Satellite Navigation: The Need to Avoid Interference
Recent news stories have weighed the potential for disruptions to global positioning system (GPS) services that may be caused by a terrestrial communications network planned for construction by the company LightSquared. Many end-users and policymakers are concerned about the impact these disruptions could have on such things as aircraft safety and in-car navigation. In addition, as new satellite navigation systems are deployed by various countries, there is the potential for interference between the various signals that are broadcast to users on the ground. Whether the source of interference is terrestrial or space-based, the ubiquity of positioning, navigation and timing services means it is essential to ensure that any interference is swiftly ended or mitigated.
To many, GPS has become shorthand for the overall concept of satellite-based navigation, but this acronym only refers to one specific network. The U.S. Navigation Signal Timing and Ranging Global Positioning System (NAVSTAR-GPS) was developed by the U.S. Air Force and remains the only fully operational global satellite-based navigation network. Operational since 1995, the system nominally requires 24 satellites and at least four reserves in orbit. The network has 32 satellites, including 30 active satellites and two spares, of which at least half have either exceeded, or will soon exceed, their design lives. A new replacement fleet is being deployed, starting with the May 2010 launch of the first of the GPS IIF satellites. Improvements include an extended design life of up to 12 years, faster processors with more memory and a new civil signal on a dedicated frequency. This added signal could provide sub-meter accuracy for the civilian GPS signal without the need for augmentation systems. There are 11 more spacecraft in this series scheduled for launch, with three expected to be deployed in 2011. The United States enhances the GPS system with the Wide Area Augmentation System (WAAS), which uses dozens of ground stations as well as capacity on commercial FSS satellites to increase the accuracy, integrity and availability of GPS positioning data.
Other nations are developing alternatives to GPS to stimulate innovation and provide new business opportunities. Russia's GLONASS, like GPS, is designed for both military and civilian use. However, due in part to Russia's economic collapse in the 1990s, the Soviet-era network fell into disrepair. In 2001, President Vladimir Putin ordered a 10-year, $3 billion network modernization program to create a fully operational alternative to GPS. The GLONASS system design requires 21 active satellites for full global coverage, with three in-orbit spares. A launch failure in December 2010, and the loss of three GLONASS satellites on board, means that the constellation is now scheduled for completion in mid-2011. While there are relatively few GLONASS-compatible receivers, Russian manufacturers are increasing production as the country pursues agreements with foreign companies including Nokia, Motorola and Qualcomm.
The European system, a 30-satellite constellation called Galileo, is under development by the European Union. In January 2010, the first contracts for satellite construction were awarded, and the first launches are planned for 2012, leading to a partial operational capability by 2015. The EU continues to proceed despite budgetary overruns, schedule delays, and the possibility that the system may need to be reduced from 30 satellites to 24. Such a reduction could limit Galileo's use in safety-of-life services proposed for commercial airlines. Further, an October 2010 European Commission (EC) report indicated that Galileo will be unprofitable for at least the first several years of operation. Europe also has a space-based GPS-augmentation system called the European Geostationary Navigation Overlay Service (EGNOS), which became operational in October 2009. The network is similar to the U.S. WAAS system, incorporating ground stations and transponders aboard three GEO satellites.
Since 2000, China has been building a system called Compass (Beidou, in Chinese). China launched five satellites in 2010 as part of Compass, which is planned to consist of 30 MEO satellites for global coverage augmented regionally by five GEO satellites. The regional system is expected to become functional by 2012, when the first 12 satellites are expected to be operational. The full constellation providing global coverage is expected to become operational between 2015 and 2020.
In September 2010, JAXA successfully launched the Michibiki spacecraft, the first satellite of the Japanese Quasi-Zenith Satellite System (QZSS). This network will be comprised of three GEO satellites operating over Japan and surrounding areas. The orbit of these satellites was selected so that they will be easier to see from urban and mountainous regions where the terrain often obstructs the line-of-sight to PNT satellites. Japan also has a GPS augmentation system that is based on two GEO Multi-functional Transport Satellites (MTSAT). These satellites were launched in 2005 and 2006 and form the core of the country's MTSAT Satellite-based Augmentation System (MSAS).
The Indian Regional Navigation Satellite System (IRNSS) will include seven satellites providing coverage primarily for South Asia. The complete constellation is planned to be in place by 2014, with the first satellite launch anticipated in 2012. India had planned to include a transponder for its GPS-Aided Geo Augmented navigation (GAGAN) system as part of the GSAT-4 communications satellite. In April 2010, the GSAT-4 crashed into the Indian Ocean shortly after the failure of the GSLV rocket carrying it. An additional satellite with a GAGAN payload is to be launched in early 2011 and another in 2012, helping to provide coverage over India and the surrounding region.
This information, and many more interesting and informative facts about space and the space industry, can be found in The Space Report 2011: The Authoritative Guide to Global Space Activity. Click here to learn more.