In the late 1950s, the former Soviet Union launched the first man-made
earth satellite. American scientists discovered the Doppler frequency shift
phenomenon in their tracking research, and used this principle to promote the
Doppler satellite navigation and positioning system. The completion of TRANsIT
has achieved great success in military and civilian applications, and is a leap
in the history of navigation and positioning. my country has also introduced a
number of Doppler receivers, which are used in the fields of island joint
testing and earth exploration. However, due to the low orbital height and low
signal carrier frequency of Doppler satellites, it is difficult to improve the
orbital accuracy, so that the positioning accuracy is low, so as to meet the
requirements of geodetic survey or engineering survey, and it is even less
likely to be used for astronomical geodynamics research. In order to improve the
accuracy of satellite positioning, the United States began to build the Global
Positioning System GPS (Global Positioning System) in 1973. After entering the
stage of program demonstration and system testing, the official working
satellite was launched in 1989, and it was all completed and put into use in
1994. The space part of the GPS system consists of 21 satellites, which are
evenly distributed on 6 orbital planes. The ground height is more than 20,000
kilometers, the orbit inclination is 55 degrees, the flat heart rate is about 0,
and the period is about 12 hours. The carrier signal of two bands, the carrier
signal frequencies are 1575.442 MHz (L1 band) and 1227.6 MHz (L2 band), and a
high-precision atomic clock is installed on the satellite to ensure the
stability of the frequency. Broadcast ephemeris of satellite position, C/A code
and P code for ranging, and other system information, can provide
high-precision, all-weather, continuous, real-time three-dimensional speed
measurement to any multi-user on a global scale , 3D positioning and timing.
The control part of the GPS system consists of 5 monitoring stations
located in the continental United States. These stations continuously observe
the GPS satellites, and update the satellite information with the information of
calculation and forecast by the injection station.
The user of the GPS system is very hidden, it is a one-way system, the user
only receives and does not have to transmit the signal, so the number of users
is also unlimited. Although the GPS system was originally established for
military purposes, it soon developed greatly in civilian use, and various GPS
receivers and processing software emerged one after another. At present, the
receivers that appear in the Chinese market mainly include NovAtel, ASHTECH,
TRIMBLE, CMC and so on. A receiver that can observe two frequencies is called a
dual-frequency receiver, and a receiver that can only observe one frequency is a
single-frequency receiver, and they are quite different in accuracy and
price.
For users in the field of surveying and mapping, GPS has caused a
revolutionary change in the field of surveying and mapping. At present, the
control network or deformation monitoring network with a range of several
kilometers to several thousand kilometers, with an accuracy of 100 meters to
millimeters of positioning, generally GPS is the first choice. With the maturity
of RTK technology, GPS has begun to penetrate into the fields of decimeter and
even centimeter-level staking and high-precision dynamic positioning.
Since 1992, IGS, the international GPS geodetic and geodynamic service, has
established a number of data storage and processing centers and more than 100
perennial observation stations around the world. my country has also established
Shanghai Yushan, Wuhan, Xi'an, and Lhasa. , Taiwan and other perennial
observation stations. The observation data of these stations are transmitted to
the data storage center in the United States through the Internet every day. IGS
also integrates the results of various data processing centers in almost real
time, and participates in the global earth rotation service IERS. Coordinate
reference system maintenance and publication of Earth rotation parameters. Users
can also obtain observation data and precise ephemeris products from the
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 the satellite position in the broadcast ephemeris,
reduce the accuracy of the correction number of the star clock, and add high
frequency to the satellite reference frequency. Jitter (reduces the measurement
accuracy of pseudorange and phase), and then implemented the A-S policy, that
is, changing the P code to the Y code, which further restricts precision
pseudorange measurement, and the US military and authorized users are not
affected by these policies. Influence, but in order to obtain greater commercial
interests in the United States, these policies will eventually be cancelled.
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