The New GPS 24+3 Constellation

In January, the U.S. Air Force announced it is reconfiguring the GPS constellation. The Air Force is changing the constellation from a 21+3 configuration to a 24+3 configuration. The result will be more satellites in view, on average.

This is great news for the GPS surveying and GIS mapping user. In my? opinion, it was the only achievable short/medium-term solution to the GPS “brownout” problem that has plagued GPS surveying and mapping users for years, and has worsened in recent months.

In short, a GPS ?brownout? is a time of the day when a GPS user is unable to utilize his or her GPS receiver because there aren?t enough satellites in view to achieve the desired accuracy. GPS ?brownouts? primarily affect high-precision RTK users because that technology requires that the GPS receiver is tracking at least six satellites for a reliable position. With the current GPS constellation, there are times during the day when this is not possible given the satellite configuration and local conditions (obstructions such as trees, buildings, and terrain). This problem puts a serious damper on GPS productivity.

Even though there are currently 30 operational GPS satellites, they are configured in a 24-satellite constellation. Essentially, several satellites are ?paired up? so they add no value to users on the ground. They are designated as back-up satellites in case of a failure. I wrote a detailed article on this subject in October 2009 titled GPS Constellation Management: Playing Not to Lose that summarizes the problem.

The New 24+3 Configuration

Announcements from various publications and online newsgroups have different interpretations of the Air Force announcement. Some are emphasizing increased accuracy and others are citing increased coverage in Afghanistan. While both are correct, the major benefit to the surveying/mapping user community is increased worldwide satellite visibility. In other words, more GPS satellites will be in view at a given time during the day.

More satellites in view = greater RTK and mapping productivity.

The reason that increased accuracy is mentioned in the announcement is because PDOP values will be lower in general due to the increase of satellites in view?and there?s a direct correlation between accuracy and PDOP. Just how many more satellites will be in view is not clear yet. I?m working on producing some mission planning charts that will illustrate the benefits of 24+3 compared to 21+3.

The three satellites being repositioned are SVN24, SVN26, and SVN49. SVN24 and SVN26 are two of the oldest satellites (Block-IIA) in the GPS constellation. SVN24 was declared operational in August 1991. SVN26 was declared operational in July 1992. SVN49 is a newer Block II-RM that was launched last March and has never been declared operational due to an anomaly discussed here before. More on SVN49 further down.

The time to reposition each satellite is significant. SVN24, with the furthest distance to travel, began its journey last week and will take 12 months to reach its destination slot according to the Air Force. SVN49 will begin its journey on January 21, 2010, and will take four months (May 2010). SVN26 will begin its transition on February 8, 2010, and will reach its destination slot in approximately three months (May 2010), according the Air Force.


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