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GNSS USER ASSESSMENT OF GPS/Galileo Interoperability*

GNSS USER ASSESSMENT OF GPS/Galileo Interoperability*. Dr. A.J. Van Dierendonck, AJ Systems * The comments in this paper represent the opinions of the author and are not necessarily those of any other person or organization. Introduction – What is addressed?.

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GNSS USER ASSESSMENT OF GPS/Galileo Interoperability*

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  1. GNSS USER ASSESSMENT OF GPS/Galileo Interoperability* Dr. A.J. Van Dierendonck, AJ Systems * The comments in this paper represent the opinions of the author and are not necessarily those of any other person or organization. CGSIC

  2. Introduction – What is addressed? • An assessment of Signal Interoperability of proposed GPS and Galileo signals • Civil Signals Only • Signal interoperability issues with regard to design decisions already made, or to be made, on both GPS and Galileo CGSIC

  3. GPS and Galileo Interoperability and Compatibility CGSIC

  4. Interoperability and Compatibility • What do these words mean? According to the New Space-Based PNT National Policyas follows: • ‘Interoperable’ refers to the ability of civil U.S. and foreign space-based positioning, navigation, and timing services to be used together to provide better capabilities at the user level than would be achieved by relying solely on one service or signal • ‘Compatible’ refers to the ability of U.S. and foreign space-based positioning, navigation, and timing services to be used separately or together without interfering with each individual service or signal, and without adversely affecting navigation warfare CGSIC

  5. Translation and Opinion • Interoperable • As stated, means that the different GNSS systems provide the same answer, within the advertised accuracy of each individual system – Call this “System Interoperability” • Should also mean that different GNSS systems provide signals, and required use of those signals, that are relatively similar so as to simplify GNSS receivers that use combined GNSS systems (or make them feasible) – Call this “Signal Interoperability” or “Optimized System Interoperability” • Compatible • Means that GNSS systems do not interfere with each other, and that non-military signals can be jammed without adversely affecting the military signals CGSIC

  6. Signal Interoperability at L1 • Galileo and GPS are essentially interoperable at L1 from a “system” point of view • From a user point of view, there is still a lack of Signal Interoperability • Symbol rates on L1 Galileo SOL/OS signal are 5 times higher that GPS C/A code signal • Perceived to cause problems for indoor users • As a consequence, Galileo has defined E5a with lower symbol rates as their indoor user signal -- Creates problems for both indoor and aviation users (indirectly) CGSIC

  7. Signal Interoperability at L5/E5 • Should be a new topic of US/EU negotiations • Certainly has been a topic at EUROCAE (Europe’s version of RTCA) • E5a is signal interoperable with L5 • E5b is not interoperable with any GPS signal • Unfortunately, required for European aviation to receive Galileo integrity information • However, it is “compatible” in that it doesn’t interfere with any GPS signal CGSIC

  8. Interoperability/Compatibility at L2 • There is no Galileo L2 signal – thus not “signal interoperable” with GPS, but, certainly, the GPS L2 signal does not interfere with any Galileo signal – thus, “compatible” • Of course, L2 is not in an ARNS band, so its relationship to Galileo does not affect aviation • Given future availability of Galileo, L2C would probably not be ideal for indoor use CGSIC

  9. GPS/Galileo Signal Interoperability Issues Details CGSIC

  10. L2/L5/E5 Signal Interoperability Details • Precise Land-Based Users • For these users, not correct to discuss L5/E5 signal interoperability without including L2 • Combination will provide ultimate performance for land-based users (coupled with L1) • L2 is adjacent to E5 band (See Figure) • ~75 MHz receiver front-end covers all three bands, centered at about 1202 MHz – technologically feasible for land-based receivers • See ION-GPS/GNSS-2003 paper by Issler, et al • Not feasible for aviation use • Not all ARNS and too much visible interference CGSIC

  11. L2C M P(Y) L5 E5a* E5b* L2 (1227.6 MHz) L5, E5a (1176.45 MHz) E5b (1207.14 MHz) *BOC(15,10) modulation option shown for E5a/E5b. L2/L5/E5 Spectral Relationships ~1202 MHz ~75 MHz ~50 MHz Not to Scale CGSIC

  12. L5/E5 Signal Interoperability Details – Aviation Users - 1 • L5/E5 band covers about 50 MHz • DME/JTIDS/MIDS/Radar pulse reception up to 100% duty cycle at some defined saturation level • Receiver processing limited to individual L5/E5a and E5b bands • Antenna/LNA may still have to cover entire 50 MHz band because of inability to separate bands at RF frequencies • Spectral separation would then be at IF frequencies • Some pulse (radar and onboard) interference will still saturate LNA – thus, interference in one band would affect the other CGSIC

  13. L5/E5 Signal Interoperability Details – Aviation Users - 2 • Signal Reception simultaneously at L5/E5a and E5b, even if possible in interference environment, complicates aviation receiver design • Wideband antenna likely will not meet ARINC size and profile requirements • This problem has not been solved – being investigated by Chelton (UK – parent of COMANT in US) for Galileo • E5b close to radar bands • LNA saturation would cause signal loss in both bands using wideband LNA approach • Separate IF path required • Increases aviation receiver cost CGSIC

  14. A Few Words About Galileo Integrity • Of course, very expensive to implement • Aviation community in Europe being forced to use Galileo integrity as opposed to receiving integrity information from SBAS systems • Not all problems have been solved – still marginal with respect to aviation requirements • Still have to receive GPS integrity from SBAS/GBAS • Increases signal data rate to 125 BPS • Perceived to be not interoperable with indoor operations • May be encrypted to separate from OS signal???? • Not feasible for GPS – must maintain legacy CGSIC

  15. Indoor Receiver Considerations 1 • Galileo signal designs seem to give Indoor GNSS Reception priority over aviation • Because of high volume sales • Dominates frequency/bandwidth considerations • Galileo integrity broadcast presents a conflict • High rate integrity data (250 sps) broadcast on L1 SOL signal also appears on OS signal • Prevents indoor data recovery at low SNR • 10 dB degradation relative to 50 sps on E5a • Does not affect tracking threshold because of data-less (pilot) carrier on Galileo signals – only affects data recovery • If data is received via cell phone link, not required for indoor use CGSIC

  16. Indoor Receiver Considerations 2 • Because of high data rate at L1, Galileo proposes moving indoor operation to L5/E5a • Taking advantage of low data rate on L5 • Assign integrity data broadcast to E5b • Aviation users in Europe required to use Galileo integrity data • Requires reception of E5b • At the same time, reception at L5 required for GPS use and for reception of GPS integrity from EGNOS and WAAS (and other SBASs) • Galileo by itself considered a degraded mode in the future • At present, broadcast of Galileo integrity not planned for EGNOS, but most likely will be on WAAS and other SBASs CGSIC

  17. Indoor Receiver Considerations 3 • How does Galileo expect that use of GNSS in cell phones will migrate from L1 to L5/E5a? • Especially since “dual constellation” will be available at L1 long before it will be available at L5/E5a • Dual frequency receivers not likely • Required antenna and receiver technology not commensurate with low-cost/low-power requirements • Migration to L5/E5a would have to be a clean break • In the interim, what is the tradeoff between more satellites and a the lower data rate on Galileo E5a? • I would think that manufacturers and users would pick the advantages of more satellites CGSIC

  18. Signal Interoperability Problems at E5/L5 • Obvious that priority for Indoor Users has causes GPS/Galileo signal interoperability problems at E5/L5 for Aviation Users • Is it correct to compromise Safety-of-Life applications for new technology that is dubious at best? • & Just to be able to improve data recovery performance for data that should be available via the cell link anyway? CGSIC

  19. Indoor Use Data Recovery Issues • Why is data recovery from GPS/Galileo signals required? • Receiver integration with cell phone (or other communications device) is required to connect to E911 • So why not collect data from E911 network? • Real reason not to may be related to intellectual property – in the near term, most manufacturers may have to pay royalties • Any patents will probably expire in another 10 years • Certainly not an integrity issue – integrity data would require the higher data rate anyway CGSIC

  20. Summary & Conclusions • Definitions of GPS/Galileo Interoperability and Compatibility were reviewed • Official definition of Interoperability does not account for user signal processing requirements • Consequently, GPS and Galileo are not optimally interoperable • Optimal Interoperability inconsistencies shown for Aviation and Indoor Users • Galileo (and GPS) Indoor Use seems to have taken precedence • Major E5/L5 interoperability problem for Aviation Users CGSIC

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