Comprehensive GPS Knowledge
With continuous improvements to the Global Positioning System and ongoing enhancements in hardware and software, its application areas continue to expand. It now spans various sectors of the national economy and is gradually becoming integrated into people’s daily lives. The characteristics of the GPS system are: 1. Global, all-weather operation: It provides users with continuous, real-time three-dimensional position, three-dimensional velocity, and precise time. It is not affected by weather conditions. 2. High positioning accuracy: Single-unit positioning accuracy is better than 10 meters, and with differential positioning, accuracy can reach the centimeter or even millimeter level. 3. Multiple functions, wide applications: As understanding of GPS deepens, its use has expanded beyond measurement, navigation, speed measurement, and timing, continually broadening its application fields.
Before the advent of satellite positioning systems, long-range navigation and positioning primarily relied on radio navigation systems. 1. Radio Navigation Systems ● LORAN-C: Operates at 100KHZ, consisting of three ground-based navigation stations, covering a working area of 2000KM with general accuracy of 200-300M. ● Omega: Operates at tens of kilohertz. Comprises eight ground-based navigation stations and can cover the entire globe. Accuracy is a few miles. ● Doppler System: Utilizes the Doppler shift principle. By measuring frequency shifts, it calculates moving object parameters (ground speed and drift angle) to deduce the aircraft’s position. It is a self-contained dead reckoning system. Errors accumulate with increased travel distance. Disadvantages: Limited coverage area; radio wave propagation is affected by the atmosphere; positioning accuracy is not high. 2. Satellite Positioning Systems.
The earliest satellite positioning system was the U.S. Transit system, developed in 1958 and officially put into use in 1964. Due to the small number of satellites (5-6), low orbital altitude (average 1000KM), and long intervals between satellite observations from ground stations (average 1.5h), it could not provide continuous real-time three-dimensional navigation and had relatively low accuracy. To meet the urgent needs of both military and civilian sectors for continuous, real-time, and three-dimensional navigation, the U.S. Department of Defense formulated the GPS plan in 1973. 3. GPS Development History The GPS implementation plan is divided into three phases: The first phase was the conceptual design and preliminary design stage. From 1973 to 1979, four experimental satellites were launched. Ground receivers were developed, and a ground tracking network was established. The second phase was the full-scale development and testing stage. From 1979 to 1984, seven more experimental satellites were launched, and various types of receivers were developed. Experiments showed that GPS positioning accuracy far exceeded design standards. The third phase was the operational networking stage.
On February 4, 1989, the first operational GPS satellite was successfully launched, marking the start of the GPS system’s engineering construction phase. By the end of 1993, the operational GPS constellation (21+3 satellites) had been completed, and failed satellites will be replaced according to the plan in the future. GPS Principles 1. Composition of the GPS System GPS consists of three independent parts: ● Space Segment: 21 operational satellites and 3 backup satellites. ● Ground Support System: 1 master control station, 3 upload stations, and 5 monitoring stations. ● User Equipment Segment: Receives signals transmitted by GPS satellites to obtain necessary navigation and positioning information. Data is processed to complete navigation and positioning tasks.
The hardware of a GPS receiver generally consists of a main unit, an antenna, and a power supply. 2. GPS Positioning Principle The basic principle of GPS positioning is to use the instantaneous position of high-speed-moving satellites as known reference data and apply the space-based resection method to determine the location of the target point. As shown in the diagram, assume that at time t, a GPS receiver is placed at the ground target point. It can measure the time △t it takes for the GPS signal to reach the receiver. Combined with other data such as satellite ephemeris received by the receiver, the following four equations can be determined: In these four equations, the coordinates x, y, z of the target point and Vto are unknown parameters, where di = c△ti (i=1, 2, 3, 4). di (i=1, 2, 3, 4) are the distances from satellite 1, 2, 3, and 4 to the receiver, respectively. △ti (i=1, 2, 3, 4) are the times taken for the signals from satellite 1, 2, 3, and 4 to reach the receiver. c is the propagation speed of the GPS signal (i.e., the speed of light). The meanings of the parameters in the four equations are as follows: x, y, z are the spatial rectangular coordinates of the target point. xi, yi, zi (i=1, 2, 3, 4) are the spatial rectangular coordinates of satellite 1, 2, 3, and 4 at time t, which can be obtained from the satellite navigation message. Vti (i=1, 2, 3, 4) are the clock errors of the satellite clocks for satellites 1, 2, 3, and 4, provided by the satellite ephemeris. Vto is the clock error of the receiver. The coordinates x, y, z of the target point and the clock error Vto of the receiver can be solved from the above four equations.
GPS Principles Currently, the positioning accuracy provided by the GPS system is better than 10 meters. To achieve higher positioning accuracy, Differential GPS (DGPS) technology is commonly used: A GPS receiver is placed at a reference station to make observations. Based on the known precise coordinates of the reference station, the distance correction values from the reference station to the satellites are calculated, and these correction values are sent out in real time by the reference station. While conducting GPS observations, the user receiver also receives the corrections from the reference station and adjusts its positioning results accordingly, thereby improving positioning accuracy. Differential GPS is divided into two main categories: pseudorange differential and carrier phase differential. 1. Pseudorange Differential Principle This is the most widely used form of differential. At the reference station, all satellites are observed. Based on the known coordinates of the reference station and the coordinates of each satellite, the true distance from each satellite to the reference station at any given moment is calculated. This is then compared with the measured pseudorange to derive the pseudorange correction value, which is transmitted to the user receiver to improve positioning accuracy. This type of differential can achieve meter-level positioning accuracy, such as the "beacon differential" widely used along coastlines. 2. Carrier Phase Differential Principle Carrier phase differential technology, also known as RTK (Real Time Kinematic) technology, is a method for real-time processing of the differences in carrier phase observations from two stations. That is, the carrier phase collected by the reference station is sent to the user receiver for differential calculation to determine coordinates. Carrier phase differential can achieve centimeter-level positioning accuracy. It is extensively used in fields requiring dynamic high-precision positioning.
GPS Applications ---- Categorized by function, GPS is used in navigation Primarily for positioning and navigating moving objects such as ships, cars, and airplanes. Examples: 1. Oceanic navigation and harbor pilotage for ships, 2. Airway guidance and landing approach for airplanes, 3. Autonomous car navigation, 4. Ground vehicle tracking and urban intelligent traffic management, 5. Emergency lifesaving, 6. Personal travel and outdoor exploration, 7. Personal communication terminals (integrated with mobile phones, PDAs, electronic maps, etc.) GPS used for timing and frequency calibration. 1. Time synchronization for networks such as power, telecommunications, and communications, 2. Provision of accurate time, 3. Provision of accurate frequency GPS used for high-precision measurement, 1. Various levels of geodesy, control surveying, 2. Road and various line layout, 3. Underwater topographic surveying, 4. Crustal deformation measurement, dam and large structure deformation monitoring, 5. GIS applications, 6. Construction machinery (such as tire cranes, bulldozers) control 7. Precision agriculture Categorized by application field: a. Military Use U.S. Navy nuclear submarine personnel equipped with GPS GPS-guided shipborne missiles b. Civilian Use Vehicle navigation management Aircraft positioning and navigation Agricultural monitoring Police equipped with GPS Research on the uplift of Mount Everest Land surveying and engineering Forestry surveys, resource management Self-guided travel Ecological research GPS collars for animals
As the saying goes: "The application of GPS is only limited by human imagination." Since its inception, GPS has demonstrated its dominant position in the fields of navigation and positioning. Many fields have undergone revolutionary changes due to the emergence of GPS. Today, nearly all users worldwide who require navigation and positioning are attracted by GPS’s high accuracy, all-weather capability, global coverage, convenience, flexibility, and excellent cost-performance.
The development of GPS applications in China has been rapid. In just a few years, GPS use in China has expanded rapidly from a few scientific research institutions and military departments to various civilian sectors. The widespread use of GPS has changed work methods, improved efficiency, and brought significant economic benefits. It can be said that the application prospects of GPS in China are limitless. Netizen Discussions on GPS Netizen Talk:
First, purchase based on your purpose and budget. For example, if you only need it for a laptop, then just buy a gps-mouse gm210. If you enjoy hiking and traveling, I recommend the gm101 handheld device. It can also connect to a computer via USB to function as a receiver, and its maps can even be customized.
Second, brand is also important—brand represents quality.
Third, it’s best to find a reputable company because when you’re just starting out with GPS, you might not know how to use it. A reliable company is more dependable if you run into usage issues.
Fourth, if possible, test it on-site. 1. You can learn how to use it, 2. Prevent GPS setup issues from wasting your time, 3. Check whether the GPS you’re buying is user-friendly.
Fifth, I suggest not buying a PDA with a built-in GPS system. I’ve tried several integrated PDAs on the market, and they’re not very good. 1. PDAs with integrated GPS often have subpar PDA performance compared to mainstream models. 2. The integrated GPS cannot be used with other devices like laptops, so your investment isn’t fully utilized. 3. Integrated devices usually come with pre-installed software, which may not be the software you prefer, leaving no room for choice.
Sixth, another important aspect of using GPS is the map. Nowadays, well-known brand GPS units offer decent accuracy, but domestic maps may have manual deviations due to national controls, making them somewhat inaccurate. Therefore, after purchasing a GPS, the map becomes another factor to consider. A good GPS plus a good map will maximize its effectiveness.
Seventh, when buying a GPS device, also consider multi-functionality—for example, whether it can work with your old laptop, new computer, or without needing additional purchases. For instance, my 270u works on an old laptop (which can be bought for just a few hundred yuan on the market), a new laptop, and even a PCC handheld device. The gm101 comes with a USB cable for standalone use or can connect to a computer via USB to function as a receiver. The key is to maximize the GPS’s potential.
Eighth, the casing of the GPS receiver is also a consideration. GPS receivers are typically used outdoors, in the field, or in vehicles, so environmental resistance is important. It’s best for the GPS receiver casing to be made of engineering plastic. Don’t just focus on looks—practicality is key. Ninth, don’t overlook the internal components. The core of a GPS is its main chip. There are several types of main chips, and it’s best to choose one from the U.S.-based SiRF company. With so many GPS receiver brands on the market, it’s essential to recognize this crucial “heart.”
HowToTrip, Your Outdoor Adventure and Self-Drive Travel Website!