GPS and RTK Principles and Operation Methods

I. GPS Overview

1. Definition:

The Global Positioning System (GPS), fully named NAVSTAR GPS (NAVigation Satellite Timing And Ranging Global Position System), is a space-based all-weather navigation system developed by the U.S. Department of Defense to meet military requirements for obtaining position, velocity, and time information in a common reference system on land, sea, or in near-Earth space. It is a navigation system capable of time transfer and space intersection positioning through ranging, providing continuous, real-time, high-precision three-dimensional position, three-dimensional velocity, and time information to global users.

2. GPS System Components

GPS Space Segment:

Consists of 24 GPS satellites, including 21 operational satellites and 3 in-orbit spares. The 24 satellites are evenly distributed across 6 orbital planes with an orbital inclination of 55°. The orbital planes are spaced 60° apart in longitude of ascending node. Each plane contains 4 satellites separated by 90° in argument of latitude. Satellites in adjacent orbital planes are staggered by 30° in argument of latitude.

Ground Control System:

Precision Observation: Due to UAVs' special capabilities, they can collect data over larger areas and higher densities, such as monitoring urban traffic, forests, and farmland conditions.

Real-time Monitoring: UAV ground monitoring systems can monitor target areas in real-time, which is crucial for emergency response situations.

Enhanced Deployment Flexibility: UAV ground monitoring systems can easily access difficult or hazardous locations for observation, not only avoiding personal injury but also achieving better monitoring results.

Improved Safety: Widely used in public security, firefighting, environmental protection and other related departments, UAV ground monitoring systems enable more precise and effective safety monitoring.

In summary, the application of UAV ground monitoring systems across multiple fields can significantly improve monitoring standards and governance efficiency, providing more accurate and timely services.

3. Positioning Principles

① Absolute Positioning Principle UAV GPS absolute positioning uses satellite signals and triangulation principles to calculate precise position. During flight, the GPS receiver captures signals from different satellites to determine the UAV's longitude, latitude, and altitude.

Triangulation Principle: This positioning method assumes that a target point and two reference points with known coordinates form a triangle. By calculating the length of reference edges and measuring angles between reference points and target point, the distance and coordinates of the target point can be determined.

② Relative Positioning Principle
GPS relative positioning calculates position by analyzing time differences between signals from different satellites. By placing two receivers at baseline endpoints, maintaining static positions, and synchronously observing four or more GPS satellites, the relative positions of baseline endpoints in the Earth-Centered Earth-Fixed (ECEF) coordinate system can be determined.

③ Static Relative Positioning

Multiple GPS receivers are placed at baseline endpoints, remaining stationary while simultaneously observing four or more satellites. Multiple observation sessions are conducted, each lasting from several minutes to about one hour. Observation data is then processed by software to calculate coordinates.

④ Dynamic Relative Positioning A base station with continuous satellite observation is established. Another receiver observes for several minutes at the first point, then moves to other points for brief observations. Data processing yields coordinates. The operational range for dynamic relative positioning generally should not exceed 15km.

II. RTK

1. Overview

Real-Time Kinematic (RTK) is a high-precision GNSS technology providing centimeter-to-millimeter level positioning accuracy. Through real-time data transmission between base stations and mobile devices, RTK enables rapid, high-precision position calibration, with applications in surveying, agriculture, construction, and autonomous navigation.

2. Principles

① RTK implementation relies on two key elements: base station and rover. The technology primarily measures phase differences between satellite signals received by the rover and those received by the geographically known base station, enabling high-precision positioning.

The core of RTK technology lies in differential positioning between base station and rover. The base station receives satellite signals, records its position, and transmits this data to the rover. The rover compares this information with its own satellite signals, calculating phase differences to achieve centimeter-level accuracy.

Differential Positioning: By comparing satellite signals measured at a known reference station with those measured by another receiver, more precise position measurements are achieved. The reference station calculates precise position from satellite signals and transmits this data to the rover. The rover uses this information to calibrate its own measurements, obtaining enhanced accuracy.

Differential positioning can be real-time or post-processed. Real-time differential positioning provides immediate high-precision positioning, typically used in time-sensitive applications. Post-processed differential positioning performs offline data processing after collection using recorded data from both reference station and rover.

② RTK Base Station Components:

Components: GPS receiver with satellite antenna, radio data link with transmitter antenna, DC power supply

Function: Calculates real-time phase differential corrections and transmits them via data link to refine rover's GPS observations for improved real-time positioning.

③ RTK Rover Components:

Components: GNSS receiver, communication module, antenna, power supply

Functions: Real-time positioning accuracy enhancement, rapid positioning feedback.

④ Limitations of RTK Technology:

High Cost: Requires real-time data transmission equipment and services between base station and rover.

Environmental Sensitivity: Signal quality and stability requirements make performance susceptible to adverse weather or environments with obstructions like urban canyons or dense foliage.

Base Station Dependency: Requires continuous communication with base station. Positioning accuracy degrades beyond base station coverage.

Technical Complexity: Requires specialized equipment and technical expertise for setup and maintenance.

Multipath Effects: Signal reflections/refractions in certain environments can affect measurement accuracy.