GeoWorld

GeoWorld April 2012

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control to maintain equipment operating tempera- tures; electrical power generation and battery storage; attitude control to maintain satellite orientation and conduct orbital maneuvers; propulsion to obtain and maintain the proper orbit; and a telemetry, tracking and commanding system to process ground-control commands and telemetry as well as manage bus sig- nals and data. Leaning Forward for GPS Navigation The GPS III team developed the world's most capable navigation satellite to meet rigorous customer needs. The U.S. Air Force carefully collected user require- ments from the civilian community, led by the Federal Aviation Administration and NASA, as well as warfighters. Next, it selected a winning team (Lockheed Martin, ITT Exelis, General Dynamics, Honeywell and Infinity Systems) to develop the GPS III satellites. In GPS navigation, location is all about time. GPS III atomic clocks maintain enhanced clock stability with improved filtering of a spectral absorption line of the Rubidium atom. Excelitas' rubidium clocks have pro- vided the "heartbeat" used to synchronize GPS signals for more than half the GPS constellation. With GPS III satellites, each atomic clock is com- pared with two other clocks: a high-performance clock (voltage-controlled crystal oscillator) and a less-precise clock (temperature-controlled crystal oscillator). This comparison provides a means to validate clock timing and isolate faults. Improved GPS III timekeeping will improve navigation performance as their signals blanket Earth in near-perfect synchronization. Better timing accuracy through improved atomic clocks is only part of the story. If the atomic clock is the "heart," the mission data unit is the "brain" that determines the GPS III satellite location (with assis- tance from GPS ground control) and formulates GPS messages for transmission to Earth. GPS signals carry navigation messages with precise satellite-clock and ephemeris information. There are three unencrypted civilian messages for all users and two encrypted military messages for specific military operations. Of the three civilian navigation messages, one is the legacy navigation message, and two are modernized messages, the latter of which adds data bits to improve satellite-location accuracy. The improved accuracy in satellite location translates directly to improved accuracy for all users. The mis- sion data unit has been developed by ITT Exelis and currently is undergoing robust tests. Next Steps The dedicated GPS III team is developing the first GPS III pathfinder satellite to ensure seamless pro- duction of subsequent GPS III satellites, significantly Mobility/GPS Special Issue Jim Keyser, manager of Lockheed Martin's GPS Processing Facility, stands in the Anechoic test chamber where the company will perform tests on the GPS III spacecraft to ensure all its signals and interfaces work properly. lowering risk for the first GPS III satellite launch in 2014. All elements of the GPS III pathfinder are under- going rigorous tests. With the first GPS III launch in 2014, users world- wide will begin to experience unsurpassed navigation performance. This cutting-edge technology, combined with international agreements to share the new L1C civil signal, creates robust navigation performance with multisystem receivers. Thanks to GPS, "stopping to ask for directions" no longer is a road-trip tradition. New applications are constantly evolving and appear to be unlimited: GPS tracking chips to find lost pets; golf-game enhance- ment devices that offer the insights of a PGA caddie; researchers tracking aquatic and land-based wildlife in real time; and tsunami centers floating GPS units on ocean buoys to increase coastline warning times. We've come a long way since the lanterns in the steeple of Boston's Old North Church. Mark Crews is director of PNT Programs, ITT Exelis Geospatial Systems; e-mail: mark.crews@exelisinc.com. APRIL 2O12 / WWW . GEOPLA CE . COM 17

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