Definition
BeiDou (BDS, BeiDou Navigation Satellite System) is China's independently developed global satellite navigation system, which provides not only positioning and timing, but also short message communication capabilities. Industrial terminals often adopt GPS+BeiDou dual-mode, which can achieve better coverage and accuracy in China and surrounding regions.
System Composition and Evolution
The BeiDou system consists of the space segment (satellite constellation), ground segment (master control stations, upload stations, monitoring stations), and user segment (receivers). BeiDou-1 (BDS-1) was a regional experimental system providing short message and positioning services; BeiDou-2 (BDS-2) covers the Asia-Pacific region and provides public signals such as B1I, B2I, and B3I; BeiDou-3 (BDS-3) is a global system that introduces modern signals like B1C and B2a, and is compatible and interoperable with GPS and GNSS other systems. BeiDou-3 MEO satellites use inter-satellite links, which enable autonomous orbit determination and time synchronization, reducing reliance on ground stations.
Positioning and Timing
BeiDou provides Standard Positioning Service (SPS) and Precise Positioning Service (PPS). The civilian SPS has a horizontal positioning accuracy of about 10m (95%) and a vertical accuracy of about 20m, with a timing accuracy of about 20ns. Centimeter to decimeter level positioning can be achieved through SBAS (Satellite-Based Augmentation) or ground-based augmentation (CORS). Timing applications are widely used in industries requiring high time synchronization, such as power, telecommunications, and finance. BeiDou/GPS dual-mode or multi-mode receivers can simultaneously receive signals from multiple systems, increasing the number of visible satellites and obtaining more robust positioning results in obstructed environments.
Short Message Communication
BeiDou's unique short message function can achieve two-way communication in remote areas without mobile network coverage. Users send short messages (up to 120 Chinese characters or 480 bytes) through a BeiDou terminal, which are forwarded via satellite to the ground center, and then relayed to the target user or platform. It is suitable for scenarios such as maritime search and rescue, field operations, and emergency communication. BeiDou-3 enhances short message service capacity, supporting larger volumes and wider coverage. Industrial IoT terminals can integrate BeiDou short message modules to achieve critical communications such as fault reporting and status feedback in areas without public network access.
Industrial Applications
In the industrial IoT, BeiDou is mainly used for vehicle/ship/person positioning, logistics tracking, power inspection, and geological disaster monitoring. Terminals using GPS+BeiDou dual-mode or multi-mode chips can receive more visible satellites in China and surrounding areas, improving positioning availability and accuracy. High-precision applications (such as autonomous driving and precision agriculture) need to be combined with enhancement technologies like RTK and PPP. BeiDou timing can provide a reliable time reference for industrial control systems and communication base stations, meeting the requirements for high-precision synchronization.
Short messages and the position solution are not the same link
The position solution uses the navigation message and pseudorange. A short message uses a separate inbound and outbound channel. A chip that supports both does not mean one service can stand in for the other. Positioning reports longitude, latitude, height, speed, and a one-pulse-per-second output. A short message carries a limited user payload and needs a terminal identity, a service rate, and a receipt. At sea or in the mountains, with no public network, a short message is suited to an alarm or a brief status. It is not suited to a track of points or a vibration waveform. After the allowed rate is used up, further sends are rejected. The device may still report a normal position solution. If operations staff watch only the positioning indicator, they will think communication is healthy too. The length and rate of a civil short message follow the service that was actually opened. Do not size the application to the longest frame tried in the lab.
Timing has to be judged from the pulse or from the lock state of the time message. "Position valid" does not mean time is usable. Power-system synchronization and communication base stations are sensitive to a phase step. When the receiver switches satellites it may relock, and the pulse can step. When a dual-mode module switches its time source between BeiDou and GPS, define a primary constellation and a hold strategy so the two time bases do not jump back and forth. If the position still has to reach a platform, a public or private network is still required. The short message is only the fallback when there is no public network. The two arrival delays are not in the same range.
Antenna elevation and multipath
The elevation and the time of day when BeiDou satellites are visible change with latitude and terrain. An antenna should not keep the habit of being stuck to the side of a metal box. It should face open sky. A metal surface below it can act as a ground plane. A metal wall, a photovoltaic panel, or a cable tray beside it creates multipath. The receiver can still solve a coordinate, but the horizontal position drifts along the reflection. A vehicle antenna left inside the cab loses low-elevation satellites to the glass and the roof. In an urban canyon the geometry worsens and the track becomes a broken line. Height is more easily hurt by geometry than the horizontal position. If height is used to decide whether a vehicle is on a bridge or has entered a basement, the threshold needs margin.
The output coordinate also has a datum question. A receiver's raw result is often longitude and latitude in an earth-centered frame, while the map may use another plane projection. Drawing a fence on the map without a conversion shifts the boundary as a whole, which looks like a positioning-module fault. Differential or precise point positioning needs a correction link. When corrections stop and the receiver falls back to a standalone solution, that step in accuracy has to be marked in the status word. Enhanced and ordinary status must not be written as the same quality. A feeder that is too long, a wet connector, or an active antenna whose supply voltage is too low all drop the carrier-to-noise ratio and lengthen a cold start. Those signs look like an expired ephemeris. Measure the voltage at the antenna before blaming the constellation.
Ephemeris lifetime and reacquisition
After a long power-off the ephemeris expires, and the first fix has to collect the navigation message again. Output during that interval should be marked invalid. The coordinate from before shutdown must not be reused. Reacquisition after a short blockage is much faster. The two durations must not be written as one acceptance figure. When the voltage at the antenna is low, the carrier-to-noise ratio falls across the board, which looks like an expired ephemeris. Measure the feeder supply before replacing the module.
When a vehicle enters and leaves a tunnel, the position estimated inside the tunnel and the satellite position after the exit jump once. That jump has to be marked in the status. The jump distance must not be treated as speed. A late short-message receipt does not mean the position solution failed. An invalid position does not mean a short message cannot be sent. Record the two states separately so the platform does not use one indicator in place of the other service.