1. space part
The space part of GPS is composed of 24 working satellites [1], which are
located 20,200 km above the surface, evenly distributed on 6 orbital planes (4
in each orbital plane), and the orbital inclination is 55°. In addition, there
are 4 active backup satellites in orbit. The distribution of satellites makes it
possible to observe more than 4 satellites anywhere in the world and at any
time, and to maintain a geometric image with good positioning accuracy. This
provides continuous global navigation capability in time. GPS satellites
generate two sets of codes, one is called C/A code (Coarse/Acquisition Code
11023MHz); the other is called P code (Procise Code 10123MHz), P code is not
easy to be interfered with due to its high frequency, and its positioning
accuracy is high, so It is controlled by the US military and has a password that
cannot be read by ordinary people. It mainly serves the US military. After the
C/A code people deliberately reduce the accuracy in order to take measures, they
are mainly open to private use.
2. ground control section
The ground control part consists of a main control station, 5 global
monitoring stations and 3 ground control stations. The monitoring stations are
equipped with sophisticated cesium clocks and receivers capable of continuously
measuring all visible satellites. The satellite observation data obtained by the
monitoring station, including ionospheric and meteorological data, will be
transmitted to the main control station after preliminary processing. The main
control station collects tracking data from each monitoring station, calculates
the satellite's orbit and clock parameters, and then sends the results to three
ground control stations. The ground control station injects the navigation data
and the command of the main control station into the satellite when each
satellite runs to the sky. This injection is performed once a day for each GPS
satellite, and a final injection is made before the satellite leaves the range
of the injection station. If a ground station fails, the navigation information
pre-stored in the satellite can still be used for a period of time, but the
navigation accuracy will gradually decrease.
3. User Equipment Section
The user equipment part is the GPS signal receiver. Its main function is to
capture the satellites to be tested selected according to a certain satellite
cut-off angle, and to track the operation of these satellites. When the receiver
captures the tracked satellite signal, it can measure the pseudo-distance from
the receiving antenna to the satellite and the rate of change of the distance,
and demodulate the satellite orbit parameters and other data. According to these
data, the microprocessor computer in the receiver can perform positioning
calculation according to the positioning solution method, and calculate the
longitude, latitude, altitude, speed, time and other information of the user's
geographic location. The receiver hardware and on-board software, as well as the
post-processing software package for GPS data, constitute a complete GPS user
equipment. The structure of the GPS receiver is divided into two parts: the
antenna unit and the receiving unit. The receiver generally uses two kinds of DC
power sources, inside and outside the machine. The purpose of setting the
internal power supply is to replace the external power supply without
interruption of continuous observation. The battery is automatically charged
when the external power supply is used. After shutdown, the internal battery
powers the RAM memory to prevent data loss. At present, various types of
receivers are getting smaller and smaller in size and lighter in weight, which
is convenient for field observation.
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