Automatic Packet Reporting System (APRS) is an amateur radio-based system for real-time digital communications of information of immediate value in the local area
Data can include object Global Positioning System (GPS) coordinates, weather station telemetry, text messages, announcements, queries, and other telemetry. APRS data can be displayed on a map, which can show stations, objects, tracks of moving objects, weather stations, search and rescue data, and direction finding data. APRS (Automatic Packet Reporting System), is a digital communications protocol for exchanging information among a large number of stations covering a large (local) area, often referred to as "IP-ers". As a multi-user data network, it is quite different from conventional packet radio. Rather than using connected data streams where stations connect to each other and packets are acknowledged and retransmitted if lost, APRS operates entirely in an unconnected broadcast fashion, using unnumbered AX.25 frames.
APRS packets are transmitted for all other stations to hear and use. Packet repeaters, called digipeaters, form the backbone of the APRS system, and use store and forward technology to retransmit packets. All stations operate on the same radio channel, and packets move through the network from digipeater to digipeater, propagating outward from their point of origin.
So these types of devices are often used during events so that radio amateurs who are moving around and reporting accidents or other problems can have their exact location pin-pointed for quick dispatch of emergency response personnel to the right place without having to waste time describing the exact location where there are no street addresses or where the event stretches over a long distance.
These APRS trackers can be built from parts and will often be used using radio where the digipeaters cover the area being covered. But cellular phones can also be used with an app that also reports the location to the same server.
\* APRS itself at at https://en.wikipedia.org/wiki/Automatic_Packet_Reporting_System.
\* How to build a typical tracker at https://www.hotarc.org/about/trackers2.php
\* Google Maps view of various real-time trackers from around the world at https://aprs.fi/