At the end of the 1950s, the former Soviet Union launched the first
man-made earth satellite of mankind. American scientists discovered the
phenomenon of Doppler frequency shift in their tracking and research, and used
this principle to facilitate the establishment of the Doppler satellite
navigation and positioning system TRANsIT, which has achieved great success in
military and civil aspects. It is a leap in the history of navigation and
positioning. China has also introduced several Doppler receivers, which are used
in island joint survey Earth exploration and other fields. However, due to the
low orbit height and signal carrier frequency of the Doppler satellite, it is
difficult to improve the orbit accuracy, which makes the positioning accuracy
low to meet the requirements of geodesy or engineering survey, and it is
impossible to apply it to the research of astrogeodynamics. In order to improve
the accuracy of satellite positioning, the United States began to build the
Global Positioning System (GPS) in 1973. After entering the phase of scheme
demonstration and system test, the official working satellite was launched in
1989, and was completed and put into use in 1994. The space part of the GPS
system is composed of 21 satellites, which are evenly distributed on 6 orbital
planes. The ground height is more than 20000 km, the orbital inclination is 55
degrees, the flat heart rate is about 0, and the period is about 12 hours. The
satellite transmits carrier signals of two bands to the ground. The carrier
signal frequencies are 1575.442 MHz (L1 band) and 1227.6 MHz (L2 band)
respectively. The satellite is equipped with atomic clocks with high accuracy to
ensure the stability of the frequency, The broadcast ephemeris representing
satellite position, C/A code and P code used for ranging, and other system
information are modulated on the carrier wave, which can provide high-precision,
all-weather, continuous, real-time three-dimensional velocity measurement,
three-dimensional positioning, and time service to any number of users
worldwide.
The control part of the GPS system consists of five monitoring stations
located in the United States. These stations continuously observe the GPS
satellites and update the calculated and predicted information from the
injection station to the satellite information.
The users of the GPS system are very invisible. It is a one-way system, and
users only receive signals without transmitting them. Therefore, the number of
users is unlimited. Although the GPS system was built for military purposes at
the beginning, it has been rapidly developed in civilian areas, and various GPS
receivers and processing software have emerged. At present, the receivers in the
Chinese market mainly include NovAtel, ASHTECH, TRIMBLE, CMC, etc. The receiver
that can observe two frequencies is called dual frequency receiver. The receiver
that can only observe one frequency becomes single frequency receiver. They are
quite different in accuracy and price.
For users in the surveying and mapping field, GPS has caused revolutionary
changes in the field of surveying and mapping. At present, the control network
or deformation monitoring network covering several kilometers to thousands of
kilometers, with positioning accuracy ranging from hundreds of meters to
millimeters, generally use GPS as the preferred means. With the maturity of RTK
technology, GPS has begun to penetrate into decimeter and even centimeter level
lofting, high-precision dynamic positioning and other fields.
Since 1992, IGS, an international GPS geodetic and geodynamic service, has
established multiple data storage and processing centers and more than 100
perennial observation stations around the world. China has also set up a number
of perennial observation stations in Shanghai Yushan, Wuhan, Xi'an, Lhasa,
Taiwan, etc. The observation data of these stations are transmitted to the data
storage center in the United States every day through the Internet. IGS also
integrates the results of each data processing center almost in real time, And
participate in the global coordinate reference system maintenance of the
international earth rotation service IERS and the release of the earth rotation
parameters. Users can also obtain observation data, precise ephemeris and other
products from INTERNET for free.
The real-time navigation and positioning accuracy of the GPS system is very
high. The United States has implemented the so-called SA policy since 1992, that
is, to reduce the accuracy of satellite positions in broadcast ephemeris, to
reduce the accuracy of clock correction numbers, to add high-frequency jitter to
the satellite reference frequency (reducing the measurement accuracy of pseudo
range and phase), and then to implement the A-S policy, that is, to change the P
code to Y code, which further restricts the precision pseudo range measurement,
The US military and licensed users are not affected by these policies, but in
order to obtain greater commercial benefits, these policies will eventually be
cancelled.
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