Motion tracker

Spartans and Elites have a Motion Tracking Sensor built into the heads-up display 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. Unmanned vehicles appears as large gray 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. In Halo: Reach, the motion tracker also takes into account vertical positioning. Allies, enemies, and vehicles will appear as properly colored triangles when they are a considerable height above, and appear as faded-out dots when they are a considerable height below.

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 motion sensor, indicating 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 motion sensor.
 * The Hornets on The Storm and The Covenant -- while you are on the ground -- show up on your motion sensor as yellow blips, indicating 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 does not necessarily mean that unit can fight, either partially or completely. Surprisingly, the Banshees can be destroyed.

Trivia

 * In Halo 3 custom games or Forge, the motion sensor's range can be set to 15 meters, 25 meters, 50 meters, 75 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 the player's shields are critical, the motion sensor will flicker on and off.
 * 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 at the center of the Motion Sensor's display is the player himself. If the player isn't moving, this dot will disappear, then 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), will appear as manned by a friendly.
 * In Halo: Reach, the motion sensor tells players in what area of the map they are currently in.
 * In Prototype from Halo Legends, the HRUNTING/YGGDRASIL Mark I Prototype Exoskeleton was equipped with a 1400m+ sensor (with map mode).
 * In Halo 2, if a player was to look up or down, the motion sensor "plane" will adjust to be perpendicular to the player view. (i.e. looking straight up will allow you to see how far someone is above/below you)