How Do I Choose the Best GPS For Geocaching?

Global Positioning System (Global Positioning System, GPS) is a high-precision radio navigation positioning system based on aerial satellites. It can provide accurate geographic location, driving speed, and precise location anywhere in the world and near-Earth space. Time information. Since its inception, GPS has attracted many users with its high accuracy, all-weather, global coverage, convenience and flexibility. GPS is not only the patron saint of cars, but also the intelligent star of logistics industry management. With the rapid development of the logistics industry, GPS plays a pivotal role, becoming the second largest consumer group after the automotive market. GPS is a new generation of satellite navigation and positioning system developed by the United States from the 1970s. It cost 20 billion US dollars for 20 years and was fully completed in 1994. ^[1] .

GPS is a new generation of satellite navigation and positioning system developed by the United States from the 1970s. It cost 20 billion US dollars for 20 years and was fully completed in 1994. . 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 all-weather, high-precision, automation, and high-efficiency features. , Vehicle navigation and control, crustal movement monitoring, engineering deformation monitoring, resource exploration, geodynamics and other disciplines, which has brought a profound technological revolution to the field of surveying and mapping.

GPS is the second-generation satellite navigation system in the United States. It was developed on the basis of the satellite meridian satellite navigation system, which adopted the successful experience of the satellite meridian system. According to the current plan, the space part of GPS uses 24 satellites with a height of about 20,200 kilometers to form a satellite constellation. The 24 satellites are all near circular orbits, with an operating period of about 11 hours and 58 minutes. They are distributed on 6 orbital planes (4 per orbital plane) with an orbital inclination of 55 degrees. The distribution of satellites allows more than 4 satellites to be observed anywhere in the world at any time, and can maintain a good geometry of the positioning accuracy. This provides continuous global navigation capabilities in time.

GPS consists of three major components: space, ground monitoring, and user equipment. GPS system has the characteristics of high accuracy, all-weather, wide use, etc. ^[1]

In the 1970s, the U.S. Department of Defense began to develop "navigation satellites" in order to provide real-time, all-weather and global navigation services to the three major areas of land, sea and air, as well as military purposes such as intelligence collection, nuclear explosion monitoring, and emergency communications. Timing and Ranging Global Positioning System ", referred to as Global Positioning System. In 1973, the United States Department of Defense began designing and testing. On February 4, 1989, the first GPS satellite was successfully launched. By the end of 1993, a practical GPS network, namely the (21 + 3GPS) constellation, had been built, and commercial operations had begun. After more than 20 years of research and experiments, which cost US $ 30 billion, by March 1994, 24 GPS satellite constellations with global coverage of up to 98% have been deployed ^[2]

GPS refers to a radio navigation positioning system that uses GPS satellites to provide three-dimensional position, three-dimensional speed, and other information in real time around the world. The predecessor of GPS was a Transit satellite positioning system developed by the U.S. military in 1958. It was officially put into use in 1964. The system works with a star network composed of 5-6 satellites and bypasses the earth at most 13 times a day. And it can not give height information, and it is not satisfactory in terms of positioning accuracy. However, the meridian satellite positioning system enables the R & D department to obtain preliminary experience in satellite positioning and verify the feasibility of positioning by satellite system, which paves the way for the development of GPS. Because satellite positioning shows great advantages in navigation and the meridian system has huge flaws in navigation of submarines and ships, the US Army, Navy, and Air Forces and the civil sector all feel the urgent need for a new satellite navigation system.

In the 1970s, the Army, Navy, and Air Force jointly developed a new generation of global positioning system. The main purpose of GPS is to provide real-time, all-weather and global navigation services for the Army, Navy, and Air Force. It is also used for intelligence collection and nuclear explosion monitoring. For some military purposes such as emergency communications, and after more than 20 years of research and experiments, which cost US $ 30 billion, by 1994, 24 GPS satellite constellations with a global coverage rate of 98% had been deployed. GPS has gone through the first and second generations, and has now been upgraded to the third generation to maintain its dominance in navigation and positioning systems. From the current point of view, GPS is the most accurate and widest-range navigation in the world. GPS.

The original GPS plan was developed under the leadership of the United States Joint Planning Office, which puts 24 satellites in three orbits forming 1,200 each other. There are 8 satellites in each orbit, and 6-9 satellites can be observed at any point on the earth. In this way, the coarse code accuracy can reach 100m, and the fine code accuracy is 10m. Due to the compression of the budget, the GPS plan had to reduce the number of satellite launches and instead distribute 18 satellites in 6 orbits of 600 each other. However, this solution cannot guarantee the reliability of the satellites. In 1988, the last modification was made: 21 working satellites and 3 spare satellites work in 6 orbits of 600 each other. This is also the working method currently used by GPS satellites ^[3]

Positioning principle

GPS positioning includes pseudo-range single-point positioning, carrier phase positioning, and real-time differential positioning.

1. Pseudo-range measurement and pseudo-point single-point positioning

Pseudo-range measurement is to measure the distance from the satellite to the receiver, that is, the distance obtained by multiplying the propagation time of the ranging code signal transmitted by the satellite to the GPS receiver multiplied by the speed of light. Pseudo-range method for single-point positioning is to use a GPS receiver to measure the pseudo-ranges with more than 4 GPS satellites at a certain time, and the instantaneous coordinates of the satellites obtained from the satellite navigation message. Three-dimensional coordinates in a coordinate system.

2. Carrier phase measurement and carrier phase positioning

Carrier phase measurement measures the phase delay between the GPS satellite carrier signal and the receiver antenna. The ranging satellite code and navigation message are modulated on the GPS satellite carrier. After receiving the satellite signal, the receiver first removes the ranging code and satellite message on the carrier wave, and obtains the carrier again, which is called reconstruction carrier. The GPS receiver compares the satellite reconstructed carrier with the local oscillator signal generated by the oscillator in the receiver through a phase meter to obtain a phase difference.

3 Real-time differential positioning

The principle of GPS real-time differential positioning is to place a GPS receiver (referred to as a base station) on an existing precise geocentric coordinate point, use known geocentric coordinates and ephemeris to calculate the correction value of GPS observations, and communicate by radio The device (called the data link) sends the corrections to a moving GPS receiver (called a rover). The rover uses correction values to correct its own GPS observations to eliminate the above errors, thereby improving real-time positioning accuracy. There are many kinds of GPS dynamic difference methods, including position difference, pseudo-range difference (RTD), carrier phase real-time difference (RTK), and wide area difference.

The global positioning system consists of the following three parts: the space part (GPS satellite), the ground monitoring part, and the user part. GPS satellites can continuously broadcast ranging signals and navigation messages to users for navigation and positioning, and receive various information and commands from ground monitoring systems to maintain the normal operation of the system. The main function of the ground monitoring system is to track GPS satellites, measure distances to them, determine the satellite's orbit and satellite clock corrections, make predictions, then compile navigation messages according to the prescribed format and send them to the satellites through injection stations. The ground monitoring system can also issue various instructions to the satellite through the injection station, adjust the satellite's orbit and clock readings, repair faults or enable spare parts. The user uses a GPS receiver to measure the distance from the receiver to the GPS satellite, and obtains his own three-dimensional position, three-dimensional motion speed, and clock offset based on information such as the position of the satellite in space at the instant of observation given by the satellite ephemeris. . At present, the United States is working to further improve the functions of the entire system, such as determining satellite orbits through mutual tracking between satellites, in order to reduce dependence on ground monitoring systems and enhance system autonomy.

GPS is currently the most successful satellite positioning system and is known as a milestone in human positioning technology. To sum up, the system has the following characteristics:

(1) Global, continuous navigation and positioning capabilities around the clock. GPS can provide continuous, round-the-clock navigation and positioning capabilities for all types of users in any place or near-Earth space. Users do not need to transmit signals, so they can meet multi-user use.

(2) Real-time navigation, high positioning accuracy and short observation time. When using GPS positioning, position data can be obtained several times within 1 second. This near real-time navigation capability is of great significance to highly dynamic users, and can provide users with continuous 3D position, 3D speed, and accurate time information. At present, the real-time positioning accuracy using C / A code can reach 20-50m, the speed accuracy is 0.1m / s, the special processing can reach 0.005m / s, and the relative positioning accuracy can reach millimeter level.

With the continuous improvement of the GPS system and the continuous updating of the software, the relative static positioning within 20km currently only takes 15-20min. When performing fast static relative positioning measurement, when each rover is within 15km from the reference station, the rover observation time It only takes 1-2min, and then can be positioned at any time, and each station observation only takes a few seconds.

(3) The station does not need to see through: GPS survey only requires the sky above the station to be open, and does not require the stations to communicate with each other, so it can save a lot of bid-making costs (general bid-making costs account for 30%, 50% of the total funding) ). Because there is no need to look through the points, the position of the points can be sparse and dense as required, which makes the selection of the point work very flexible, and it can also save the measurement of the calculation points and transition points in the classic measurement.

(4) Globally uniform 3D geocentric coordinates can be provided: GPS measurement can accurately determine the station's plane position and geodetic elevation at the same time. At present, the GPS level can meet the accuracy of fourth-level level measurement. In addition, the GPS positioning is calculated in the globally unified WGS-84 coordinate system. Just now, the measurement results of different locations around the world are interrelated.

(5) The instrument is easy to operate: With the continuous improvement of GPS receivers, the degree of automation of GPS measurement is getting higher and higher. In the observation towel, the surveyor only needs to set up the instrument, connect the cable, measure the antenna height, and monitor the working status of the instrument. Other observation tasks, such as satellite capture, tracking observation and recording, are automatically completed by the instrument. You only need to turn off the power and close the receiver to complete the field data collection task.

If a long-term continuous observation is required at a station, the collected data can also be transmitted to the data processing center through data communication to achieve fully automated data collection and processing. In addition, the size of the receiver is getting smaller and smaller, and the corresponding weight is getting lighter and lighter, which greatly reduces the labor intensity of the measurement author and makes field work easier.

(6) Strong anti-interference ability and good confidentiality: GPS uses spread-spectrum technology and pseudo-code technology. Users only need to receive GPS signals, they will not transmit signals themselves, and they will not be interfered by other external signal sources.

(7) Multi-function and wide application: GPS is a military and civilian dual-use system, and its application range is very wide. Specific application examples include: car navigation and traffic management, line inspection vehicle management, road engineering, personal positioning, and navigators, etc. ^[7]

The main function of the GPS receiver is to receive GPS satellite signals and process the signal amplification, frequency conversion and phase-locking, measure the propagation time of the GPS signal from the satellite to the receiver antenna, interpret the navigation message, and calculate the GPS antenna location (3D coordinates) in real time And running speed. GPS receiver is a passive radio positioning device, which is divided into navigation receiver, geodesic receiver, timing receiver and attitude measurement receiver according to different purposes; it can be divided into multi-channel receivers according to the number of receiver channels. , Sequential channel receiver and multiplexed channel receiver.

GPS receiver is mainly composed of GPS receiving antenna, GPS receiver host and power supply.

1. GPS receiver antenna

The GPS receiver antenna consists of an antenna unit and a preamplifier. The function of the antenna is to convert the weak electromagnetic wave energy of the GPS satellite signal into the corresponding current, and amplify the received GPS signal through a preamplifier.

2. GPS receiver host

The receiver host consists of a frequency converter, a signal channel, a microprocessor, a memory, and a display. The main task of the frequency converter is to make the received L-band radio frequency signal into a low frequency signal. The signal channel is a circuit combining software and hardware, and is the core part of the receiver. Its role is to search, tow, and track satellites, despread and demodulate broadcast message signals into broadcast messages, perform pseudo-range measurements, carrier phase measurements, and more Doppler frequency shift measurement. The memory is used to store satellite ephemeris, satellite almanac, code phase pseudorange observations, carrier phase observations, and Doppler frequency shifts collected by the receiver. The microprocessor is the core of the GPS receiver's work. The GPS receiver's work is carried out under the unified cooperation of microcomputer instructions. The GPS receiver has a liquid crystal display screen to provide the GPS receiver's working information, and is equipped with a control keyboard for users to control the receiver's work.

3 GPS receiver power

There are two types of GPS receiver power supply. One is internal power supply, generally using lithium battery, which mainly supplies power to RAM memory; the other is external power supply, commonly used rechargeable 12V DC nickel-cadmium battery pack ^[6]

The global positioning system generally refers to GPS. Although Russia, Europe, and China have also established their own global positioning systems, as far as technology is concerned, the GPS technology established in the United States is in an advanced position. The application of the positioning system referred to in this section is mainly based on the application of GPS. We believe that in the near future, other positioning systems will also play their role and can also achieve applications in the corresponding fields.

(1) Application in precision engineering, measurement and deformation monitoring

The control network established by applying GPS satellite positioning technology is called GPS network. GPS network is divided into two categories, one is global or national high-precision GPS network; the other is regional GPS network. The scientific research task of geodesy is to study the shape of the earth and its changes over time, and use the high-precision GPS network with global coverage to establish a high-precision dynamic coordinate frame. Regional GPS network refers to national C, D, E class GPS networks or engineering GPS networks specially designed for engineering projects.

(2) Application in transportation system

The positioning of the current position and the positioning of the target are two key technologies of the ground vehicle navigation system. The former requires GPS to obtain the point basis, while the latter focuses on digital maps to determine the point location, which is actually a map-related analysis problem.

With the expansion of the scale of urban construction in our country, more and more vehicles, traffic management and reasonable dispatch, the command and safety management of police vehicles have become an important issue for public security and transportation systems. The advent of GPS navigation and positioning technology provides specific real-time positioning capabilities for the navigation and positioning of vehicles, ships, and other vehicles. Mainly used in public security and transportation systems are: command management system combining vehicle GPS positioning and radio communication system; command management system using GPS differential technology. GPS vehicle navigation has a wide range of applications, such as transportation line navigation, emergency vehicle navigation, and vehicle dispatch. Generally, intelligent vehicles and highway systems can be divided into 4 types according to their functions: automatic systems, fleet management systems, Consulting navigation system and census system. The fleet management system can operate multiple vehicles and has a center. The consulting navigation system integrates the automatic system and the fleet management system. The census system usually consists of automatic vehicle loading cameras or digital cameras, which can be used for Get road information of time and point characteristics,

(3) Application in geodynamics

Use GPS to monitor global and regional plate movements, monitor regional crustal movement, cause to the earth and power engine

System research. Investigate underground fault activity patterns and changes in stress fields, and estimate and predict seismic danger values. In order to carry out crustal deformation monitoring in China, the Nine-Five Years major scientific engineering project Zhongtong crustal movement monitoring network project undertaken by the Seismological Bureau, the General Staff Survey and Mapping Bureau, the National Surveying and Mapping Bureau, and the Chinese Academy of Sciences was completed in 2000. Wuhan University of Surveying and Mapping Technology used two phases of GPS monitoring data from western Yunnan and Yunnan to invert the low fault activity pattern of the Honghe fault zone, and made a relatively accurate mid-term forecast for the 1996 Lijiang earthquake in Yunnan. -6 km. The magnitude is completely accurate, revealing the possibility of using GPS monitoring data for medium-term earthquake prediction.

(4) Application in meteorology

The use of GPS theory and technology to remotely measure the earth s atmosphere and conduct meteorological theories and methods is called GPS meteorology (GPSIMET). The study of GPS meteorology began in the late 1910s. After the test results, the application of GPS in meteorology has gradually begun in other countries such as Japan. GPS / MET detection data has the characteristics of wide coverage (global), high vertical resolution, high accuracy and high long-term stability.

The earth s atmosphere is equivalent to a lens. The transmission path of electromagnetic wave signals (such as carrier waves) in the atmosphere will be tortuous due to atmospheric refraction. The greater the gas density, the stronger the signal refraction, the slower the wave transmission, and the low-orbit GPS receiver receives. The refracted signal can be used to calculate the composite signal refractive index. After proper conversion, the vertical structure of the refraction path of electromagnetic waves in each layer of the atmosphere can be reconstructed. The radio uses a microwave converter to transmit the signal from the GPS satellite to the receiver. It needs to pass through a certain medium during the propagation. The nature of the medium can be investigated by studying the nature of the signal transmission. There are three kinds of GPS signal refraction sources in the atmosphere, namely dry matter, water vapor and ionosphere material.

(5) Military applications

The military can be used for coordinated operations, missile guidance, search and rescue personnel field positioning. In terms of cooperative operations, GPS can provide various levels of command systems with the time and place of various targets and events. In terms of missile guidance, about 70% of the US-Iraq war used GPS-assisted guidance, so that Tomahawk cruise missiles accurately hit the daily standard of a small house from 1,600 km. In terms of field positioning of search and rescue personnel, in the vast desert, there are no signs, and navigation satellites are mainly used for positioning to know where they are.

(6) Application in agriculture

The GPS system is extremely important in the implementation of fine agriculture: it can give accurate positioning to various information on farmland, including navigation of agricultural machinery and vehicles, flat land, precise seeding, spraying, fertilizer application, data management, and crop vitality detection and variable control. The more mature and more profitable applications in precision agriculture include: autonomous driving, fertilization, spraying, and seeding. GPS surveys soil nutrient distribution, detects crop yields, and farmland management is much more efficient and accurate than human management. It is equipped with a monitor and GPS receiver on a combine harvester to form a crop yield monitoring system. Comprehensive analysis of can find out the relevant factors that affect crop yields, so as to carry out specific field fertilization and other management tasks. Using the difference in the wavelength of light reflected from brown soil and green crop leaves

Recognizes soil, crops and weeds. The difference in reflected light waves is used to identify crop leaves that are deficient in nutrition or infected by pests. There are two methods for applying herbicides: Collecting weed information in the field by using weed detection sensors, and controlling the amount of herbicide sprayed by the control system of variable spraying equipment; Weed patches in the field are drawn in advance by the weed sensor The distribution map, the prescription is output from the electronic map, and implemented by variable spraying machinery.

(7) Application in wildlife protection

The U.S. Bureau of Land Development developed a GPS-based Geographic Information System (GIS) in the mid-1910s. The U.S. Land Bureau collects information about wildlife resources quarterly, such as ecological conditions and distribution of wildlife habitats, Number of wild animals, etc. Thousands of wild horses and bison live in the western United States. The main purpose of establishing a wildlife management system is to use voice input instead of the old manual input method; to display the current and historical flight routes, and to accurately locate the distribution of wildlife. In actual work, the system is installed on an airplane or vehicle, looking for wild animals in the air, recording relevant information by means of voice recording, and GPS point data is automatically stored in the database. If necessary, a comparative analysis of different periods can be carried out according to historical routes, and the current and historical data can be used for dynamic monitoring of wild animal activity areas.

(8) Application in emergencies

The length of the response time to an emergency event affects the degree of loss of the event to some extent. GPS and GIS play a decisive role in the rapid response of emergencies such as medical, fire, and traffic accidents. In general, the establishment of an emergency rescue system can roughly include: vehicle parts. , With GPS equipment, and processed by appropriate software to obtain geographic coordinates; communication system. A dual-channel cellular communication network is used. After the incident, the driver activates the help-seeking system, requesting the type of help (alarm, medical, etc.) and the degree of injury; the processing center. Can receive user signals, with the help of GIS, quickly query and locate the incident, and determine the corresponding first-aid measures according to the type of help information provided by the user. GIS includes digital map and attribute information. It can query and locate the location of the emergency on the digital map and the dispatcher's choice of the shortest route. GPS can make the information such as the location of emergency vehicles timely feedback to the main control Center for scheduling.

(9) Application in tourism

With the improvement of people's quality of life, GPS services are gradually applied to private tourism and field trips. Travel to scenic areas, field trips to virgin forests, snow mountain canyons or desert areas. GPS installed in the car The receiver will take full advantage of its global positioning function to become the driver's most faithful guide. While driving the scenery, the rider can know the location of the vehicle, the speed and direction of walking at any time, so as to avoid getting lost. Even if trouble occurs on the way, the GPS monitoring service center will promptly provide instructions, connect with the nearest rescue agency, and take active action. Through the friendly remote service provided by the GPS monitoring center, even if the rider travels thousands of miles, he still feels at home. Providing travel route planning is an important auxiliary function of car navigation systems, including automatic route planning and route design. Automatic route planning is designed by computer software to automatically calculate the optimal driving route, including the fastest route, the simplest route, and the route that passes the least number of times on highway sections. The manual route design is to automatically create a route library by designing the starting point, ending point, and passing point according to the destination of the driver.

GPS applications also include aerial photogrammetry, line surveys and tunnel penetration surveys, topographic, cadastral and real estate surveys, marine surveying, engineering construction surveys, bridge construction control network establishment, offshore exploration platform settlement monitoring, bridge dynamic real-time deformation monitoring, Real-time deformation monitoring of high-rise buildings. More efficient GPS services will be applied in various areas of production and life ^[12] .

The application of GPS in the field of logistics is mainly reflected in the following aspects:

1. Cargo tracking GPS computer information management system can collect the real-time dynamic information of trains, locomotives, vehicles, containers, and cargoes transported through GPS and computer networks to achieve tracking and management of land and water cargoes. As long as you know the model, car number or ship number of the truck, you can immediately find the truck or ship from the railway network or waterway network, know their location, the distance to the transportation destination, and information about all shipments. The use of this technology can greatly improve the accuracy and transparency of operations and provide high-quality services to cargo owners.

2. Combined with geographic information system (GIS) to solve logistics distribution. Logistics includes order management, transportation, warehousing, loading and unloading, delivery, customs declaration, return processing, information services and value-added services. The whole process control is the core issue of logistics management. Suppliers must comprehensively, accurately and dynamically grasp the flow of products scattered in various transit warehouses, dealers, retailers, and various transportation links such as cars, trains, aircraft, and ships nationwide, and formulate production and sales plans accordingly. Adjust market strategies in a timely manner. Therefore, for large suppliers, it is impossible to establish an effective distribution network without the entire process of logistics management; for large chain retailers, it is impossible to establish a supply and distribution system without the entire process of logistics management; The three-party logistics service providers and warehouse logistics centers cannot obtain customers' logistics business without the whole-process logistics management service. For ordinary users, there is no fast, accurate, safe, and reliable logistics distribution service. Online purchase is almost unimaginable. The process of logistics distribution is mainly the process of transferring the spatial location of goods. In the process of logistics distribution, it involves the business links of transportation, warehousing, loading and unloading, and delivery of goods. The issues involved in each link, such as the choice of transportation routes and warehouse locations Effective management and decision-making analysis such as the selection of warehouse capacity, reasonable loading and unloading strategies, transportation vehicle scheduling, and delivery route selection will help logistics and distribution companies to effectively use existing resources, reduce consumption, and improve efficiency. In fact, a careful analysis of the problems in each of the above links reveals that the above problems involve geographical elements and geographical distribution. GIS technology can be applied to all areas related to geographical distribution. GPS / GIS technology is an indispensable part of the entire process logistics management ^[13] .