Motion tracker

Spartans and Elites have a Motion Tracking Sensor built into the HUD of their helmets and armor. The sensor shows individual allies as yellow dots and allied vehicles as larger yellow dots. Enemies are shown as red dots, with Hunters and enemy vehicles represented by larger red dots. Flood Infection Forms are shown as small red dots, the only exception being in the level The Covenant, when they are allies. They are then shown as small yellow dots. The motion tracker shows how close the other characters are in relation to your current position, up to 25 meters. When someone on the other team in multiplayer speaks, their red blip is surrounded by a white glow and it becomes bigger. Nav points will also show up, shown only as a small white triangle at the end of the sensor.

The motion tracker is not a radar, however, and can be countered by crouching, which makes you undetectable on other player's sensors.

Trivia

 * In Halo 3 custom games or Forge, the motion sensor can increase its range by 15 metres, 25 metres, 50 metres or even 150 metres.
 * In the novel Halo: Ghosts of Onyx, the motion tracker sensor is shown to be vulnerable to sea water, when Fred's motion tracker froze due to a crusting of sea salt.
 * In Halo 2, if your shields are critical, your motion sensor will flicker on and off.