1. What is Global Positioning System (GPS)?

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The Global Positioning System (GPS) was developed in the United States in the 1970s. It lasted for 20 years and cost 20 billion US dollars. It was fully built in 1994 and has full-scale real-time 3D navigation and positioning in sea, land and air. A new generation of satellite navigation and positioning systems. After nearly 10 years of use in China's surveying and mapping departments, GPS has won the trust of surveying and mapping workers with its remarkable features such as all-weather, high-precision, automation and high efficiency, and has been successfully applied to geodesy, engineering survey, aerial photogrammetry, Vehicle navigation and control, crustal motion monitoring, engineering deformation monitoring, resource exploration, geodynamics and many other disciplines have brought a profound technological revolution to the field of surveying and mapping.

The Global Positioning System (GPS) is the second generation of satellite navigation systems 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 constellation of satellites. The 21+3 satellites are all near-circular orbits with an operating period of approximately 11 hours and 58 minutes, distributed over six orbital planes (four per orbital surface) with an orbital inclination of 55 degrees. The distribution of satellites allows for the observation of more than four satellites at any time anywhere in the world, and maintains a good positioning resolution geometry (DOP). This provides continuous global navigation capabilities over time.

The ground monitoring part includes four monitoring rooms, one uplink injection station and one master station. The monitoring station is equipped with a GPS user receiver, an atomic clock, sensors for collecting local meteorological data, and a computer for preliminary processing of data. The main task of the monitoring station is to obtain satellite observation data and transmit it to the master station. The main control station is located at Vandenberg Air Force Base. It exercises full control over the ground monitoring department. The main task of the main control station is to collect all the observation data of the GPS satellites of each monitoring station, and use these data to calculate the orbit and satellite clock correction value of each GPS satellite. The upstream injection station is also located at Vandenberg Air Force Base. Its mission is mainly to inject such navigation data and instructions from the master station into the satellite as each satellite runs to the sky. This injection is performed once a day for each GPS satellite and for the final injection before the satellite leaves the range of the injection station.

The GPS 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. Now it has spread to various sectors of the national economy and has begun to gradually deepen people's daily lives.

2. How to locate GPS

The GPS receiver can receive time information accurate to the nanosecond level that can be used for timing; a forecast ephemeris used to predict the approximate location of the satellite in the next few months; and a broadcast ephemeris used to calculate the satellite coordinates required for positioning. The accuracy is from a few meters to tens of meters (different satellites, changing at any time); and GPS system information, such as satellite status.

The GPS receiver measures the distance from the satellite to the receiver. It is called pseudorange because it contains the error of the receiver satellite clock and the atmospheric propagation error. The pseudorange measured by the 0A code is called the UA code pseudorange, and the precision is about 20 meters. The pseudorange measured by the P code is called the P code pseudorange, and the precision 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 recover the carrier. Strictly speaking, the carrier phase should be referred to as the carrier beat frequency phase, which is the difference between the received satellite signal carrier phase affected by the Doppler shift and the phase of the receiver local oscillator generated signal. Generally, the epoch time determined by the receiver clock is measured, and the tracking of the satellite signal is kept, and the phase change value can be recorded, but the initial phase values ​​of the receiver and the satellite oscillator at the start of observation are unknown. The phase integer of the 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 is as high as millimeter, but the premise is that the whole-circumference ambiguity is solved. Therefore, the phase observation value can be used only when the relative positioning is performed and there is a continuous observation value, and the positioning accuracy is better than the meter level. 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 the receiver based on the observation data of a receiver. It can only be measured by pseudo-range observation and can be used for rough navigation and positioning of vehicles and ships. Relative positioning (differential positioning) is a method of determining the relative position between observation points based on observation data of two or more receivers. It can use both pseudo-range observation and phase observation. Geodesy or engineering measurement should be performed. Phase observations are used for relative positioning.

The GPS observations include errors such as the clock difference of the satellite and the receiver, the atmospheric propagation delay, and the multipath effect. They are also affected by the satellite broadcast ephemeris error during the positioning calculation. Most of the common errors are caused by relative positioning. Offset or weaken, so the positioning accuracy will be greatly improved. The dual-frequency receiver can offset the main part of the ionospheric error in the atmosphere according to the observation of two frequencies. When the accuracy is high and the distance between receivers is far away (the atmosphere is obviously different) ), should use dual-frequency receiver.

In the positioning observation, if the receiver moves relative to the surface of the earth, it is called dynamic positioning, such as pseudo-single point positioning with accuracy of 30-100 meters for rough navigation and positioning of vehicles and ships, or for urban vehicle navigation and positioning. Pitch-precision differential positioning with meter-level accuracy, or centimeter-level phase differential positioning (RTK) for measuring stakeouts. Real-time differential positioning requires a data link to transmit observations from two or more stations in real time. In the positioning observation, if the receiver is stationary with respect to the surface of the earth, it is called static positioning. When performing control network observation, it is generally observed by several receivers in this way. It can play GPS to the least extent. Positioning accuracy, a receiver dedicated to this purpose is called a terrestrial receiver and is the best performing class in the receiver. At present, GPS has been able to meet the accuracy requirements of crustal deformation observation, and IGS's perennial observing stations have been able to form a millimeter-scale global coordinate framework.

3. How does the GPS system make up

The GPS system consists of three parts: the space part—the GPS satellite constellation; the ground control part—the ground monitoring system; the user equipment part—the GPS signal receiver.

GPS satellite constellation

The GPS working satellite and its constellation consist of a constellation of 21 satellites and three in-orbit spare satellites, which are recorded as (21+3) GPS constellations. Twenty-four satellites are evenly distributed in six orbital planes with an orbital inclination of 55 degrees, and each orbital plane is 60 degrees apart, that is, the ascending points of the orbits are 60 degrees apart. The elevation angles between the satellites in each orbital plane are 90 degrees apart, and the satellites in one orbital plane are 30 degrees ahead of the corresponding satellites on the adjacent orbital planes in the west.

At a height of 20,000 kilometers, when the Earth rotates for a week, the Earth orbits the Earth for two weeks, that is, the time around the Earth is 12 stars. Thus, for ground observers, the same GPS satellite will be seen 4 minutes in advance every day. The number of satellites above the horizon varies with time and location, with at least 4 visible and up to 11 visible. In the navigation and positioning with GPS signals, in order to settle the three-dimensional coordinates of the station, it is necessary to observe four GPS satellites, called positioning constellations. The geometric position distribution of these four satellites during the observation process has a certain influence on the positioning accuracy. For a certain place, even accurate point coordinates cannot be measured. This time period is called "gap segment". However, this time interval is very short, and it does not affect the all-weather, high-precision, continuous real-time, real-time, and high-speed, continuous real-time GPS satellites. The number of the GPS working satellites is basically the same as that of the test satellites.

Ground monitoring system

For navigation positioning, the GPS satellite is a dynamic known point. The position of the star is calculated from the ephemeris transmitted by the satellite, a parameter describing the motion of the satellite and its orbit. The ephemeris broadcasted by each GPS satellite is provided by the ground monitoring system. Whether the various equipment on the satellite is working properly and whether the satellite is always operating along a predetermined orbit is monitored and controlled by the ground equipment. Another important role of the ground monitoring system is to keep each satellite at the same time standard - GPS time system. This requires the ground station to monitor the time of each satellite and find the clock difference. It is then sent to the satellite by the ground injection station, which is then sent to the user equipment by the navigation message. The ground monitoring system of the GPS working satellite includes a master station, three injection stations and five monitoring stations.

GPS signal receiver

The task of the GPS signal receiver is to capture the signals of the satellites to be tested selected according to a certain satellite height cut-off angle, and track the operation of these satellites, and transform, amplify and process the received GPS signals to measure The propagation time of the GPS signal from the satellite to the receiver antenna, the navigation message sent by the GPS satellite is interpreted, and the three-dimensional position, position, and even three-dimensional speed and time of the station are calculated in real time.

In static positioning, the GPS receiver is fixed in the process of capturing and tracking GPS satellites. The receiver measures the propagation time of the GPS signal with high precision, and uses the known position of the GPS satellite in orbit to solve the position of the receiver antenna. Three-dimensional coordinates. Dynamic positioning is the use of a GPS receiver to determine the trajectory of a moving object. The moving object on which the GPS signal receiver is located is called a carrier (such as a ship in navigation, an airplane in the air, a traveling vehicle, etc.). The GPS receiver antenna on the carrier moves relative to the earth during the tracking of the GPS satellite, and the receiver uses the GPS signal to measure the state parameters (instantaneous three-dimensional position and three-dimensional velocity) of the motion carrier in real time.

The receiver hardware and in-flight software as well as post-processing software packages for GPS data form a complete GPS user equipment. The structure of the GPS receiver is divided into two parts: an antenna unit and a receiving unit. For geodetic receivers, the two units are generally divided into two separate components. When observing, the antenna unit is placed on the station, the receiving unit is placed in the appropriate place near the station, and the two are connected by cable. A whole machine. Some antenna units and receiving units are also made into a whole, and they are placed on the test site during observation.

GPS receivers typically use a battery as a power source. At the same time, two kinds of DC power sources are used inside the machine. The purpose of setting the internal battery is to not interrupt continuous observation when replacing the external battery. In the process of using the battery outside the machine, the battery inside the machine is automatically charged. After shutdown, the internal battery powers the RAM memory to prevent loss of data.

In recent years, many types of GPS geodetic receivers have been introduced in China. When various types of GPS geodesic 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. It is used for differential positioning with an accuracy of sub-meters to centimeters. At present, various types of GPS receivers are getting smaller and smaller, and the weight is getting lighter and easier to observe in the field. GPS and GLONASS compatible global navigation and positioning system receivers have been introduced.

4. How to classify GPS receivers

The navigation and positioning signal transmitted by the GPS satellite is an information resource that can be shared by countless users. For users of land, sea and space, as long as the user has a receiving device capable of receiving, tracking, transforming and measuring GPS signals, ie a GPS signal receiver. GPS positioning signals can be used for navigation and positioning measurements at any time. Depending on the purpose of use, the GPS signal receivers required by the user also vary. At present, there are dozens of factories in the world that produce GPS receivers, and there are hundreds of products. These products can be classified according to principles, uses, functions, and the like.

Classified by receiver usage

Navigation Receiver This type of receiver is mainly used for navigation of motion vectors, which can give the position and speed of the carrier in real time. Such receivers generally use C/A code pseudorange measurement, and the single-point real-time positioning accuracy is low, generally ±25mm, and ±100mm when there is SA influence. These receivers are inexpensive and widely used. Depending on the field of application, such receivers can be further divided into: on-board type - for vehicle navigation and positioning; navigation type - for navigation and positioning of ships; aviation type - for aircraft navigation and positioning. Due to the fast speed of the aircraft, the receivers used in aviation are required to adapt to high-speed motion. Spaceborne type - used for navigation and positioning of satellites. Since the speed of the satellite is as high as 7km/s or more, the requirements for the receiver are higher.

Geodetic receiver

Geodesic receivers are mainly used for precision geodesy and precision engineering measurements. High positioning accuracy. The instrument is complex in structure and expensive. The receivers of the timing receiver mainly use the high-precision time standard provided by the GPS satellite for timing, and are often used for time synchronization in the observatory and radio communication.

Classified by carrier frequency of receiver

The single-frequency receiver single-frequency receiver can only receive the L1 carrier signal and measure the carrier phase observation value for positioning. Single-frequency receivers are only suitable for precision positioning of short baselines (<15km) due to the inability to effectively eliminate ionospheric delay effects.

The dual-frequency receiver dual-frequency receiver can receive L1 and L2 carrier signals simultaneously. The use of dual-frequency is different for the ionospheric delay, which can eliminate the influence of the ionosphere on the delay of the electromagnetic wave signal, so the dual-frequency receiver can be used for precise positioning of thousands of kilometers.

Classified by receiver channel number

The GPS receiver can simultaneously receive signals of multiple GPS satellites. In order to separate the received signals of different satellites to achieve tracking, processing and measurement of satellite signals, a device having such a function is called an antenna signal channel. According to the type of channel that the receiver has, it can be divided into: Multi-channel receiver, through-channel receiver, multi-channel receiver

Classified by receiver operating principle

The code-dependent receiver code-dependent receiver obtains pseudorange observations using code correlation techniques.

The square-type receiver square-type receiver uses the square technique of the carrier signal to remove the modulated signal to recover the complete carrier signal. The phase difference between the carrier signal generated in the receiver and the received carrier signal is measured by the phase meter to determine the pseudo. Distance observation.

Hybrid receiver This kind of instrument combines the advantages of the above two receivers to obtain both code phase pseudorange and carrier phase observation.

Interferometric Receiver This type of receiver uses a GPS satellite as a radio source and uses an interferometric method to measure the distance between two stations.

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Properties and Classification

General description of material properties

Material Code

Properties

Application

Soft PZT ceramic

PZT-51

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PZT-52

low-frequency sound transducers

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PZT-5H

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PLiS-51

low-frequency vibration measurements

PMgN-51

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PSnN-5

Actuators

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PZT-41

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PZT-42

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PZT-43

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PZT-82

piezomotor

PCrN-4

 

PBaS-4

 

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BaTiO3

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