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Development status and components of GPS satellite positioning system
GPS, or the Global Positioning System, is a satellite-based navigation system that has gained widespread use in recent years, driving a growing demand for GPS-related products. As technology continues to evolve and new applications emerge, GPS is expected to become an integral part of everyday life.
While GPS is primarily developed and managed by the U.S. military, other global systems are also playing significant roles. China's Beidou Navigation Satellite System is contributing to national development, while Russia’s GLONASS has a long history of reliability. The European Union's Galileo system is designed to be compatible with GPS, offering enhanced accuracy and additional features, and it is set to further enrich the global positioning landscape in the coming years.
The GPS system comprises three main components: the space segment, the control segment, and the user segment. The space segment consists of a constellation of satellites orbiting the Earth at an average altitude of about 20,200 kilometers. These satellites follow elliptical orbits with the Earth located at one of the foci, and they complete an orbit approximately every 12 hours. There are nearly 30 operational satellites distributed across six orbital planes, each inclined at around 55 degrees, with some serving as spares. The U.S. military can adjust the number of active satellites through ground control stations.
Each GPS satellite acts as a known reference point, transmitting ephemeris data that describes its position and movement in real time. This data is crucial for users to calculate their location accurately. Civil GPS modules receive a forecasted version of this data, known as the broadcast ephemeris, which is transmitted via different frequencies and includes both the civilian C/A code and the more secure P code.
The ground control segment is the core of the entire GPS system. It consists of five monitoring stations that track satellite signals, collect data, compute navigation messages, and perform orbit corrections. This extensive data processing ensures the high accuracy and reliability of the GPS system.
The user segment, often referred to as the GPS module, functions like a receiver, capturing and decoding signals from GPS satellites operating at a frequency of 1575.42 MHz. These modules do not transmit signals but instead calculate their position based on the distance to multiple satellites using a method called trilateration. A minimum of four satellites is required for 3D positioning, while three satellites can provide 2D positioning, though with lower accuracy. The module continuously outputs data in NMEA format through a serial port, allowing users to select relevant information for their applications.
The performance of a GPS module is typically measured by parameters such as receiving sensitivity, positioning time, accuracy, power consumption, and time precision. Different startup modes affect the time it takes to achieve a fix. A cold start, where no prior data is available, may take up to one minute, while a warm start, with updated ephemeris data, can occur in under 45 seconds. A hot start, where the module has been recently used, can happen in as little as 10 seconds. If the module is moved over a long distance or left idle for a long time, the ephemeris data may become outdated, leading to longer positioning times. To ensure optimal performance, manufacturers usually clear internal data before shipping.
Positioning accuracy varies depending on conditions. Dynamic positioning generally offers better results than static positioning. The nominal accuracy provided by manufacturers is measured under ideal open-sky conditions, making it difficult to achieve these values in real-world scenarios. Two common metrics for horizontal accuracy are CEP (Circular Error Probable), which indicates a 50% probability of being within a certain radius, and 2DRMS (Two-Dimensional Root Mean Square), which represents a 95.5% probability.
Several factors influence GPS accuracy, including satellite clock and orbital errors, the number and geometry of visible satellites, atmospheric interference, and multipath effects. Additionally, antenna quality, placement, environmental conditions, PCB design, and even small variations between identical modules can affect performance.
In practical applications, GPS modules are often used as a time reference, complementing the internal RTC (Real-Time Clock) to provide highly accurate timing, which is beneficial for product design. Speed measurement is another application, derived from changes in latitude and longitude over time.
Common GPS antennas are ceramic flat panel types, known for their low cost and high gain. However, they are sensitive to temperature changes and require careful design. The typical size is 25 x 25 x 4 mm³, and they perform best when mounted vertically.
The signal transmission path, including external feeders and PCB traces, must maintain a 50Ω impedance to ensure proper signal integrity. Designing RF traces on a PCB can be simplified using specialized software tools.