NovAtel's Annual Journal of GNSS Technology Solutions and Innovation

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Navwar 2014 For more Solutions visit velocity 43 path between the satellites and the user anten- na. Examples include blockages due to the large safety fences at car races, tall buildings causing "urban canyons", signal tracking under bridges and through tunnels, and operation under foliage. Signal blockage decreases signal availability, so the best line of defense is to maximize the number of satellites tracked. With GPS and GLONASS con- stellations at full operational capability, Galileo starting to come on line, and systems like BeiDou, QZSS and IRNSS providing regional coverage, multi-constellation GNSS receivers can now pro- vide continuous PNT solutions in environments where GNSS was previously denied. NovAtel's SPAN technology integrates GNSS and IMUs to offer seamless navigation during GNSS denials due to signal blockage. The inertial navigation solution continues to provide mea- surements and a continuously available solution in the total absence of GNSS. The INS solution will drift over time due to errors in the IMU data. This drift error can be corrected when the GNSS signals become available again, even if fleeting, as the IMU-based solution aids the GNSS receiver to reacquire the satellite signals. The tightly-cou- pled SPAN system can even correct for IMU er- rors when only one satellite is in view. "GPS Denied"– Constellation Failure The doomsday scenario of a GNSS constella- tion becoming unusable is very unlikely. This isn't to say that the risk of a common mode failure affecting an entire GNSS constella- tion or even multiple constellations cannot be ruled out, and the possibility of extreme solar events also have to be taken into account. But the potential for catastrophic failure is miti- gated by the constellation owners, e.g. the U.S. Space-Based PNT Policy of December 2004 states that the U.S. Government shall provide: uninterrupted access to U.S. space-based PNT services for U.S. and allied national se- curity systems and capabilities through GPS; continuous civil space-based PNT services free of charge; and improved capabilities to deny hostile use of any space-based PNT ser- vices without unduly disrupting civil access to PNT outside of an area of military operation or for homeland security purposes. In com- mon with other GNSS, there is redundancy in the number of satellites. Loss of service from a few of them would not have a great effect on total-system resilience. Disruptions to the ground control segment cannot be ruled out, of course, and the ground stations could be attacked or subject to natural disaster. Again, the mitigation in place for such events includes multiple redundancies. So, while GNSS system failure is unlikely, it is prudent to address the risk by ensuring that re- versionary modes are in place and that operators are trained in their use. And, as with the other potential risks to GNSS-based PNT discussed in this article, a multi-layer, multi-sensor approach is recommended. Trouble-free PNT Discussions about operating in GPS or GNSS denied conditions are important and useful. They point out the ubiquitous reliance on GPS and other GNSS that has resulted in the casual assumption that accurate PNT is a constant "given". Users are now becoming much more aware of the use of GNSS embedded in systems, such as those enabling secure communications through precise GPS timing which are not im- mediately obvious—but are just as vulnerable to GNSS failure. This growing awareness of vulnerability is leading to better analysis and definition of PNT requirements. There is also growing understand- ing that while GNSS brings huge benefits, it also has vulnerabilities that must be managed appro- priately. Mr. Gerein concludes, "We need to adopt an eyes-open approach, recognising and mitigat- ing risks and using the best of the wide range of available sensors with their complementary ben- efits and ability to compensate for others' short- comings." " " the best line of defense is to maximize the number of satellites tracked

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