The System: New Math for GPS

February 1, 2010  - By

New Math for GPS: the Geometry of 27

The U.S. Air Force GPS Wing and 50th Space Wing have begun repositioning GPS satellites in space to fly what they call the 24+3 or Expandable 24 constellation plan. The initiative will take up to 24 months to fully implement. Benefits to users will be slowly realized during that time, as the number of GPS satellites in view will increase, potentially increasing GPS receiver accuracy.

The plan significantly alters the current configuration of 30 GPS satellites on orbit. Several newer satellites now fly in tandem, side by side, with older satellites, as a hedge against their eventual failure. This policy has effectively limited constellation geometry to that of 24 satellites.

The policy change was driven at least in part by the desire to improve satellite visibility for U.S. and allied military operations in Afghanistan and Iraq, where mountainous terrain can hamper signal coverage for troops on the ground.

The first GPS space vehicle (SV) to move, SVN24, began its long journey on January 13. This satellite has the farthest to travel, and will not reach in its new slot for approximately 12 months (January 2011). The two others, SVN49 and SVN26, will affect the geometry much sooner. SVN49 started its four-month journey on January 21, destined to reach its new home in May 2010. SVN26 will stir on February 8 and should also find itself in its new slot by that time, if all goes as planned.

SVN24 will take a full 12 months because the operators must have maneuvering fuel onboard when it reaches its final orbit location for station keeping and Delta-V maneuvers; and they must conserve fuel for end-of-life in as much as 15 years to boost the satellite into a safe retired orbit.

Civil Benefits. 24+3 will especially benefit surveyors and other professionals using real-time kinematic (RTK) positioning. These users currently require six satellites in view for a very precise (centimeter accuracy) position.

In the last few years, some users that require long dwell times and experience high mask angles have been forced to use GLONASS satellites as an augmentation, which works, although GLONASS satellites have historically been less accurate than the GPS. When 24+3 is fully implemented, GLONASS augmentation may no longer be necessary for these users.

New Ground Software. The GPS Wing also trumpeted the advent of improved capabilities through a new ground-system software release. These include telemetry, tracking, and commanding for the new GPS IIF space vehicle — as yet unlaunched. On-orbit capabilities planned to arrive with the IIF Block span the new L5 navigation signal for civil users, continued and more robust security-wise deployment of the encrypted military-only code known as M-Code, on-orbit crosslink (between GPS satellites) improvements, and overall signal power increases.

In November and December 2009, the new software uploaded operational GPS IIA and IIR space vehicles with navigation data and completed normal operational functions.

Penny-Wise, Pound-Foolish U.S. Coast Guard to Pull Loran Plug

The U.S. Coast Guard announced on January 7 that the it will cease broadcasting the North American Loran-C signal on February 8.
“As a result of technological advancements during the last 20 years and the emergence of the U.S. Global Positioning System, Loran-C is no longer required by the armed forces, the transportation sector, or the nation’s security interests.”

The force, and President Barack Obama’s fiscal year 2010 budget which it cited, go against the unanimous recommendation of the Independent Assessment Team, empanelled by a previous administration and led by Bradford Parkinson, founding program director for GPS: “complete the eLoran upgrade and commit to eLoran as the national backup to GPS for 20 years.” To pay for expert advice and then ignore it is a time-honored tradition of U.S. government.

Senator Susan Collins, ranking member of the Senate Homeland Security and Governmental Affairs Committee, called the Coast Guard plan a “mistaken decision,” adding that “A lone system is problematic and ill-advised on so many levels. We need Loran as a backup to GPS.
“Pulling the plug on Loran now will likely prove penny-wise and pound foolish, because there is no other system, or constellation of systems, that offers a more robust and cost-effective backup to GPS than eLoran. It is my urgent request that the Secretary reconsider this ill-informed decision.”

Galileo Satellites Awarded to OHB

The European Commission awarded on January 7 contracts for deployment of Galileo’s initial operational capability in space. The first order of 14 satellites goes to OHB System AG of Bremen, Germany (as indicated, but not confirmed, in these pages last month).

The EC bestowed a contract for system support services upon ThalesAleniaSpace of Italy, and one for launch services to Arianespace of France. Initial deployment and service provision of Europe’s satellite navigation system is now envisioned for early 2014.

Procurement contracts for ground mission infrastructure, ground control infrastructure, and operations should be awarded by mid-2010.
The OHB order carries a value of €566 million (U.S. $811 million), with delivery of the first satellite in July 2012. One satellite is expected every 1.5 months thereafter, with the last one scheduled to be delivered in March 2014.

Compass at Three

China launched a third Beidou-2 or Compass navigation satellite on January 17, destined for geostationary (GEO) orbit. A previous middle-Earth orbit (MEO) craft went up in April 2007 and a GEO in April 2009. By January, that GEO had drifted about 16 degrees from its initial slot, possibly indicating it is uncontrollable although some reports indicate the satellite is still usable.

Eventually, China plans five GEOs and 30 MEOs. The initial Compass system will provide the Asia-Pacific region with navigation, timing, and short-message communication services as early as 2015, with a plan for global coverage by 2020.

Compass will offer an open service (free positioning and timing services, positioning accuracy 10 meters, timing 10 nanoseconds), and an authorized service, with “more secure” position, velocity, timing, and communications data as well as a higher level of integrity.

The government also unveiled an official, Chinese-language-only Compass website,


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About the Author:

Alan Cameron is editor-in-chief and publisher of GPS World magazine, where he has worked since 2000. He also writes the monthly GNSS Design & Test e-Newsletter and the Wide Awake blog.

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