Global Positioning System

1. What is a global positioning system (GPS)
The Global Positioning System (GlobalPosiTIoningSystem-GPS) was developed by the United States since the 1970s. It took 20 years and cost 20 billion US dollars. It was fully completed in 1994 and has the ability to carry out all-round real-time 3D navigation and positioning in sea, land and air. A new generation of satellite navigation and positioning system. The use of Chinese surveying and mapping departments in the past 10 years has shown that GPS has won the trust of the majority of surveying and mapping workers with its all-weather, high-precision, automation, and high-efficiency features. It has been successfully used in geodesy, engineering survey, aerial photogrammetry, Various disciplines such as vehicle navigation and control, crustal movement monitoring, engineering deformation monitoring, resource survey, and geodynamics have brought a profound technological revolution to the field of surveying and mapping.
Global Positioning System (GlobalPosiTIoningSystem, abbreviated GPS) is the second generation satellite navigation system in the United States. It was developed on the basis of the meridian satellite navigation system, which adopted the successful experience of the meridian system. Like the meridian system, the global positioning system consists of three parts: the space part, the ground monitoring part and the user receiver.
According to the current plan, the space part of the global positioning system uses 24 satellites with a height of about 20,200 kilometers to form a satellite constellation. The 21 + 3 satellites are all near circular orbits, with an operating period of about 11 hours and 58 minutes, distributed on six orbital planes (four per orbital plane), with an orbital inclination of 55 degrees. The distribution of satellites makes it possible to observe more than four satellites at any time anywhere in the world, and can maintain a geometric figure (DOP) with good positioning and resolution accuracy. This provides continuous global navigation capability in time.
The ground monitoring part includes four monitoring rooms, an upstream injection station and a main control station. The monitoring station is equipped with a GPS user receiver, an atomic clock, sensors to collect local meteorological data, and a computer for preliminary data processing. The main task of the monitoring station is to obtain satellite observation data and transmit these data to the main control station. The main control station is located at Vandenberg Air Force Base. It exercises overall control of the ground monitoring department. The main task of the main control station is to collect all the observation data of the GPS satellites from each monitoring station, and use these data to calculate the correction value of the orbit and satellite clock of each GPS satellite. The upward injection station is also located at Vandenberg Air Force Base. Its mission is mainly to inject such navigation data and commands from the main control station into the satellites when each satellite is in the sky. This injection is performed once a day for each GPS satellite, and the final injection is made before the satellite leaves the injection station.
The global positioning system has the characteristics of good performance, high precision and wide application, and is the best navigation and positioning system to date. With the continuous improvement of the global positioning system, the continuous improvement of hardware and software, the application field is constantly expanding. At present, it has spread to various sectors of the national economy and has gradually penetrated into people's daily life.

2. The GPS system includes three parts:
Space part-GPS satellite constellation; ground control part-ground monitoring system; user equipment part-GPS signal receiver.
GPS satellite constellation; GPS working satellite and its constellation are composed of 21 working satellites and 3 spare satellites in orbit, which is denoted as (21 + 3) GPS constellation. The 24 satellites are evenly distributed in 6 orbital planes, the orbital inclination angle is 55 degrees, and the orbital planes are 60 degrees apart, that is, the ascension points of the orbits differ by 60 degrees. The ascending angle distances between the satellites in each orbital plane differ by 90 degrees, and the satellites in one orbital plane are 30 degrees ahead of the corresponding satellites in the adjacent orbital plane in the west. In GPS satellites at a height of 20,000 kilometers, when the earth rotates for a star for one week, they orbit the earth for two weeks, that is, the time for a circle around the earth is 12 stars. In this way, the ground observer will see the same GPS satellite 4 minutes in advance every day. The number of satellites above the horizon varies with time and location, with a minimum of 4 satellites and a maximum of 11 satellites. When navigating and positioning with GPS signals, in order to settle the three-dimensional coordinates of the station, four GPS satellites must be observed, called positioning constellations. The geometric position distribution of the four satellites during the observation process has a certain influence on the positioning accuracy. For a certain place at a certain time, even accurate point coordinates cannot be measured, this time period is called "gap period". But this time interval is very short, and it does not affect the all-weather, high-precision, continuous real-time, most of the world's places. The number of GPS working satellites is basically the same as the test satellites. Ground monitoring system: For navigation and positioning, the GPS satellite is a dynamically known point. The position of the star is calculated based on the ephemeris emitted by the satellite-a parameter describing the movement of the satellite and its orbit. The ephemeris broadcast by each GPS satellite is provided by the ground monitoring system. Whether the various equipment on the satellite is working normally, and whether the satellite has been running along the predetermined orbit, must be monitored and controlled 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 to find the clock difference. It is then sent to the satellite by the ground injection station, and the satellite is then sent to the user equipment by the navigation message. The ground monitoring system of GPS working satellites includes a main control station, three injection stations and five monitoring stations.
GPS signal receiver: The task of the GPS signal receiver is to be able to capture the signals of the satellites under test selected according to a certain satellite height cutoff angle, and track the operation of these satellites, transform, amplify and convert the received GPS signals Processing in order to measure the propagation time of the GPS signal from the satellite to the receiver antenna, interpret the navigation message sent by the GPS satellite, and calculate the three-dimensional position, position, even three-dimensional velocity and time of the station in real time. In static positioning, the GPS receiver is fixed during the process of capturing and tracking GPS satellites. The receiver measures the propagation time of GPS signals with high precision, and uses the known position of the GPS satellites in orbit to solve for the location of the receiver antenna. Three-dimensional coordinates. In dynamic positioning, a GPS receiver is used to measure the running trajectory of a moving object. The moving object where the GPS signal receiver is located is called a carrier (such as a ship in navigation, an airplane in the air, a walking vehicle, etc.). The GPS receiver antenna on the carrier moves relative to the earth in the process of tracking the GPS satellites. The receiver uses GPS signals to measure the state parameters (the instantaneous three-dimensional position and three-dimensional velocity) of the moving carrier in real time.
The receiver hardware and on-board software and the post-processing software package of 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. For geodetic receivers, the two units are generally divided into two independent components. During observation, the antenna unit is placed on the measuring station, the receiving unit is placed in an appropriate place near the measuring station, and the two are connected by a cable A whole machine. Some also make the antenna unit and receiving unit as a whole, and place it on the measuring station during observation.
GPS receivers generally use batteries as power sources. At the same time, two DC power sources are used inside and outside the machine. The purpose of setting the internal battery is to continue observation without interruption when replacing the external battery. In the process of using the external battery, the internal battery is automatically charged. After shutdown, the internal battery powers the RAM memory to prevent data loss.
In recent years, many types of GPS geodesic receivers have been introduced in China. When various types of GPS geodetic receivers are used for precise relative positioning, the accuracy of the dual-frequency receiver can reach 5mm + 1PPM.D, and the accuracy of the single-frequency receiver can reach 10mm + 2PPM.D within a certain distance. For differential positioning, the accuracy can reach sub-meter level to centimeter level. At present, various types of GPS receivers are getting smaller and lighter in weight, making them easier to observe in the field. GPS and GLONASS compatible GNSS receivers have been released.

3. GPS positioning
The GPS receiver can receive accurate nanosecond time information that can be used for timing; the forecast ephemeris for forecasting the rough position of the satellite in the next few months; the broadcast ephemeris for calculating the satellite coordinates required for positioning , The accuracy is a few meters to tens of meters (different satellites, changing at any time); and GPS system information, such as satellite status.
The GPS receiver can measure the code to get the distance from the satellite to the receiver. Because it contains the error of the receiver's satellite clock and the atmospheric propagation error, it is called pseudorange. The pseudo range measured for the 0A code is called the UA code pseudo range, and the accuracy is about 20 meters. The pseudo range measured for the P code is called the P code pseudo range, and the accuracy is about 2 meters.
The GPS receiver decodes the received satellite signal or uses other techniques to remove the information modulated on the carrier, and then the carrier can be restored. Strictly speaking, the carrier phase should be called the carrier beat frequency phase, which is the difference between the received satellite signal carrier phase affected by the Doppler shift and the signal phase generated by the receiver's local oscillation. Generally measured at the epoch time determined by the receiver clock and keeping track of the satellite signal, the phase change value can be recorded, but the initial value of the phase of the receiver and the satellite oscillator at the beginning of the observation is unknown. The phase integer of the initial epoch is also unknown, that is, the ambiguity of the whole week can only be solved as a parameter in data processing. The accuracy of the phase observation value is as high as millimeters, but the premise is to solve the ambiguity of the entire circumference, so the phase observation value can only be used when there is a relative observation and a continuous observation value, and the positioning accuracy that is better than the meter level is only Phase observations can be used.
According to the positioning method, GPS positioning is divided into single-point positioning and relative positioning (differential positioning). Single-point positioning is a way to determine the position of a receiver based on the observation data of a receiver. It can only use pseudorange observations and can be used for rough navigation and positioning of vehicles and ships. Relative positioning (differential positioning) is a method to determine the relative position between observation points based on the observation data of more than two receivers. It can use either pseudorange observations or phase observations. Geodetic or engineering measurements should be used. Use phase observations for relative positioning.
GPS observations include errors such as satellite and receiver clock differences, atmospheric propagation delay, multipath effects, etc., and are also affected by satellite broadcast ephemeris errors during positioning calculations. Most common errors are caused by relative positioning Cancel or weaken, so the positioning accuracy will be greatly improved. The dual-frequency receiver can cancel the main part of the ionospheric error in the atmosphere based on the observations of the two frequencies. When the accuracy requirements are high and the distance between the receivers is long (the atmosphere has a significant difference ), Dual-frequency receivers should be used.
In positioning observation, if the receiver moves relative to the earth's surface, it is called dynamic positioning. For example, it is used for pseudo-range single-point positioning with an accuracy of 30 to 100 meters for rough navigation and positioning of vehicles and ships, or for navigation and positioning of urban vehicles. Pseudorange differential positioning with meter-level accuracy, or centimeter-level phase differential positioning (RTK) for measuring lofts, etc. Real-time differential positioning requires a data link to transmit observation data from two or more stations in real time for calculation. In positioning observation, if the receiver is stationary with respect to the surface of the earth, it is called static positioning. In the observation of the control network, it is generally observed by several receivers at the same time in this way. It can maximize the use of GPS. Positioning accuracy, receivers dedicated to this purpose are called earth-type receivers, and are the best performers among the receivers.
At present, GPS has been able to meet the accuracy requirements of crustal deformation observation, and IGS's perennial observation stations can already form a millimeter-level global coordinate frame.

4. What is the conventional GPS measurement method of RTK technology, such as static, fast static, and dynamic measurement all need to be solved afterwards to obtain centimeter-level accuracy, and RTK is a measurement method that can obtain centimeter-level positioning accuracy in the field in real time. Carrier phase dynamic real-time differential (Real-TImekinemaTIc) method is a major milestone in GPS application. Its emergence has brought new dawn for engineering lofting, topographic mapping, and various control measurements, which has greatly improved the efficiency of field operations.
Carrier phase observations must be used for high-precision GPS measurement. RTK positioning technology is a real-time dynamic positioning technology based on carrier phase observations. It can provide real-time 3D positioning results of the measurement station in a specified coordinate system and achieve centimeter-level accuracy. In RTK operation mode, the base station transmits its observation value and station coordinate information to the rover through the data link. The rover not only receives the data from the reference station through the data link, but also collects the GPS observation data, and composes the differential observation value in the system for real-time processing, and at the same time gives the centimeter-level positioning result, which takes less than one second. The rover is in a static state or in a moving state; it can be initialized at a fixed point before entering a dynamic operation, or it can be directly turned on under dynamic conditions, and the search and solution of weekly ambiguity can be completed in a dynamic environment. After the knowledge solution is fixed throughout the weekend, each epoch can be processed in real time. As long as the tracking of the phase observations of more than four satellites and the necessary geometry can be maintained, the rover can always give centimeter-level positioning results.
The key of RTK technology is data processing technology and data transmission technology. During RTK positioning, the base station receiver is required to transmit the observation data (pseudorange observation value, phase observation value) and known data to the rover receiver in real time. Large, generally require a baud rate of 9600, which is not difficult to achieve on the radio.

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