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GPS Future and Evolutions
| GPS | |
|---|---|
| Title | GPS Future and Evolutions |
| Author(s) | GMV |
| Level | Basic |
| Year of Publication | 2011 |
The GPS is a space-based global navigation satellite system (GNSS) that provides reliable positioning, navigation, and timing services to civilian and military users on a continuous worldwide basis. GPS is a U.S.-owned utility, developed by the U.S. Air Force starting with the program in 1978.
During the 1990s, a lot of civilian and commercial GPS applications appeared, and also another GNSS systems competitive with GPS were envisaged. Taking into account this, the policy of the United States policy is to maintain U.S. leadership in the service, provision, and use of satellite navigation systems. The U.S. government has additional policy goals to meet growing demands by improving the performance of GPS services, and to remain competitive with international satellite navigation systems.[1]
GPS Modernization Program
To meet growing demands and stay competitive internationally, the U.S. government is committed to a long-term modernization program that will improve GPS performance, including more robust resistance to interference. The GPS modernization program is an ongoing, multibillion-dollar effort to upgrade the GPS space and control segments with new features to improve GPS performance. These features include new civilian and military signals.[2]
Ending Selective Availability
The first step in GPS modernization took place in May 2000, when President Bill Clinton directed the Department of Defense to turn off the GPS Selective Availability (SA) feature. SA was an intentional degradation of civilian GPS accuracy, implemented on a global basis through the GPS satellites. During the 1990s, civil GPS readings could be incorrect by as much as a football field (100 meters). On the day SA was deactivated, civil GPS accuracy improved tenfold, benefiting civil and commercial users worldwide.
In 2007, the U.S. government announced plans to permanently eliminate SA by building the GPS III satellites without it.
New Civil Signals
The new signals are phasing in incrementally as new GPS satellites are launched to replace older ones. Most of the new signals will be of limited use until they are broadcast from 18 to 24 satellites.
Aimed at improving the performance for civilian users, the GPS Modernization Program will introduce the new signals designed for civilian use: L2C, L5, and L1C. The legacy civil signal, called L1 C/A or C/A at L1, will continue broadcasting in the future, for a total of four civil GPS signals.[3]
- L2C (1227.6 MHz): It enables the development of dual-frequency civil GPS receivers to correct the ionospheric group delay. For professional users with existing dual-frequency operations, L2C delivers faster signal acquisition, enhanced reliability, and greater operating range. L2C broadcasts at a higher effective power than the legacy L1 C/A signal, making it easier to receive under trees and even indoors. This signal is available since 2005, with the launch of the first IIR-M satellite[4]. The current L2C broadcast consists of a default message (Message Type 0) that does not provide full navigational data. L2C will remain in this state until the GPS Advanced Control Segment (OCX) comes online. The full availibility capacity will be managed with 24 satellites, around 2016.
- L5C (1176.45 MHz): L5 is broadcast in a radio band reserved exclusively for aviation safety services. Future aircraft will use L5 in combination with L1 C/A to improve accuracy (via ionospheric correction) and robustness (via signal redundancy). Beyond transportation, L5 will provide users worldwide with the most advanced civilian GPS signal, as it transmits at a higher power than current civil GPS signals, and has a wider bandwidth. Its lower frequency may also enhance reception for indoor users. It will be compatible with other GNSS systems, with the goal to be interoperable as well. This signal is available since the launch of the Block IIF satellites (May 28th 2010), and like L2C, the L5 broadcast will not include a data message until OCX comes online.
- L1C (1575.42 MHz): It has been designed for interoperability with Galileo. It will be backward compatible with the current civil signal on L1, be broadcast at a higher power level, and include advanced design for enhanced performance. The design will improve mobile GPS reception in cities and other challenging environments. Other satellite navigation providers are adopting L1C as a future standard for international interoperability. Japan's Quasi-Zenith Satellite System (QZSS), the Indian Regional Navigation Satellite System (IRNSS), and China's Compass system all plan to broadcast L1C. The United States will launch its first L1C signal with GPS IIIA (planned in 2014).
Moreover, in order to improve the anti-jamming and secure access of the military GPS signals, a new military signal (M-code) will be transmitted in L1 and L2 frequencies.
Once L2C and L5 are fully operational, their features will obviate the need for codeless or semi-codeless GPS receivers, which many GPS professionals use today to attain very high accuracy. Such receivers work by exploiting characteristics of the encrypted military P(Y) signal at the L2 frequency to achieve dual-frequency capability. Then, the U.S. government encourages all users of codeless/semi-codeless GPS technology to plan on using the modernized civil signals by December 31, 2020, as P(Y) may change after that date.
New Control Segment
Regarding the Ground Segment, the new Operational Control Segment (OCX) will replace the current GPS Operational Control System placed at Schriever Air Force Base.[5] The OCX will maintain backwards compatibility with the Block IIR and IIR-M constellation satellites, providing command and control of the new GPS IIF and GPS III families of satellites, and enabling new modernized civil signal capabilities. The OCX will provide new anti-jamming technologies, more advanced predictive algorithms, and more frequent clock and ephemeris updates. In addition, OCX will provide — for the first time ever — monitoring of civil GPS signals as well as command and control (C2) of new capabilities associated with the new GPS III generation of satellites. OCX will also support enhanced mission planning by Department of Defense users and be connected with a classified facility from which advanced military GPS capabilities can be planned and executed. Raytheon Corporation’s Intelligence and Information Systems (IIS) business, has won the contract to build the Next Generation GPS Control Segment (OCX) on March 2010.[6]
New GPS satellites
The GPS constellation is a mix of new and legacy satellites. The new generations, or blocks, of GPS satellites under development as part of the GPS modernization program are descrived below:
- GPS Block IIR(M)
The IIR(M) series of satellites are an upgraded version of the IIR series, completing the backbone of today's GPS constellation. The "M" in IIR(M) stands for modernized, referring to the new civil and military GPS signals added with this generation of spacecraft. Developed by Lockheed Martin, there are eight IIR(M) satellites: SVN-48 through SVN-50, SVN-52, SVN-53, SVN-55, SVN-57, and SVN-58. The first IIR(M) was launched in September 2005, and the last launch occurred in August 2009. As of April 2011, there were seven healthy IIR(M) satellites in the GPS constellation. The key improvements of this block are: the inclusion of second civilian GPS signal (L2C) for improved performance in commercial applications, two new military signals providing enhanced military jam-resistance and flexible power levels for military signals.
- GPS Block IIF
The IIF series expand on the capabilities of the IIR(M) series with the addition of a third civil signal in a frequency protected for safety-of-life transportation. The "F" in IIF stands for follow-on. Compared to previous generations, GPS IIF satellites have a longer life expectancy and a higher accuracy requirement. Each spacecraft uses a mix of rubidium and cesium atomic clocks to keep time within 8 billionths of a second per day. The IIF series will improve the accuracy, signal strength, and quality of GPS. Developed by Boeing, the IIF series includes a total of 12 satellites: SVN-62 through SVN-73. The first IIF satellite launched in May 2010. As of April 2011, there was one operational IIF satellite in the GPS constellation. The key improvements of this block are: operational version of the third civilian GPS signal (L5) for transportation safety, 12-year design lifespan and extremely accurate atomic clocks.
- GPS Block III (Increment IIIA and beyond)
The GPS III series is the newest block of GPS satellites (SVN-74 and up). GPS III will provide more powerful signals in addition to enhanced signal reliability, accuracy, and integrity -- all of which will support precision, navigation, and timing services. The first set of these satellites, under development by Lockheed Martin, is designated GPS IIIA. Two GPS IIIA satellites will be produced with an option to purchase an additional 10 satellites. Future versions will feature increased capabilities to meet demands of military and civilian users alike. The key improvements of GPS IIIA are: fourth civilian GPS signal (L1C) for international interoperability, 15-year design lifespan. The future GPS III will include Distress Alerting Satellite System (DASS) for search and rescue, and Satellite crosslinks for rapid command and reduced age of data. The first launch is expected as of 2016.
Modern technologies
In addition to the specific new features noted above, GPS modernization is introducing modern technologies throughout the space and control segments that will enhance overall performance. For example, legacy computers and communications systems are being replaced with a network-centric architecture, allowing more frequent and precise satellite commands that will improve accuracy for everyone.
Program Schedule
The GPS modernization program involves a series of consecutive satellite acquisitions, including GPS IIR(M), GPS IIF, and GPS III. It also involves improvements to the GPS control segment, including the Architecture Evolution Plan (AEP) and the Advanced Control Segment (OCX). The schedule for the parallel space and control segment upgrades is shown in next figure (the information on the schedule is correct as of May 2011).
GPS Future on Interoperability
As new and restored global and regional spacebased navigation systems emerge, interoperability continues to be the key to GNSS future. International cooperation on satellite navigation issues is a priority for the U.S. Government. The U.S. actively engages in
- bi-lateral cooperation: Europe, Japan, India, Russia ;
- multi-lateral cooperation: International Committee on GNSS (ICG), Asia Pacific Economic Cooperation (APEC) , ICAO, IMO, and ITU.
The main objective of the International Committee on Global Navigation Satellite Systems (ICG) is to ensure Compatibility and Interoperability between the GNSS systems. The ideal interoperability allows navigation with one signal each from four or more systems with no additional receiver cost or complexity. A dedicated ICG Working Group on Inteoperability has been created.