GPS Positioning Principle

The GPS system consists of three parts: the spatial part-GPS satellite constellation;
Ground control part-ground monitoring system;
User device part-GPS signal receiver.

GPS satellite constellation
The GPS working satellite and its constellation are composed of 21 working satellites and 3 in-orbit standby satellites, which are recorded as (21 + 3) GPS constellations. 24 satellites are evenly distributed in six orbital planes, with an orbital inclination of 55 degrees. Each orbital plane is 60 degrees apart, that is, the ascending point of the orbit is 60 degrees different from each other. The ascending Angle Deviation between satellites in each orbital plane is 90 degrees different, and the satellite in one orbital plane is 30 degrees ahead of the corresponding satellite in the adjacent orbital plane in the West.
GPS satellites at an altitude of 20 thousand kilometers run around the Earth for two weeks when the earth rotates for a week, that is, when the week around the Earth is 12 stars. In this way, the ground observer will see the same GPS satellite four minutes in advance every day. The number of satellites located above the horizon varies with time and location, with at least four and up to 11 satellites visible. When GPS signal is used for navigation and positioning, four GPS satellites must be observed to settle the three-dimensional coordinates of the station.

Ground Monitoring System
GPS satellite is a known dynamic point for navigation and positioning. The location of a star is calculated based on the satellite-defined satellite motion and orbit parameters. The calendars broadcast by each GPS satellite are provided by the ground monitoring system. The monitoring and control of various satellite equipment and the operation of the satellite along the scheduled orbit must be carried out by the ground equipment. Another important role of the ground monitoring system is to keep each satellite in the same time standard-GPS time system. This requires the ground station to monitor the time of each satellite and find the clock difference. The satellite is then sent from the ground injection station to the satellite, and then from the navigation message to the user's device. The ground monitoring system of GPS working satellite includes one master station, three injection stations, and five monitoring stations.

GPS signal receiver
the task of GPS signal receiver is to capture the signals of the satellite to be tested selected based on the cutoff angle of a certain satellite height and track the operation of these satellites, transform, zoom in, and process the received GPS signals to measure the transmission time of GPS signals from the satellite to the receiver antenna, and extract the navigation messages sent by the GPS satellite, calculate the 3D location, location, and even 3D speed and time of the test site in real time.
in static positioning, GPS receivers are fixed during GPS satellite capturing and tracking, and GPS signal propagation time is measured with high precision. uses the known location of the GPS satellite in orbit to calculate the three-dimensional coordinates of the receiver antenna . Dynamic Positioning uses a GPS receiver to determine the running trajectory of a moving object. The moving object in which the GPS signal receiver is located is called a carrier (such as a ship sailing, an airplane in the air, or a walking vehicle ). The GPS receiver antenna on the carrier moves relative to the Earth while tracking the GPS satellite. The receiver uses the GPS signal to measure the state parameters of the motion carrier in real time (instantaneous three-dimensional position and three-dimensional speed ).
the receiver hardware, built-in software, and post-processing software package of GPS data constitute a complete GPS User device. The structure of GPS receivers is divided into two parts: antenna unit and receiving unit. For a ground-based receiver, the two units are generally divided into two independent components. During observation, the antenna unit is placed on the test station, and the receiving unit is placed near the test station, use a cable to connect the two into a whole machine. Some also make the antenna unit and the receiving unit into a whole, and place it on the test site during observation.