Headlines
Galileo Goes Forward
The European Union (EU) has decided to press forward with plans to develop Galileo, a European version of the Global Positioning System (GPS). The European Commission approved funding for the project despite resistance from the United States, which sees "no compelling need" for it, according to a U.S. State Department announcement.
The development phase of Galileo is expected to run from 2002 to 2005, allowing researchers to test the technology on orbit before implementing the complete 30-satellite constellation. A deployment phase will follow, leading to a full operational capability in 2008.
 (European Space Agency) |
The Aerospace Corporation has been helping define U.S. position with respect to Galileo. For example, Aerospace analyzed potential interference to GPS from Galileo's proposed navigation signal structure and assessed options for making the time and space reference frames interoperable. These reference frames define time and position calculations for system users. The navigation signals provide ranging signals, tied to the time and space reference frames, that allow a receiver to determine its position and time. The Aerospace work had two goals: to prevent GPS and Galileo from adopting signal designs that interfere with each other, and to identify opportunities for making the signals and reference frames interoperable. By making them interoperable, the United States and EU would enable manufacturers to build inexpensive receivers that can simultaneously use signals from both systems.
After identifying a range of approaches and assessing their technical and practical impact, Aerospace recommended that each system develop and maintain its own reference frames but provide users with the data needed to remove intersystem errors. Greater levels of coordination were viewed as technically desirable but would have required revisions of U.S. and EU policy. Aerospace also assessed several alternative Galileo signal designs in light of technical and national policy goals. The assessment identified candidate signals that would be compatible with existing GPS civil signals and that provide the opportunity for establishing a new common standard structure for future civil satellite navigation signals. These recommendations were provided to the GPS program office for eventual use by the Defense and State Departments.
The EU has pledged that Galileo will be a civil program under civil control, independent of, but interoperable with, the civil components of GPS. Although the initial funding approval freed up 4.5 million euros, the total system cost is estimated at 3.4 billion euros.
Air Force Approves Purchase of GPS IIF
The U.S. Air Force asked Boeing Space and Communications in March to proceed with production of the GPS IIF satellites. The Block IIF program will function as a bridge to eventual implementation of GPS III. The satellites will transmit new civilian and military codes with greater accuracy, integrity, availability, and antijam performance. They will also be compatible with the Evolved Expendable Launch Vehicle.
(The Boeing Company) |
Aerospace has been assisting the Air Force throughout its GPS modernization efforts. For example, Aerospace reviewed Boeing's system specification to ensure that it reflected the proper technical baseline. Based on Aerospace analyses, Boeing increased the satellite design life from 8 to 12 years, added more accurate rubidium frequency standards, and increased the L-band signal power with only modest cost growth.
Aerospace also influenced the selection of the solar array design. The choice came down to an expensive oversized two-panel design or a cheaper standard-sized three-panel design. The deciding factor was the reaction wheel required for critical maneuvers. Aerospace assessed reaction-wheel performance for both designs using customized deployment simulations. Based upon the Aerospace findings, the Air Force recommended the three-panel design, which Boeing subsequently adopted.
Aerospace recognized that the Block IIF program needed to achieve launch capability sooner than originally planned. By revising on-orbit satellite reliability estimates, Aerospace helped support a decision to move the first planned launch from January 2006 to October (or potentially March) 2005. In response to government concerns about the risk associated with early long-lead part procurement, Aerospace assessed alternatives with an eye toward constellation sustainment, technical feasibility, and translation of program needs into meaningful requirements. The effort resulted in a decision to proceed with an incremental long-lead approval option.
Collision Prevention for GPS
GPS satellites can help prevent collisions on Earth, but apparently, they need help preventing collisions in space.
In fact, GPS satellites placed in disposal orbits could collide with the primary operational constellation within 20 to 40 years, according to recent Aerospace studies. Revised procedures for decommissioning the old satellites are therefore needed to reduce the risk of collision.
The problem, explained Aerospace researcher Chia-Chun (George) Chao, is that the disposal orbits "start out circular but degrade over time into more eccentric orbits as a result of the resonance induced by sun/moon gravitational forces and the Earth oblateness effects."
Besides jeopardizing the GPS constellation, these satellites could pose a threat to operational satellites in low Earth (LEO) and geosynchronous (GEO) orbits, Chao said. To reduce the probability of collisions, the decommissioned satellites must be inserted into disposal orbits at least 500 kilometers higher than the GPS constellation. Moreover, the initial eccentricity of the disposal orbit must be minimized as much as possible, and its perigee must be optimally oriented with respect to Earth's equatorial plane.
The Aerospace report, commissioned by the Air Force, revealed other dangers as well. Future GPS satellites will be launched on the Evolved Expendable Launch Vehicle, which, unlike current GPS launch vehicles, may leave an upper stage near the constellation. Aerospace is participating in an Air Force study to develop a disposal procedure for these upper stages.
The Russian GLONASS navigation constellation, which already includes about 100 failed satellites, may also pose a collision risk in 40 years, the studies show. A similar problem applies to Galileo, the planned European navigation constellation.
The most recent study by Chao revealed that the newly recognized resonance effect is strongly dependent on orbit inclination and altitude. "The effect becomes more pronounced for Galileo orbits due to a higher altitude—3000 kilometers above GPS," Chao explained. Understanding the dependence on initial inclination may help the designers of GPS III and Galileo systems select the proper inclination for minimizing the large eccentricity growth. "The maximum eccentricity growth for GPS and Galileo can be significantly reduced by selecting inclinations a few degrees from the current nominal values for both programs," he said.
FCC Rules on Ultrawideband Devices
The Federal Communications Commission (FCC) has authorized the use of ultrawideband devices above 3.1 gigahertz and imposed strict technical limits on those below this frequency. The decision was intended to protect national security systems from frequency interference while allowing commercial deployment of new technologies.
Aerospace has taken an active role in U.S. evaluations of ultrawideband devices, recognizing that they could potentially interfere with GPS receivers if not properly regulated. Aerospace assisted the National Telecommunications and Information Administration (NTIA) in selecting operational scenarios to check and planning the appropriate tests. Aerospace also evaluated and critiqued NTIA documentation of the tests and inspected the testing site.
The Department of Defense supported the FCC decision, concluding that the technical restrictions on ultrawideband devices would be sufficient to protect spectrum-dependent military systems, including GPS. Such restrictions were the minimum required to avoid interference.
The Pentagon will monitor regulatory and market developments to ensure that national security is maintained and that ultrawideband devices, as deployed, do not jeopardize mission-critical operations supporting public safety, national security, and homeland defense.
Ultrawideband devices emit low-energy signals across very wide bandwidths. They are used for detection and surveillance as well as short-range communications.
A Fine System
The U.S. Department of Consumer Protection ordered a Connecticut car-rental agency in February to stop imposing speeding fines on its customers. The agency reportedly used GPS to track the speed of its customers and charged a $150 fine to their credit cards each time they drove more than 79 miles per hour for more than two minutes.
The consumer protection commission did not take issue with the use of GPS or the fine itself, but only with the agency's failure to disclose the full details of its policy to renters.
The GPS component used in the cars is part of a system known as AirIQ OnBoard, which gathers ranging information about a host vehicle and transmits it wirelessly to a processing station—in this case, the rental company. The manufacturer of AirIQ OnBoard is a member of the Intelligent Transportation Society of America, a broad-based organization created by Congress in 1991 to coordinate the development of intelligent transport systems.
The rental agency plans to maintain its policy, but with better disclosure to its customers. Other agencies reportedly use AirIQ OnBoard—to locate lost or stolen cars, to provide driving directions, even to unlock car doors remotely for customers—but none imposes a surcharge for speeding. At least not yet.
To Summer 2002 Table of Contents
