4.1
Introduction
The concept of Digital Earth was proposed by former US vice president Al Gore in
1998. At the 6th International Symposium on Digital Earth in Beijing, Digital Earth
was defined as an integral part of other advanced technologies, including earth obser-
vation, geo-information systems, global positioning systems, communication net-
works, sensor webs, electromagnetic identifiers, virtual reality, and grid computa-
tion. Satellite navigation and positioning technology can provide precise position
and time information, which are key elements of the Digital Earth.
In satellite navigation and positioning technology, the radio signals transmitted
by navigation satellites are received by the user terminal. By measuring the time
delay of the signal propagated from the navigation satellite to the receiver, naviga-
tion, positioning and timing services can be realized. Compared with conventional
navigation and positioning techniques, satellite navigation and positioning technol-
ogy can provide precise three-dimensional positions, velocity and time for users. It is
an all-weather, all-time and globally available technology. Great progress has been
made in recent decades, and many countries and consortia have established their
own global navigation satellite systems. Global satellite navigation and positioning
technology has been widely applied in navigation for vehicles, offshore ships, aero
craft and aerospace vehicles and in the fields of geodesy, oil exploration, precision
agriculture, precise time transfer, and earth and atmospheric sciences.
4.2
Global Navigation Satellite System
Before the advent of man-made satellites, navigation and positioning mainly
depended on ground-based radio navigation systems that were developed during
the Second World War such as LORAN and Decca Navigator, shown in Fig.
4.1
. On
October 4th 1957, the former Soviet Union (Union of Soviet Socialist Republics,
or USSR) launched the first man-made satellite. Based on the Doppler shift of the
radio signal, Dr. Guier and Dr. Wiffenbach from Johns Hopkins University success-
fully calculated the orbit of the satellite. This laid the foundation for the scientific
idea of navigation and positioning with the use of man-made satellites. In 1958,
the US military began to develop the first (generation) satellite navigation and posi-
tioning system in the world—the Transit navigation satellite system (TRANSIT),
which was formally put into military use in 1964. The USSR also began to estab-
lish the CICADA system in 1965, and the first CICADA satellite was launched in
1967. Using the Doppler shift method, the first-generation satellite positioning sys-
tem needed long-term observations to realize navigation and positioning, and the
positioning accuracy was also unsatisfactory. To overcome these limitations, the
joint development of a new generation satellite navigation system—the Global Posi-
tioning System (GPS)—by the US army, navy and air force was formally approved
4
Satellite Navigation for Digital Earth
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