New 24+3 GPS Configuration Will Increase Accuracy

---

Moving three existing GPS satellites to new orbit locations will have a profound effect on GPS capabilities for all civil, commercial, and military users worldwide.

The U.S. Air Force Global Positioning Systems Wing and the 50th Space Wing have announced a reconfiguration: the 24+3 GPS constellation plan or “Expandable 24,” with timeline as follows:

“The [24+3] initiative will take up to 24 months to fully implement as satellites are repositioned within the constellation based on constellation health. The beneficial impact to all GPS users, including civilian users, will be slowly realized during that time period. Over the next two years, the number of GPS satellites in view from any point on earth will increase, potentially increasing accuracy of GPS receivers.”

The plan will significantly alter the current configuration, which consists of indeed 30 GPS satellites in MEO or medium earth orbit that are used globally, however many of the additional satellites are currently flown in tandem, side by side, with considerably older satellites and effectively limit the constellation geometry to that of 24 satellites.

Further details will come in this week’s Defense PNT newsletter, in a column by GPS World contributing editor Don Jewell.

Meanwhile, the Wing and GPS 50th Space Wing also announced that they “are ushering in improved Global Positioning System (GPS) capabilities through a new ground system software release. New capabilities include telemetry, tracking and commanding for the new GPS IIF space vehicle and robust security improvements. The planned transition at Schriever Air Force Base on January, 11, 2010, is the result of extensive testing to ensure this upgrade is transparent and has no impact to military and civil users."

The Space Wing continued, "With the pending mid-2010 launch of the first GPS IIF space vehicle, the ground system is prepared to command the new on-orbit GPS IIF capabilities which include a new navigation signal for civil users, encrypted military code, crosslink enhancements, improved navigation signal accuracy and signal power increases. The new software also provides robust security improvements to include 'over-the-air' distribution of encryption keys to properly equipped military users. Preparation for activation of the new software included rigorous developmental and operational testing events including five transition exercises. The new ground system software commanded current individual GPS satellites during numerous testing events and rehearsals. In November and December 2009, the new software successfully uploaded operational GPS IIA and IIR space vehicles with navigation data and completed normal operational functions. This improvement initiative continues the Air Force’s commitment to the global community of GPS users."

The GPS Wing statement continued, "The U.S Air Force and Air Force Space Command have been the diligent stewards of GPS since its conception in the 1970s and continue its commitment to this critical component of our National Infrastructure. The current GPS constellation has the most satellites and the greatest capability ever. We are committed to maintaining our current level of service, as well as striving to improve service and capability through on-going modernization efforts. The Air Force will continue to pursue an achievable path maintaining GPS as the premier provider of positioning, navigation and timing for military and civilian users around the world."

The Air Force Space Command’s Space and Missile Systems Center, located at Los Angeles Air Force Base, Calif., is the U.S. Air Force’s center of acquisition excellence for acquiring and developing military space systems including six wings and three groups responsible for GPS, military satellite communications, defense meteorological satellites, space launch and range systems, satellite control network, space based infrared systems, intercontinental ballistic missile systems and space situational awareness capabilities.

source: GPS World

Opinion: GPS L2P(Y) Phase Shift Causes Needless Consternation January 6, 2010 By: Don Jewell

Roughly three years ago, the U.S. military conducted the first flex-power test on the L2 GPS codeless signal. Almost immediately, the civilian GPS community expressed concern that future changes to the L2P(Y) signal power levels might cause a signal phase shift; such a phase shift would be incompatible with equipment using the P(Y) signals in a codeless/semicodeless fashion for extremely accurate positioning applications.

Civilian users were naturally upset because they had invested millions of dollars in systems that might not be usable — even if the unusable periods were of a very short duration.

The National Positioning, Navigation, and Timing (PNT) Executive Committee responded by tasking the National PNT Engineering Forum (NPEF) to look at the problem. Within a few months, the NPEF announced a solution: flex power could be used in such a manner that it would not cause a phase shift. At the same time, the military reminded civilian users that the codeless use of L2P(Y), as accurate as it might be, was never intended and should not be a long-term solution.

An agreement was reached between the U.S. government and civilian users that the civilian users of this codeless/semicodeless technique would migrate from using the L2P(Y) carrier to using the new L2C signal to achieve not only the same, but better results. To codify this agreement, a Federal Register Notice was issued in 2008 identifying the terms of this agreement, which guaranteed the phase stability of the current L2P(Y) signal until 2020. This gives civilian users 12 years to figure out a migration plan and to obtain adequate use of the equipment they already have on hand.

In addition, 2020 is not a drop-dead date, but a date when the use of L2P(Y) codeless signals will no longer be guaranteed, though may well still work. Who knows what PNT advancements will take place between now and then? This could very well be a moot point by then, and in my opinion should be one now.

Problem Solved? Apparently not. A lag between the issuance of this national policy and analogous adjustments to interface specifications caused consternation within the civilian community. Misunderstandings added to this perceived impasse. Various solutions were identified to work around this looming quandary. However, given the national policy to support codeless/semicodeless use until 2020, the Air Force Space Command commitment to that policy, and the recommendations of the NPEF, these solutions seem wholly unnecessary to me.

The U.S. government has gone well beyond what is required to ensure civilian codeless and semi-codeless users are accommodated.

For the foreseeable future, users will be able to employ L2P(Y) codeless/semicodeless techniques for very accurate position determination and will not have to worry about phase shifts disrupting their work.

— Don Jewell, GPS World Defense PNT Contributing Editor