The End of the Beginning
December 1, 2005 By: Per Enge GPS World<
font color=burgundy>The single greatest challenge for GPS this upcoming year is to accelerate the delivery of L5 . . . The second greatest challenge is to foster the inertial and terrestrial radio technologies needed to usher in the golden age of navigation.
GPS is not the end. It is not even the beginning of the end. But it is, perhaps, the end of the beginning. (With apologies to Winston Churchill, who spoke similarly about El-Alamein and World War II in 1942.) This sentiment is certainly on target for navigation applications of GPS such as aircraft flight in instrument flight rules (IFR), marine navigation at night or in the fog, car navigation, and personal navigation.
All these users will certainly benefit from the new GPS signals. The forthcoming diversity of signals (L1/L2/L5) will obviate the danger due to accidental radio frequency interference (RFI), do much to tame the ionosphere, and mitigate multipath. This year, GPS has launched the first of the Block IIRM satellites that includes the civil signal and code on the L2 frequency. Within a few years, GPS satellites will have three civil frequencies. The third signal, L5, will be the most effective of all, 5 or so dB more powerful than L1, with a chipping rate of 107 chips per second (or 10 Mcps) compared to the 1-Mcps codes used by L1 and L2.
The single greatest challenge for GPS this upcoming year is to accelerate the delivery of L5. Like the current L1 and unlike L2, L5 will reside in an Aeronautical Radionavigation System (ARNS) band of the radio spectrum. Thus, it is much more useful to the international civil aviation community. With its high chipping rate, L5 will also provide more protection against RFI and multipath. All told, it has very high value to safety-of-life applications like IFR flight. The civil aviation community and the FAA are working hard to ensure that suitable avionics and integrity monitors are available, and we must support their efforts.
Galileo is coming without a doubt. Certainly, some growing pains lie ahead, but in time GPS and Galileo will comprise a true Global Navigation Satellite System (GNSS), especially when we include the contributions from China, India, Japan, Australia, and other countries. The strength of this team will derive from two sources. First, we will be satellite-wealthy with some 60 on orbit. Second, all these satellites will broadcast a multiplicity of signals — some in ARNS bands and some for non-safety applications.
However, even this mighty GNSS team will not be enough. Satellite signals are weak by the time they travel from medium-Earth orbit to our users. Consequently, jamming will still be a danger, and satellite navigation will remain a tenuous thread for users downtown and indoors. To overcome these obstacles, we must develop our connection to inertial navigation and the right mix of terrestrial radio signals. Otherwise, we will be disappointed when new applications call for high accuracy indoors or in the deep forest. We will be heartbroken when hackers turn their malevolent attention to satellite navigation.
Thus, the second greatest challenge for 2006 is to foster the inertial and terrestrial radio technologies needed to usher in the golden age of navigation, where we can expect centimeter accuracy outdoors and meter accuracy in the toughest environments.
Inertial technology will make GNSS receivers tougher to jam. The GNSS tracking loops will be aided by accelerometers based on micro-electromechanical systems (MEMs). These devices will be available at low cost, because they need only reduce the tracking bandwidth of the GNSS phase-lock loops from 10 Hz to 1 Hz to be effective. In other words, these MEMs will not be designed to provide performance over tens of minutes but rather tenths of seconds. If the jammer does manage to overwhelm this tough receiver, then another variety of inertial technology will guide the navigator out of the troubled area. Specifically, cold-atom technology will enable error rates of approximately 5 meters/hour in contrast to the 500 meters/hour error growth of today’s inertial technology.
Terrestrial radio signals offer more help. The Loran system has steadily moved forward with many significant upgrades. In the future, enhanced Loran (eLoran) transmissions may include an authentication message so that users will know the signal comes from an authentic Loran tower. This signature will enable geo-encryption location using Loran signals. With geo-encryption, we will deliver messages to users with the right key, provided they are within a previously agreed time and location window. Television range measurements may provide location service over metropolitan areas even downtown and indoors. Wi-Fi signal-strength measurements may provide the key to meter-level accuracy within buildings. All these radio technologies will be well served by smart antennas that are getting smaller and less expensive. Silicon oscillators may allow low-cost, stable clocks to be integrated onto the CMOS integrated circuit carried by the user.
At Stanford, we have formed the Stanford Center for Position, Navigation and Time to speed the needed technology development and showcase the applications. See http://scpnt.stanford.edu/ for more information and to learn how industry may participate in these activities.
Per Enge is a professor of aeronautics and astronautics at Stanford University, where he is the Kleiner-Perkins, Mayfield, Sequoia Capital Professor in the School of Engineering. He is the research director of the Stanford Center for Position, Navigation, and Time, and a member of the National Academy of Engineering.
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