Definition
GPS (Global Positioning System) is a satellite navigation system established by the United States, providing worldwide position, velocity, and time (PVT) information. Industrial terminals often use GPS+BeiDou dual-mode modules to improve positioning accuracy and availability.
System Features
GPS consists of approximately 30 MEO satellites, transmitting signals at L1 (1575.42 MHz), L2 (1227.60 MHz), L5 frequencies, etc. The horizontal accuracy of the civilian Standard Positioning Service (SPS) is about 5–10m (95%), vertical about 10–20m, and timing accuracy about 20ns. Modernized GPS (L2C, L5) can improve multipath resistance and availability. Industrial GNSS modules often support GPS+BeiDou+GLONASS+Galileo multi-system; in China and surrounding areas, using GPS+BeiDou can achieve better coverage.
Industrial Applications
Vehicle/ship positioning, logistics tracking, personnel inspection, power inspection, geological disaster monitoring, precision agriculture, autonomous driving, etc. Timing applications provide a time standard for power synchronization, communication base stations, and financial transactions. When selecting, factors such as cold start time, reacquisition time, power consumption, multi-system support, and whether high accuracy (RTK) is required should be considered.
The civil code and the timing pulse
The civil service of the Global Positioning System provides position, velocity, and time. The receiver solves them from the civil ranging code and the navigation message. Position is the coordinate of the antenna phase center in some datum. Velocity comes from the carrier or from the change in position. Time can be output both as a sentence on the serial port and as a one-pulse-per-second signal. The rising edge of that pulse is what timing has to calibrate. The time sentence on the serial port is usually later than the edge, and the output delay has to be added as well. Parsing the sentence and never measuring the pulse leaves a base station or a protection device out of alignment. Satellite count, dilution of precision, fix mode, and whether the pulse is valid are four different states. They cannot be collapsed into one "GPS normal."
The condition is an antenna that can see enough satellites spread across the sky, and a continuous track longer than the time the receiver needs to solve. Under an elevated road, at the mouth of an alley, and inside filmed glass, the civil signal is still there and the geometry is poor. Differential corrections can tighten the error. As the age of the corrections grows, accuracy falls back toward a standalone fix, and the status word has to follow. A timing application also has to state the lock threshold and the hold policy: on a short loss of satellites, stop the pulse, or hold it from a local oscillator. The two choices affect downstream equipment in opposite ways. Power depends on how many satellites are tracked and on the update rate. Do not estimate an always-fixing terminal from the sleep current with the radio off.
Cold start and reacquisition after blockage
A cold start has to download the ephemeris again and search frequency, and it takes the longest. A hot start is only reacquisition after a short blockage. Requiring a vehicle to report the moment it leaves a tunnel, using the cold-start time, is a design that cannot work. Another mistake is to look only at the satellite count. A large count bunched in a small patch overhead stretches the horizontal position in one direction, and the track saws along a straight road. Smoothing that sawtooth without marking quality makes the backend think the vehicle is changing lanes again and again.
Drawing the coordinate straight onto another map produces an almost fixed offset. The offset is not random, so it is not multipath. Multipath pulls the position toward a high wall, or pushes it away, and it recovers after the vehicle leaves. The antenna amplifier is powered from the receiver up the feeder. A shorted or reversed feeder can make the receiver shut the supply off to protect itself. The sign is no satellites at all, the same as an antenna that was never connected. When one pulse drives several devices, distribute or isolate it. Wiring it directly across several optocouplers can slow the edge, and the timing error grows. The record should keep the raw status word. Longitude and latitude alone cannot later tell a standalone fix, a differential fix, and an invalid point apart.
Pulse fan-out and a fixed offset
Fanning a one-pulse-per-second signal out to several devices needs distribution or isolation. Wiring them in parallel slows the edge and grows the timing residual, while the navigation sentence still looks normal. The time in the sentence is later than the pulse edge. A calibration program that only lines up the moment the sentence was sent folds the serial delay into the clock error. Read the pulse-valid flag on its own. A valid position cannot stand in for it.
A map offset is fixed. Multipath changes with blockage. The two are handled in opposite ways. A fixed offset needs a coordinate conversion, and a smoothing algorithm will not remove it. Multipath needs a better antenna position, and a conversion formula will not remove it. A cold start at a tunnel mouth cannot be the basis for a report the moment the vehicle exits. A short blockage should be required against the reacquisition time. Inside a metal compartment the output should be invalid, not a continued report of the last point. When a shorted feeder shuts off the antenna supply, the sign is no satellites at all, the same as no antenna. Measuring the voltage separates the two. Save the raw status word with the coordinate, so a standalone fix, a differential fix, and an invalid point can be told apart later.
Civil ranging and timing can both be usable, or only one of them can be trusted. Just after a tunnel exit the position jumps first, and the pulse may still be waiting for lock. Downstream timing equipment should keep using its local hold until the pulse-valid flag appears. With the antenna under the windshield, the vehicle body blocks low-elevation satellites. The count may still be enough, dilution of precision gets worse, and straight travel is drawn as a sawtooth. Smoothing can improve the display. The archive should still keep the unsmoothed point and its status. Coordinate-conversion parameters are tied to the map revision. After the map is replaced, the old parameters produce a new fixed offset. Every extra coupler in the feeder is another place to fail. When there are no satellites, start from the voltage at the antenna. Do not replace the receiver module first. In the record, separate invalid points before a cold start completes from valid points after it, and leave the former out of an availability statistic.