Roswell consisted of two parts: the transmitter and the sat nav. The transmitter was hidden inside a Roswell alien keyring, and would transmit to the sat nav and give the sat nav its location through what appeared to be satellite map.
Tom and Duncan gave the Roswell transmitter as a supposed gift to Connor, so as to track him and learn about Connor's secret and what he was doing away from Duncan and Tom. Tom and Duncan subsequently presumably used Roswell to track and follow Connor from Abby Maitland's flat to the Central London football stadium. At the stadium, when a flock of Dodos came through an Anomaly there, one of them, Dodi, found the Roswell transmitter on Connor's bag and swallowed it, then went out into the stadium's car park, where Tom and Duncan found and captured it and took it back to their flat.
Later, when Tom was infected by a Parasite and set about finding evidence to expose the Anomaly operation which he believed to be a corrupt conspiracy, he used Roswell to track the dead Dodi to the Central Metropolitan University and take the dead Dodo along with a hostage Abby Maitland. When Nick Cutter and the team learned of this, they traced the Roswell transmitter to the football stadium to find Tom before he harmed or infected Abby.