Senior U.S. and international government and industry officials told specialists attending two meetings recently that by 2020 as many as 100 satellites could be radiating GPS-compatible navigation signals to air, sea and land users, with the overwhelming proportion of users being on land. At two separate conferences–one dedicated to GPS and the other to numerous satnav options–speakers from the DOD, DOT, Europe, Russia, Japan and elsewhere described current activities and future plans that will bring vastly improved satnav availability, integrity and accuracy to virtually everywhere around the globe.
The first meeting was sponsored by the joint DOD/DOT civil global positioning system service interface committee (CGSIC), tasked with identifying domestic and international civil GPS user needs, determining U.S. civil GPS policy and acting as a forum between worldwide users and U.S. GPS authorities.
The second meeting was the U.S. Institute of Navigation’s three-day global navigation satellite systems (GNSS) 2005 conference, where satnav specialists reviewed a wide range of GNSS applications–from measuring atmospheric water vapor to precision landings.
CGSIC chairman Michael Shaw opened his meeting by stating that the international community was “at the start of a new era in GNSS,” with the impending launch of the first of the DOD’s modernized GPS satellites (since launched successfully on September 27), and with encouraging progress on Europe’s Galileo, Russia’s Glonass and Japan’s future QZSS and JRANS satnav systems.
Shaw also noted advances with the FAA’s wide area augmentation system (WAAS) and with its EGNOS, GAGAN and MSAS WAAS equivalents in Europe, India and Japan, respectively, all of which are designed to transmit accuracy corrections and satellite alert signals compatible with U.S.-built GPS receivers.
Improving GPS Accuracy
Besides increased transmit power–which reduces vulnerability to jamming and other forms of interference–and other refinements, an important improvement in the DOD’s new generation of GPS satellites is additional signals for civil users. Until now, the major GPS error source has been the effect of the rapid and often erratic changes in the earth’s ionosphere on the single signal frequency, called L1, available to civil users. Additional civil signals at two separate frequencies allow receivers to compare and then cancel out ionospheric effects, providing significantly improved accuracy.
Encrypted dual GPS frequencies have been available to military users since the system was introduced more than 25 years ago, but the advent of Europe’s Galileo–which will offer unencrypted dual frequencies for aviation–plus other GPS enhancement techniques, has probably forced the DOD’s hand, since keeping GPS ahead of foreign satnav initiatives is a national goal, albeit one that is not widely publicized.
The most recent GPS launch sent a satellite aloft with a second civil frequency for non-critical applications, but the first satellite with an aviation-approved additional frequency, called L5, will not be launched before 2007. Launches of up to 11 L5-capable satellites will follow, to be succeeded by the DOD’s advanced, 24 to 30 next-generation, GPS III satellites, also L5-capable, the first of which is due to be launched around 2013.
But none of these future dates is firm, due to the DOD’s “launch on replacement” policy. This policy–the result of new weapons funding priorities following the first Gulf War, coupled with the DOD’s acceptance that GPS already works well–states that GPS satellites can be sent up only to replace failing units already in orbit.
So while the longevity of the current GPS satellites–some lasting as long as 13 years in orbit versus the forecast seven years–is a tribute to U.S. technology, it means that their more modern replacements could take much longer than anticipated to bring additional benefits to civil aviation. One program that could be affected by delays in the L5 satellites could be the FAA’s renewed plan to use WAAS to support 200-foot Category I approaches, since these would require dual-frequency satellite signals.
The DOD and the administration are also caught in a dilemma. The recently updated U.S. GPS joint-use policy states that the system is “critical to U.S. national security” and has been “integrated into virtually every facet of U.S. military operations.”
But the policy also recognizes that GPS has “grown into a global utility” and become “an essential element of worldwide economic infrastructure,” where it is offered, in its civil configuration, free to all users. It is the CGSIC’s task to maintain the delicate balance between these two apparently opposing philosophies.
But this dichotomy has also created concern among foreign administrations, which see total future dependence on the U.S. military’s GPS as a potentially unwise strategy. While it’s probably a mistake to describe foreign satnav initiatives as attempts to compete with GPS, other countries are anxious to be at least partially independent of the U.S. when it comes to protecting their own critical infrastructure and developing their own satnav technology.
As a result, Europe has moved ahead aggressively with Galileo and its WAAS-like EGNOS; Russia is rebuilding Glonass; India is implementing GAGAN, another WAAS clone; China is developing its BeiDou system; and Japan plans two regional satnav networks called QZSS and JRANS, as well as its own WAAS-like MSAS.
All of these systems–with the exception of China’s BeiDou–are or will be fully GPS-compatible and therefore usable by U.S. GPS/WAAS receivers. Compatibility offers important benefits. For example, an Australian study indicates that using signals from both GPS and Galileo would improve accuracy from about 20 feet with GPS alone to between seven and 10 feet when they are combined.
Speakers from Europe, Russia and Japan described their national programs at the GNSS 2005 Conference. Funded one-third by government and two-thirds by private investors, Europe’s Galileo is forecast to enter operation with a 30-satellite constellation between 2008 and 2010, according to a program official.
The relatively fast build-up of the Galileo constellation is the result of the much smaller size of its satellites relative to the USAF’s GPS units, which carry other classified equipment and are “hardened” against nuclear weapons attacks.
Galileo will have “open” signals for general and public safety signals, such as search-and-rescue but, since it is a commercial venture, users will pay for high-accuracy services, such as, perhaps, landing guidance. However, Galileo’s major revenue stream is expected to come from satellite-monitored highway tolls, although a UK attendee reported that there are already concerns about efforts to jam the signals to avoid payment.
Deployment in the East
Like GPS, Russia’s Glonass–for global navigation satellite system–is a dual-use military navigation aid. Developed and fielded by the Soviet Union during the latter part of the Cold War, its 24 satellites slowly dwindled to as few as seven before the new Russian government revived the program.
Glonass now has 13 satellites in operation and expects to have 18 in orbit by 2008. The program expects to field a full constellation of 24 by 2011. The system will be signal compatible with GPS and Galileo, according to a Russian Space Agency spokesman, and there will be no charge for civil use. Foreign operators will not be required to use Glonass in Russian airspace. However, he pointed out, Russian civil aircraft will be required to carry either Glonass receivers or combined Glonass/GPS units.
There were no presentations on India’s GAGAN–for GPS and GEO augmented navigation–program, which will provide WAAS-equivalent GPS corrections over the subcontinent. Further to the east of GAGAN is China’s BeiDou, which is a non-GPS-compatible, strictly military satnav system covering the People’s Republic, using between two and four equatorial-ranging satellites. But U.S. and other operators flying through Chinese airspace can navigate with GPS, Galileo or Glonass, and there have been no reports of their use being denied.
In a few years, an additional satnav system will be available over China. At the Institute of Navigation conference, a Japanese speaker described three separate satellite navigation initiatives that his country has planned. The first is a conventional, WAAS-like, geostationary satellite called MSAS, for multi-transport satellite augmentation system, poised over the equator to provide GPS and Glonass accuracy corrections.
The nation’s second and third satnav constellations, however, are unconventional. One, called QZSS, for the quasi-zenith satellite system, 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 high elevation angles.
The project underlines the massive demand for GPS service from land users, since QZSS is planned specifically 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 four million and five million cars per year for the domestic market, and it is intended that all future models will have GPS navigators as standard equipment.
Japan will then expand QZSS to create JRANS, Japan’s regional advanced navigation satellites, which will cover much of east and Southeast Asia, including China, both Koreas and part of Siberia, and its GPS-compatible signals will extend beyond Australia and New Zealand toward the Antarctic.
But unlike GPS and the other global satnav systems whose satellites travel in constant-speed, circular orbits around the earth, JRANS satellites will have an egg-shaped orbit that causes them to travel quickly around their southern track and slow appreciably as they curve around their northern peak, ensuring maximum exposure to users in their home country.
GPS is already a remarkable system, with millions of increasingly smaller and less expensive receivers used in a wide range of applications around the world. Yet over the next 10 or 15 years, we can expect that additional satellite systems and services will bring us even better performance.