232
4 Navigations from Ground to Space
was 4.5 years, but their actual lifetime was 8.8 years. The last Block I satellite
was decommissioned in the late 1995.
(3) From 1986 to 1994 was the rapidly deployed constellation phase. The secondgeneration GPS satellites are known as Block II, with approximately twice as
heavy as the Block I and a design life of 7.5 years. Between 1989 and 1990,
a total of nine Block II satellites were successfully launched into the orbits to
rapidly form an operational constellation. The Block II satellites could operate
up to 14 days without an upload from the control segment and the uploaded
information had been encrypted. The satellites themselves were radiation hardened, and their signals were subject to the Selective Availability (SA). Meanwhile, the advanced Block II satellites, known as Block IIA, were developed
during the phase. The Block IIA satellites could store more of the navigation
message, and operate without contact with the control segment for 6 months.
If it were actually done, of course, their broadcast ephemerides and clock bias
corrections would degrade. The first Block IIA satellite was launched in 1990,
and there were a total of 19 Block IIA satellites put into the orbits between
1990 and 1997. In December 1993, the GPS system achieved initial operational
capability, which indicated a full constellation with 24 GPS satellites available
to provide the SPS. The GPS program cost up to 1994, not including the cost
of the user equipment, had been estimated at about $5 billion.
The GPS Space and Control Segments had been developed, maintained and operated by the U.S. Air Force. In April 1995, the full operational capability of the GPS
system was declared by the U.S. Air Force Space Command, signifying full availability of the military secure PPS. The GPS constellation design originally called for
24 satellites, eight each in the three approximately circular orbits, but later it was
modified into six orbital planes with four satellites each. The six orbit planes have
approximately 55° inclination, separated by 60° right ascension of the ascending
node, with an orbital altitude of 20,184 km, which is the repeated orbit with two
circles per sidereal day. The constellation allows that at least six satellites are always
within line of sight from almost everywhere on the Earth’s surface. For this reason,
the four satellites are not evenly spaced 90° apart within each orbit. In general terms,
the angular differences between two close-to-satellites in each orbit are 30°, 105°,
120° and 105°, respectively, with a sum of 360°.
In the wake of the Block II satellites, the highest quality signals of the GPS system
were reserved for military use while the signals available for civilian use were intentionally degraded, which was so-called the SA. Afterward, with the widespread
growth of differential GPS services to improve civilian accuracy, the U.S. military
advantage getting the better positioning accuracy was being eliminated. Moreover,
the U.S. military was actively developing technologies to deny GPS service to potential adversaries on a regional basis. And thus, in 1996, the then U.S. President Bill
Clinton signed a policy directive declaring that the SA would be turned off in May
2000 to provide the same precision to civilians that was afforded to the military,
and establishing an Interagency GPS Executive Board (IGEB) to manage it as a
national asset. At midnight on May 1, 2000, as the SA software was switched off
4 Navigations from Ground to Space
was 4.5 years, but their actual lifetime was 8.8 years. The last Block I satellite
was decommissioned in the late 1995.
(3) From 1986 to 1994 was the rapidly deployed constellation phase. The secondgeneration GPS satellites are known as Block II, with approximately twice as
heavy as the Block I and a design life of 7.5 years. Between 1989 and 1990,
a total of nine Block II satellites were successfully launched into the orbits to
rapidly form an operational constellation. The Block II satellites could operate
up to 14 days without an upload from the control segment and the uploaded
information had been encrypted. The satellites themselves were radiation hardened, and their signals were subject to the Selective Availability (SA). Meanwhile, the advanced Block II satellites, known as Block IIA, were developed
during the phase. The Block IIA satellites could store more of the navigation
message, and operate without contact with the control segment for 6 months.
If it were actually done, of course, their broadcast ephemerides and clock bias
corrections would degrade. The first Block IIA satellite was launched in 1990,
and there were a total of 19 Block IIA satellites put into the orbits between
1990 and 1997. In December 1993, the GPS system achieved initial operational
capability, which indicated a full constellation with 24 GPS satellites available
to provide the SPS. The GPS program cost up to 1994, not including the cost
of the user equipment, had been estimated at about $5 billion.
The GPS Space and Control Segments had been developed, maintained and operated by the U.S. Air Force. In April 1995, the full operational capability of the GPS
system was declared by the U.S. Air Force Space Command, signifying full availability of the military secure PPS. The GPS constellation design originally called for
24 satellites, eight each in the three approximately circular orbits, but later it was
modified into six orbital planes with four satellites each. The six orbit planes have
approximately 55° inclination, separated by 60° right ascension of the ascending
node, with an orbital altitude of 20,184 km, which is the repeated orbit with two
circles per sidereal day. The constellation allows that at least six satellites are always
within line of sight from almost everywhere on the Earth’s surface. For this reason,
the four satellites are not evenly spaced 90° apart within each orbit. In general terms,
the angular differences between two close-to-satellites in each orbit are 30°, 105°,
120° and 105°, respectively, with a sum of 360°.
In the wake of the Block II satellites, the highest quality signals of the GPS system
were reserved for military use while the signals available for civilian use were intentionally degraded, which was so-called the SA. Afterward, with the widespread
growth of differential GPS services to improve civilian accuracy, the U.S. military
advantage getting the better positioning accuracy was being eliminated. Moreover,
the U.S. military was actively developing technologies to deny GPS service to potential adversaries on a regional basis. And thus, in 1996, the then U.S. President Bill
Clinton signed a policy directive declaring that the SA would be turned off in May
2000 to provide the same precision to civilians that was afforded to the military,
and establishing an Interagency GPS Executive Board (IGEB) to manage it as a
national asset. At midnight on May 1, 2000, as the SA software was switched off