GPS SYSTEM FUTURE
The GPS Block I satellites were only launched for system testing. The 24 operational or Block II satellites were due for launch from October 1986 to December 1988 but the space shuttle Challenger accident seriously delayed the programme. However, the launch programme came back on a new schedule and the full constellation of 24 satellites was operational in 1996. As the Block II satellites reach the end of their design life they are being replaced by Block IIR (replenishment) satellites.
The Block IIR satellites have the same signals in space as the Block II satellites. However, they autonomously navigate, that is they create their own navigation message and maintain full accuracy for at least 180 days. They have improved reliability and integrity of broadcast signal. There is additional radiation hardening of the satellite and cross-link ranging between satellites. This allows much greater flexibility in control of the system and some of the Control Segment’s functions have been transferred to the satellites themselves. There are two atomic clocks on at all times. The satellite has a larger fuel capacity.
In 1996, a major policy statement from the White House was issued. In effect, it stated that at some date between 2000 and 2006, Selective Availability (SA) would be removed and therefore there would be full access to both frequencies. In May 2000, SA was removed. In January 1999, the White House through the Office of the Vice President announced that a third civilian signal would be located at 1176.45 MHz. This is the L5 signal proposed for future Block IIF satellites. It was also announced that a second civilian signal would be located at 1227.6 MHz with the current military signal and that would be implemented on satellites scheduled for launch from 2003. This is the L2C signal on future IIR-M satellites.
In March 1997 Dr James Schlesinger, a former Secretary of Defense, summed up the situation with respect to GPS. ‘The nation’s reliance on GPS has become an issue of national security in its broadest sense that goes beyond merely national defense.’ Originally GPS was conceived as a Military Support System for War, but President Reagan, in 1983, decided that there should be civil access to the system. Therefore, it is now a critical dual-use US national asset. On the other hand, it has become more essential to military forces than had ever been previously imagined but it has also become indispensable to civil and commercial users as well. However, it is still funded, managed, and operated by the US Department of Defense.
The civil and military communities have different needs for the twenty-first century’s upgraded GPS system. The military require greater security of the system, a quicker fix from a cold start, more powerful signals, their own signals for fast acquisition and better security codes. These will be achieved with a new M code on the L1 and L2 frequencies. The civil market requires that the GPS signal is accurate, fully available, has full coverage, measurable integrity, redundant signals, more power and that SA remains switched off. A second civil signal is required for simpler ionospheric corrections and a third civil signal for high accuracy, real-time applications. Those signals which are required for ‘safety of life’ applications require spectrum protection. This will be achieved with the new L2C and L5 signals and codes.
The L2C code on the L2 signal and theMcode on both the L1 and L2 frequencies will be implemented on the Block IIR-M satellites and the L5 signal will be implemented on the Block IIF satellites. The L2C code is not just to be a replica of the existing L1 C/A code. In 2002 there were estimated to be about 50 000 dual-frequency receivers in use and to be worth about $1bn not counting spares, software and associated communications.
Unlike leisure industry receivers, these receivers are used to add value to society, for example to monitor earthquakes, volcanoes, continental drift, and weather. They add value for cadastral and land survey purposes, for guidance and control of mining, construction and agricultural operations and are used in land and offshore oil and mineral exploration and marine surveys.