The DOD’s Navstar GPS reached a new performance level this year, with 30 satellites in orbit versus its minimum required civil complement of 24. Four more satellites add signal coverage and reduce the number of occasional “holes,” or gaps where fix geometry can sometimes fall below navigation standards. However, the longer-term sustainability and modernization of the constellation is creating some concern among senior USAF and civil officials.

The reason for this is that 15 of the 30 satellites are early Block IIA units, 11 of them approaching twice their predicted 7.5-year design lives. While this testifies to the excellence of their design, an undisclosed number are now down to “single string” operation of one or more of their originally triplicated systems, such as their high-accuracy atomic clocks, attitude controls and other essential components. In these cases, one more failure would remove the vehicle from service, and at this point such failures are unpredictable. While the loss of all 11 satellites in the near term is improbable, such a failure would severely compromise GPS operations.

To be ready for failures, the USAF plans single replacement launches this month and in December, followed by more in March, June and September next year. Should there be no failures, the USAF can opt to replace the satellites it deems closest to failure; add the new satellites to the constellation; place them in a standby, non-operating “parking” orbit; or delay a launch.

While adding more active satellites to the constellation sounds attractive, having more than the current 30 can lead to problems for GPS receivers built to accommodate a maximum of 30 satellites within the current GPS Almanac.

A GPS design engineer told AIN such receivers could become unsure of which satellites they were tracking, resulting in position errors.

Discussions about GPS capabilities often get tangled up with the question of which generation of satellite is involved, what their  benefits are, or will be, and when those benefits will arrive.

Since 1997, failed Block IIA satellites have been replaced by Block IIR units. These satellites have similar capabilities and a predicted design life of 10 years.

In 2005 Block IIR-M units superseded the Block IIR satellites. The IIR-Ms bring somewhat improved performance, but their key modernization is in the incorporation of an updated classified military M Code, plus a second civil frequency, called L2C, that allows correction for ionospheric effects, the remaining major source of GPS errors. The L2C frequency is not approved for aviation use.

The Block IIR-M satellites have a mean life expectancy in orbit of 8.61 years. The satellites to be launched beginning this month will be Block IIR-M units.

Following the Block IIR-Ms will be a production batch of 12 Block IIF satellites, launches of which are forecast to begin in July 2009. The Block IIF units have an orbital design life of 15 years and will carry another civil frequency, called L5, that will reduce ionospheric errors and be interference-protected for aviation use.

First launch of a GPS III satellite should occur in 2013. GPS III satellites have the Block IIR-M and IIF features, and a new L1C frequency to provide compatibility with Europe’s Galileo and Japan’s JRANS/QZSS satnav systems. By 2021, all Navstar satellites should be GPS III units.