THE SPACE SEGMENT |
The basic functions of the satellites are to (PARKINSON et al, 1995):
The Block I satellites were built by the Rockwell International corporation. The follow-on operational satellites are separated into four series: II, IIA, IIR and IIF. The Block II and IIA satellites were also built by Rockwell International and are massive, weighing over 900kg each. 28 Block II/IIA satellites have been built, of which 24 have been launched and activated, with the remainder "moth-balled" until they are required. The replacement Block IIR satellites, to be launched from 1996 onwards, are to be built by the General Electric corporation (now the Martin Marietta corporation). There are to be 20 Block IIR satellites, with options to build a further six. The Block IIF series are still in the design phase and may, for example, incorporate an additional transmission frequency (LACHAPELLE, 1995). They are planned to be launched from 2005 onwards.
The Block II satellites are maintained on a once per day upload, while the Block IIA satellite incorporate features that permit the navigation message to be up to 180 days in length (compared to the Block II's maximum of 14 days). The Block IIR satellites incorporate the autonomous (180 day) navigation feature based on crosslink ranging between satellites. The Block IIF will have the same capability.
The design life of the Block II/IIA GPS satellites is 7.5 years (compared with the intended life of Block I satellites of 5 years, and the planned life of 10 years for the Block IIR satellites). There are a number of satellite numbering conventions in use, the two most widely used are the:
The current GPS satellite constellation.
The status of the present constellation, including details
of launch and official commissioning date, the orbital plane and position
within the plane, and the satellite I.D. number(s).
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© Chris Rizos, SNAP-UNSW, 1999