Setting a Course: Beyond GPS

Satellite navigation, once felt to be an almost exclusive U.S. monopoly, is now viewed by other nations as a utility that they, too, can build, own and operate. Several overseas initiatives are now under way, putting pressure on the U.S. Department of Defense (DoD) to stay ahead in this new competitive environment.

When DoD launched the first of its GPS satellites in the mid-1970s, few officials would have believed that by the turn of the century, their quasi-classified system, designed to girdle the world with a high-accuracy, weapons guidance network of 24 satellites, would become an international utility with millions of civilian and military users around the world, in nations both friendly and hostile to the United States. They could never have anticipated that civil users would one day vastly outnumber military users and that the world’s transportation, communications, financial and other critical infrastructures would become so enmeshed with GPS that the military’s original concept of denying service to "unauthorized" users essentially would be abandoned. Unknowingly, DoD, in GPS, had created much more than they bargained for.

On the other hand, GPS was an engineering triumph that has revolutionized almost every aspect of aircraft operations worldwide. It has brought navigation accuracy–and with it, safety–to levels which otherwise would have been beyond the reach of all but the most sophisticated avionics installations in the newest airliners.

This was underscored by FAA’s March announcement that GPS, supplemented by the agency’s Wide Area Augmentation System (WAAS), could be used to guide runway approaches down to 200 feet above ground–essentially equivalent to the Cat I standard of most current ILS systems.

Deep in the Cold War days, when GPS was designed, Pentagon planners expected that their Soviet adversaries inevitably would counter GPS with a system of their own. Five years later the U.S.S.R. launched its very similar Global Navigation Satellite System (GLONASS). But U.S. planners did not anticipate the move by Europe in the early 1990s to develop its own worldwide satellite system, subsequently named Galileo after the 16th century Italian astronomer. Dismissing U.S. objections that GPS could meet all their needs, the Europeans were determined to throw off dependence on either of the foreign, military-controlled systems. In December 2005, Europe launched the first Galileo test satellite from a Russian space center in a Russian rocket.

Earlier that year, Japan announced that it, too, would proceed with its own navigation constellation. Known as the Quasi-Zenith Satellite System (QZSS) and optimized over Japan, its coverage is planned to later expand into the Japanese Regional Air Navigation System (JRANS), covering a large part of Asia. And long before Europe and Japan, China had developed its own BeiDou (North Star) domestic satellite positioning system, although this is a military system, not available to others. Finally, Europe, India and Japan have followed the U.S. lead by placing, or intending to place, very high-altitude satellites into orbit to further refine GPS system accuracies.

All told, by 2020, well over 100 navigation satellites could be orbiting the globe, providing unprecedented positioning service almost everywhere on the Earth’s surface. Do we need this many satellites? Probably not. But the ever present threat of shifting international alliances and political dependencies, coupled with the now almost universal reliance by industrialized nations on the GPS satellites’ superaccurate timing signals, has brought about these separate initiatives. In fact, precise satellite time is viewed by many to be as important as navigation. Today, fixed objects like power dams, TV and radio stations, cell phone control centers and, for that matter, the whole of Wall Street, depend on high-accuracy GPS time, as does much of the critical U.S. civil and military infrastructure, including air traffic control. So whether one is flying, watching TV, or using an ATM card, GPS is ticking away in the background, in precise, billionths-of-a-second steps. Indeed, in the early days of GPS, DoD underscored this aspect by calling the system NAVSTAR–for Navigation System Timing And Ranging.

Fortunately, the proliferation of satellite navigation systems also has brought with it–through the U.N.’s International Civil Aviation Organization (ICAO) and other bodies–agreement on the need for all systems to provide compatibility and receiver interoperability with GPS. This will allow future satellite avionics units to navigate with a mix of signals from current and new constellations, albeit in a manner that is transparent to the crew. The greatly increased number of navigation satellites could eliminate today’s GPS coverage "holes" and bring close to 100 percent signal availability to almost everywhere on Earth.

Continuous signal availability is important and will become increasingly so in the future. Today, operators flying precision paths using required navigation performance (RNP) techniques must first determine whether the configuration of the GPS satellites will provide optimal accuracy at the arrival time at destination. Otherwise, the departure must be delayed, or the flight must observe higher landing weather limits, risking diversion or cancellation.

GLONASS

GLONASS was the Soviet Union’s 1982 response to GPS. Each system used constellations of 24 satellites placed in circular orbits around the Earth, and provided similar performance and accuracy. GPS satellites orbit at 10,900 miles altitude, while GLONASS satellites are slightly lower, at 10,300 miles. Both were, and still remain, under exclusive military control, although Russia in 2005 signed a technical cooperation and development agreement with India. But while GPS has attracted a vast civil user community, GLONASS has had little outside appeal, primarily due to the limited availability of civil user equipment.

With the disintegration of the U.S.S.R. and the financial disarray that followed, the GLONASS constellation dwindled at one point to as few as seven satellites, partly due to their relatively short, three-year design life, compared with 7.5 years for their GPS equivalents. Slowly, the Russian Federation has increased their number. And President Putin reportedly has directed that the system should be brought up to its full 24-space vehicle configuration by 2008. On Dec. 25, 2005, three GLONASS satellites were launched from Russia’s Baikonur Space Center in Kazakhstan aboard a single Proton-K rocket, adding to the 14 already in orbit. Future Russian launches are expected to carry as many as six newer-design GLONASS satellites in one rocket.

The original GLONASS signal formats differed, for military reasons, from those of GPS but they have subsequently been made compatible, and multimode receivers have been built that use both constellations. At recent international navigation conferences, Russian speakers have stated their intention to be competitive players in the growing satellite arena. However, while other countries have so far placed no restrictions on an operators’ preference in satellite navigation systems, Russian-registered aircraft are required to use GLONASS as their basic satnav aid, with GPS or other systems in a supplementary role.

Galileo

Europe’s commitment to its own satellite navigation system was made in 1992, when the concept of a civil-controlled system, financed by a one-third public and two-thirds private partnership, was first proposed. The system was to be compatible with, but technically more advanced and somewhat more accurate than, its GPS and GLONASS predecessors. Estimated to cost $4.6 billion, Galileo would employ 30 satellites, launched in groups of four per rocket, into three 12,700-mile-high orbits.

Galileo would have several unique features, including several separate services. Some–such as precision landing guidance–would be fee-bearing and offer performance guarantees to users, although a basic, GPS-like free positioning service would be available. Each satellite would transmit on two civil frequencies, called L1 and L5. The reception differences between the two frequencies would allow receivers to correct for accuracy-degrading ionospheric effects, thereby eliminating the need for ground-based accuracy augmentation networks, such as FAA’s Wide Area Augmentation System, or WAAS.

The satellites also would continuously monitor the international 406-MHz distress beacon frequency and immediately retransmit messages to rescue centers. And uniquely, the Galileo system would transmit an acknowledgment back down to the beacon, confirming to those in distress that their message had been received.

While Galileo’s technology was within grasp, political consensus was more elusive, requiring cooperation between 25 separate European Union member states, plus the industrial and financial communities. China, Israel and South Korea also have joined the undertaking, with the Ukraine, India and other nations expected to follow this year. Final agreements between all the players were only reached late in 2004, but negotiating delays set the program back two years. Full operation now is forecast in 2010.

On Dec. 28, 2005, a GIOVE-A satellite (Galileo In-Orbit Validation Experiment -A) was launched on a Russian Soyuz rocket from Baikonur to test several system characteristics. The larger, GIOVE-B test satellite is expected to follow later in 2006. (Giove is Italian for Jupiter, whose moons first were identified by Galileo.) Successful tests would usher in the launch of four production satellites by 2008. The remainder would follow by 2010.

QZSS and JRANS

Japan’s satellite constellations are very unconventional. QZSS places three GPS-compatible satellites in a figure eight-shaped orbit centered on the equator, to ensure that there will always be at least one, and usually two, over Japan at very high elevation angles. Interestingly, this because QZSS is intended primarily for car navigation in the urban canyons of most Japanese cities, where signals from low elevation GPS satellites are often blocked. Japanese automakers produce between 4 and 5 million cars per year for the domestic market, and it is intended that all future models will have GPS navigators as standard equipment. But the very high elevation QZSS satellites, when combined seamlessly with GPS or Galileo, will also provide aircraft operators with enhanced satellite navigation performance in the region, as will JRANS, described next.

QZSS will be complemented later by JRANS, which will cover much of East and Southeast Asia, including China, North and South Korea, and part of Siberia. Its GPS-compatible signals will extend beyond Australia and New Zealand towards the Antarctic. But unlike GPS and other global satnav systems whose satellites travel in constant speed, circular orbits around the Earth, the QZSS and JRANS orbits cause them to travel faster around their southern paths and slow down appreciably as they approach and then curve around their northern peak. This ensures maximum exposure to users in the home country. Operational dates for QZSS and JRANS have not yet been announced.

To eliminate positioning errors induced by ionospheric effects, GPS, GLONASS, QZSS and JRANS (but neither BeiDou nor Galileo) use accuracy correction signals uplinked from ground monitor networks to satellites in geostationary orbits 23,000 miles above the equator. These in turn transmit correction signals down to users. Four GPS augmentation satellite systems cover adjacent regions around the world. These are WAAS, the European Geostationary Navigation Overlay System (EGNOS), India’s GPS And Geostationary Augmentation Network (GAGAN–also Hindi for "sky") and Japan’s Multitransport Satellite Augmentation System (MSAS). Because Galileo performs its own ionospheric corrections, it does not require augmentation. This leads some to suggest that once Galileo achieves full operation, Europeans may feel less inclined to further fund EGNOS, simply to support GPS and GLONASS in their region.

Unfortunately, to support their long orbital lives, the signals from high-altitude satellites are necessarily very low-power. This makes them vulnerable to inadvertent, or deliberate, interference from earthbound emitters. Inadvertent interference is rare, but not unknown, and can usually be located and shut down. However, while only a few cases have been reported, deliberate interference, or jamming, is acknowledged by authorities as a potential threat. And since all satellite navigation systems use the same groups of frequencies for compatibility, all are vulnerable.

Consequently, civil aircraft are required always to have a backup capability, using different navigation sources, should the satellite signals be lost. FAA has proposed an eventual minimum "skeleton" nationwide network of 300 to 400 VOR/DME stations vs. 1,050 today. But these would leave significant gaps in coverage for lower-flying general aviation aircraft. Alternatively, some have proposed the new enhanced Loran system as a much lower cost, unjammable, backup solution, using small autonomous receivers embedded in GPS units.

Jamming devices can range from sophisticated military systems to homemade and easily concealed, battery-powered devices. The former, of course, would normally only be found in conflict areas. It is therefore the latter–disguised inside a soft drink can or similar container, used either by pranksters or more serious troublemakers–which could pose problems to civil operators. Turned on intermittently, they can be difficult to locate but are usually limited to only a few tens of miles. Aircraft would be exposed to them for quite short periods. The newer satellite systems also transmit at somewhat higher power than the early GPS and GLONASS units, which reduces, but does not eliminate, their threat.

But two interference sources cannot be shut down. To test its anti-jamming capabilities, DoD is permitted, with prior announcement, to jam GPS signals in various parts of the country, with jamming signals occasionally reaching as far as 400 miles at high altitudes. Less predictable are the intense magnetic storms produced by the sun. A very large event, called Solar Max, is forecast later in 2007, and physicists have already warned of its potential interference to satellites and radio communications over a wide area.

Future GPS

Unquestionably, DoD and the U.S. Departments of State, Commerce and others, are concerned about these developments. From a national security viewpoint, the proliferation of advanced, foreign-owned systems minimizes the military "force multiplier" advantages of GPS. Similarly, from an industrial viewpoint, the overseas market for commercial satnav goods and services will certainly be diluted by foreign national purchasing interests.

A recent U.S. Defense Science Board (DSB) study notes that the arrival of Galileo in 2010 could make it appear that GPS is falling behind technologically. This would be a misconception, as it is due to DoD’s strained budgets and its strategy of launching satellites only to replace failed vehicles in orbit. But the earlier satellites are substantially exceeding their predicted lives, with some lasting as long as 13 years in orbit. While this does credit to their designs, it unfortunately postpones their replacement. For example, replacing all 24 current GPS satellites with units offering Galileo’s L1/L5 accuracy-enhancing frequencies and other benefits probably will not occur until several years after the European system becomes fully operational.

This could put the State Department and FAA in a quandary since the agency’s Local Area Augmentation System (LAAS), a long awaited GPS precision landing guidance system intended to replace ILS, depends on the availability of both L1 and L5 signals. The State Department had earlier told Europeans that GPS could fulfill all their needs. So, should the U.S. allow aircraft to use foreign-controlled satellite signals to guide them to bad weather landings at U.S. airports until GPS can offer this capability? Once Galileo is operational, this could create a dilemma in Washington. Galileo’s performance guarantees, vs. DoD’s provision of GPS "at the user’s risk," could increase civil misgivings about over-reliance on GPS.

Even further away is DoD’s next generation GPS III, which is planned to offer greatly improved performance although again primarily to military users. But GPS III will be a much more expensive system, and reportedly has already consumed over $100 million in design studies, alone. While launch of the first GPS III satellite is tentatively forecast for 2013, no date for the full constellation has been given.

In fact, the DSB study may now further impact the GPS III schedule by proposing, among other things, that it use a Galileo-like, three-orbital plane, 30-satellite configuration, instead of the six orbital planes and 24 satellites of today’s GPS. However, the transition from six to three orbital planes, and from 24 to 30 satellites, could be complex, possibly mandating receiver modifications.

The study also proposed that the satellites’ size and weight be drastically reduced–by eliminating current components like nuclear explosion detection equipment–to allow cost-saving dual launches. Improved anti-jam performance was also recommended. The DSB added two other proposals which would have been unthinkable a few years ago, and may still be contentious within Pentagon circles. These were that the United States and NATO forces should be encouraged to adopt combined GPS/Galileo receivers and that there should be more civil involvement in GPS operations and control.

The foreign initiatives have galvanized U.S. government and industry thinking about satnav policy directions. But for the world’s civil aviation community, having guidance from over 100 satellites would be a major step forward in flight safety and operating efficiency.