High-Precision Solutions

　　"GPS high-precision solutions" refer to systems that utilize various technologies and methods to improve the positioning accuracy of the Global Positioning System (GPS) or other Global Navigation Satellite Systems (GNSS, such as China's Beidou, Russia's GLONASS, and Europe's Galileo) from the conventional meter-level to centimeter-level or even millimeter-level. These solutions are widely used in fields requiring precise positioning.

　　Main Solutions and Technologies for GPS High Precision:

　　Differential GNSS (DGNSS)

　　Principle: The core concept is to use a fixed reference station (base station) with a known location to calculate the error in GNSS observations (primarily pseudoranges). These error corrections (differential corrections) are transmitted in real time or post-process to nearby mobile users (rover stations). Users apply these corrections to their own observations, thereby improving positioning accuracy.

　　Main Types:

　　Position Differential: The reference station calculates its own position error and transmits this error to the user to correct its position solution. Accuracy is relatively low (meter-level to sub-meter-level). Pseudorange Differential: The reference station calculates the pseudorange error for each satellite (including common errors such as satellite orbit error, satellite clock error, and atmospheric delay) and transmits these error corrections to the user. The user uses these corrections to correct their own pseudorange observations for positioning. This is the most common real-time differential technique (such as the RTCM SC-104 format), achieving sub-meter accuracy (0.5-2 meters).

　　Carrier Phase Differential: This is the core technology for achieving centimeter-level accuracy. The reference station not only calculates pseudorange errors but, more importantly, calculates the integer ambiguities and errors of the carrier phase observations. It then transmits high-precision carrier phase corrections (or raw observation data) to the rover. The rover uses this information along with its own received carrier phase data to perform relative positioning calculations.

　　Real-Time Kinematic Positioning: The rover receives corrections from the reference station in real time and calculates its own position in real time.

　　RTK: The most popular real-time centimeter-level positioning technology. It requires a reliable wireless data link (radio, cellular network) to transmit correction data from the reference station to the rover in real time (typically achieving optimal results within a range of 10-30 kilometers). Accuracy: 1-2 cm + 1 ppm horizontally, 2-3 cm + 1 ppm vertically.

　　Network RTK/VRS: Utilizes a densely distributed network of permanent reference stations within a region (e.g., a city, province, or country). The network control center integrates the data from all reference stations and constructs a regional spatially correlated error model (e.g., atmospheric delay). The rover transmits its approximate position to the control center, which generates corrections (or regional correction parameters) for a "virtual reference station" and transmits them to the rover via the cellular network. The rover uses the VRS data just like a single RTK reference station. Advantages: Wide coverage, uniform accuracy, and no need for users to set up their own reference stations. Accuracy comparable to RTK.

　　Post-kinematic positioning: The rover and reference station simultaneously record raw observation data (carrier phase, pseudorange, ephemeris, etc.). Afterward, specialized software is used on a computer to process both data to determine the rover's high-precision position. This is generally more accurate and reliable than real-time processing.

　　PPK: The rover and reference station record data independently and process it post-processing. Commonly used for drone aerial surveys, mobile mapping, and oceanographic surveying. Accuracy: Centimeter-level or even millimeter-level. Advantages: Not dependent on real-time data links, suitable for areas with signal obstruction or communication difficulties; more rigorous data processing results in higher accuracy.

　　Precise Point Positioning (PPP)

　　Principle: No local reference station is required. Users utilize raw observation data (pseudoranges and carrier phase) received by a single receiver, combined with precise satellite orbits and clock corrections accurately calculated and broadcast by a global or regional reference station network (typically obtained via the internet or satellite link), and an accurate atmospheric delay model, to achieve high-precision absolute positioning directly at the receiver.

　　Features:

　　Global Coverage: Works anywhere in the world as long as precise ephemeris and clock products (such as those provided by the IGS) are available.

　　Convergence Time: Early PPP systems required a long time (tens of minutes to an hour) to achieve centimeter-level accuracy (referred to as "convergence time"). This was its main disadvantage.

　　Accuracy: After convergence, it can reach centimeters (static) to decimeters (dynamic). Improved Technologies:

　　PPP-RTK/PPP-AR: This combines the concepts of PPP and RTK/Network RTK. In addition to providing precise ephemeris and clock errors, it also provides regional, high-precision atmospheric delay correction information (ionosphere, troposphere) and satellite phase deviation products. This enables the receiver to quickly fix carrier phase ambiguities, significantly reducing convergence time (to minutes or even faster) and improving accuracy to centimeter-level. This is a current research hotspot and future development direction.

　　Assistive Technologies and Enhancements

　　Multi-Frequency, Multi-System Reception: Modern high-precision receivers typically support reception of multiple frequency bands (e.g., L1, L2, and L5) from multiple GNSS systems (GPS, GLONASS, Galileo, BeiDou). This offers significant advantages:

　　More satellites: Improves availability, reliability, and positioning accuracy (especially in obscured environments).

　　Ionospheric Delay Correction: Different frequency signals are affected differently by the ionosphere. Multi-frequency observations can more accurately estimate and eliminate ionospheric delay errors (a major source of error). Fast Integer Ambiguity Fixation: Multi-frequency signals help resolve carrier phase integer ambiguities more quickly and reliably.

　　Tightly Coupled Inertial Navigation System (INS): Deeply integrates a GNSS receiver with an inertial measurement unit (IMU - gyroscope and accelerometer). GNSS provides absolute position and velocity, but this can be interrupted; INS provides continuous, high-frequency position, velocity, and attitude changes, but this can drift. Tightly coupling the two through algorithms such as Kalman filtering allows the INS to maintain high-precision positioning during brief GNSS signal loss (such as in tunnels or under bridges), while the GNSS corrects for INS drift. Providing continuous, reliable position, velocity, and attitude information is crucial for autonomous driving, drones, precision agriculture, and other applications.

　　Multipath-Resistant Antennas: Specially designed antennas (such as choke antennas and multipath suppression antennas) can effectively reduce errors caused by signal reflections from the ground or surrounding objects (multipath). Advanced Data Processing Algorithms: Observation data is processed using complex algorithms, either within the receiver or via post-processing software. These algorithms include cycle slip detection and repair, fast and accurate integer ambiguity resolution, and optimized filtering, to improve accuracy and reliability.

　　Application Scenarios:

　　Surveying and Geographic Information: Topographic surveying, engineering stakeout, cadastral surveying, and GIS data collection.

　　Precision Agriculture: Autonomous tractors, variable-rate fertilization/spraying, and yield mapping.

　　Building and Construction: Machine control (bulldozers, excavators, and pavers), structural monitoring, and pile foundation positioning.

　　UAVs and Aviation: Precision drone aerial surveying, remote sensing, plant protection, and automated takeoff and landing.

　　Autonomous Drones and Intelligent Transportation: High-precision vehicle positioning, lane-level navigation, and V2X.

　　Oceans and Rivers: Hydrographic surveying, dredging, ship berthing, and buoy positioning.

　　Scientific Research: Crustal deformation monitoring (earthquakes, volcanoes), atmospheric research (water vapor inversion).

　　Robotics and Automation: Outdoor mobile robot navigation.

　　Which solution should I choose? When selecting a high-precision solution, consider the following:

　　Required accuracy: millimeter-level, centimeter-level, sub-meter-level, meter-level.

　　Application scenarios: real-time, post-processing, static, dynamic, open areas, and urban canyon/obstructed environments.

　　Coverage: local areas (such as construction sites and farms) and wide areas/global.

　　Reliability requirements: whether relevant needs exist.

　　Summary:

　　RTK (especially network RTK/VRS) is currently the most mature and widely used solution for achieving real-time centimeter-level positioning. PPK is a reliable method for achieving high accuracy through post-processing. PPP (especially PPP-RTK) is a global high-precision solution that does not require local reference stations and holds great promise as technology advances (reducing convergence time). Auxiliary technologies such as multi-frequency, multi-system, and tightly coupled INS are key components of building a robust, reliable, and high-performance high-precision positioning system. The choice of solution depends on the specific application requirements, budget, and operating environment.