The Tracer is an Intrusion Countermeasure System or "ICS" for short.
It's a motion detector that tracks hostile units in its range that have stealth and are cloaked.
__The ICS does NOT reveal stealth units on your radar. Instead, visual red circles constantly ping their location, and an audio alert will notify you. Your forces won't automatically fire on these pings—you must manually attack ground, or move units to find the hidden enemies.
The ICS only pings when cloaked and stealth units are moving. Those who hold still will remain hidden.
Unlike radar, the ICS uses 'seismic' vibrations, which means it's range is NOT blocked by terrain. You can safely build it behind natural barriers for protection.
Also great for protecting your Citadel against stealthy EMP attacks.