Motion tracker

Spartans and Elites, and ODST units 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, at least until you shoot while crouching.

Glitches

 * The rats in Crow's Nest show up as small Infection Form-like yellow blips on your radar, indication that they are allied to you, or to avoid confusion about enemies in the area.
 * The fish in the Covenant lake outside the first shield barrier control room, show up as small Infection Form-like yellow blips on your radar.
 * The Hornets on the Storm and the Covenant --while you are on the ground-- show up on your radar as yellow blips, indication that they can actually fight land enemy targets, but they don't. Strangely enough, the same does not go for the Banshees in any level. However, a blip on the motion sensor doesn't necessarily mean that unit could fight, either partially or completely.

Trivia

 * In Halo 3 custom games or Forge, the motion sensor can increase its range by 15 meters, 25 meters, 50 meters or even 150 meters with blips becoming increasingly smaller the larger the range gets.
 * 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.
 * In Halo: Combat Evolved PC. Through a mod, if you are playing as an Elite or a Grunt. The motion tracker or the shielding will not be there.
 * Even if the player isn't crouching, if he/she is moving slowly enough, they will be invisible to the Motion Sensor.
 * The yellow dot which appears at the center of the Motion Sensor's display is the player themselves'. If the player isn't moving, this dot will disappear, but reappear once the player is moving with enough speed to register.
 * Vehicles that are moving, but are not currently manned (i.e. If someone jumps out of a Warthog that keeps moving), these vehicles will appear as manned by a friendly/friendlies.