]  Simsky A., and D. Mertens, J-M. Sleewaegen, T. Willems, M. Hollreiser, and M. Crisci (2007),“Multipath and Tracking Performance of Galileo Ranging Signals Transmitted by GIOVE-A,” Proceedings of ION GNSS 2007, September 25-28, 2007, Fort Worth, Texas, USA
[6]  Stansell, T., and P. Fenton, L. Garin, R. Hatch, J. Knight, D. Rowitch, L. Sheynblat, A. Stratton, J. Studenny, and L. Weill, “BOC or MBOC? The Common GPS/Galileo Civil Signal Design: A Manufacturers dialog, Part 1,” Inside GNSS, July/August 2006, pp. 30-37

Manufacturers

The receiver used to collect the observations described in this article was the GSTB-v2 Experimental Test Receiver (GETR) designed and built by Septentrio Satellite Navigation, Leuven, Belgium under contract with the European Space Agency. The wideband antenna was the Galileo Ground Segment Reference antenna from Space Engineering S.p.A., Rome, Italy.

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