Realtime tracking is using the Garmin eTrex Legend Cx GPS to report its position frequently to a computer (typically a laptop in a car) to update its position on a map.

Older versions of Mapsource used to support realtime tracking. Around mid 2005, that functionality was moved to the free Garmin nRoute program. Note that in order to have maps displayed, you also need to own a Mapsource title.

All members of this product family support Garmin's proprietary PVT positioning protocol on USB. This protocol isn't widely supported, but is supported by a few programs like Google Earth and (obviously) Garmin nRoute.

Garmin Spanner can be used on these models on Windows systems to provide a virtual COM port that has NMEA for those programs expecting NMEA. That effectively converts Garmin PVT protocol to NMEA.

Units with serial ports such as Garmin 60Cx™, Garmin 60CSx™, Garmin 76Cx™, and GarminG 76CSx™ can output a "real" NMEA data stream on the serial port. With contemporary laptops, this usually involves a USB/Serial adapter, but works on operating systems where Garmin Spanner is not available.