A dot moving on a map looks simple. Behind it is a chain of five steps, and each can fail in its own way. Understanding the chain tells you why a tracker is sometimes late, why a position can jump, and what to ask a provider before you buy.
Step 1: the tracker works out where it is
A GPS tracker contains a receiver that listens to signals from navigation satellites. Each satellite broadcasts its position and the exact time. By measuring how long the signals took to arrive from several satellites, the receiver calculates its own position. It needs signals from at least four satellites to work out latitude, longitude and height. Most modern trackers also use other satellite systems such as Galileo, GLONASS and BeiDou, which gives more satellites in view.
GPS only works well with a view of the sky. Inside a garage, a tunnel or between tall buildings, the signal is weak or reflected, and the position becomes less accurate or disappears.
Step 2: the tracker packs the position into a message
The tracker does not only send the position. A typical message holds the time, latitude and longitude, speed, heading, and the state of other inputs: ignition on or off, battery voltage, door sensors, fuel level or temperature probes, depending on the device. The tracker saves messages in memory so that nothing is lost if the network is down.
Step 3: the mobile network carries it
The tracker has a SIM card and sends its messages over the mobile data network, usually as small packets over TCP or UDP to the address of the tracking server. Because a message is only a few dozen bytes, the data use is small. The delay is usually seconds, but it depends on the network. In areas without coverage, the tracker stores its messages and sends them when the coverage returns, which is why a vehicle’s history can fill in later.
Step 4: the server decodes, stores and checks
The server receives messages from many different makes of device, and every make has its own protocol: its own way of encoding the same facts. A platform that supports many device types needs a decoder for each. After decoding, the server stores the position, updates the live view, and checks the rules you set up: has the vehicle entered or left a zone, did it exceed a speed, has the ignition changed, has it stopped for too long.
This step is where most of the engineering is, because it has to handle messages arriving late, out of order or twice, and still keep the history correct.
Step 5: the map and the alerts reach you
The web page or app shows the stored positions on a map, and alerts are sent as notifications, emails or messages. A good platform also keeps the history, so you can replay a trip, see where it stopped and for how long, and produce reports.
Why positions sometimes look wrong
- Jumps: a weak signal can produce a few bad fixes. Good platforms filter positions that would require impossible speed.
- Drift at standstill: a parked tracker can wander by a few metres. Platforms usually ignore small movements while the ignition is off.
- Late updates: the tracker may have been out of coverage and sent its stored messages later.
- Gaps: the tracker lost power, the SIM has no data, or the device was disconnected.
How often does a tracker report?
It is a trade-off. Frequent reports give a smooth route and quick alerts, but use more data and, for battery trackers, more power. Hardwired vehicle trackers often report every few seconds while moving and less often when parked. Battery-powered asset trackers report much less often, sometimes only a few times a day, to last months.
Power: hardwired, plug-in or battery
- Hardwired: connected to the vehicle battery and ignition. It reports the ignition state and cannot be removed easily.
- Plug-in (OBD): fits the diagnostic port and needs no installation, but can be unplugged by anyone.
- Battery: for trailers, containers and equipment. It reports less often and the battery must be recharged or replaced.
What this means when you choose a system
Ask how the platform handles lost coverage, how it filters bad positions, and which device protocols it supports. Our guide to choosing a tracking system covers the questions in detail.
Written by the 22Softwares engineers who build and run GPS tracking platforms.