What is APRS?
Wikipedia says: 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.
There are presently two main frequencies dedicated to APRS work: 10.151 MHz LSB on HF (the mark/space frequencies fall within the 30-meter ham band) and 144.39 throughout North America and Canada. Monitoring the HF band will show activity across the US, because of the propagation characteristics of 30 meters.
More info on APRS and its origin can be found at the ARRL Website – APRS
Here’s the APRS Map of currently connected stations