The upgrade consists of hardware and software to replace the original command-and-control system, which has operated since GPS' inception, said Capt. Brian O'Connell, GPS Modernization Flight commander.

"The system will be capable of commanding and controlling the GPS constellation much as we do today, but planned software drops will also allow us to control the new II-F block of satellites when they're ready to fly," Captain O'Connell said.

The system also lets operators link directly into the Air Force Satellite Control Network, more than doubling the number of sites they can use for satellite command and control.

"This provides us with a greater capability to command our satellites, reduce commanding visibility gaps and potentially reduce our anomaly response time," Captain O'Connell said.

The transition process won't be as simple as turning off one system and turning on another, however.

"The core of the system is something called the Kalman Filter," Captain O'Connell said. "This system takes in data from our monitoring stations worldwide and uses this data to predict where each of the satellites will be in the future. This model is constantly updated, and the model in turn is uploaded to each vehicle."

The Kalman Filter makes sure each GPS satellite is broadcasting a precise navigation and timing signal. The new system will have a new Kalman Filter--which means the system will have to be carefully aligned with the current system's Kalman Filter.

"If we didn't do this, and we began uploading satellites with data from the AEP Kalman Filter, those vehicles would tell you that you're in a different place than the vehicles that still contain 'legacy' uploads," Captain O'Connell explained. "Clearly you wouldn't want your GPS receiver trying to tell you that you're in two different places."

In addition, each GPS ground antenna and monitoring station must migrate to the new system.

"This is a complicated piece of choreography," the captain said.

The process of converting the constellation and ground system takes about five days.

Related Links
2nd Space Operations Squadron
GPS Applications, Technology and Suppliers
GPS Applications, Technology and Suppliers