From Halopedia, the Halo wiki
The motion tracker, also known as the motion sensor, is a piece of equipment utilized by the UNSC and the Covenant. In addition to being used as UNSC field equipment, it is also a feature built into the heads-up display in a variety of combat suits, including the HRUNTING/YGGDRASIL Mark I exoskeleton, the MJOLNIR armor and the Sangheili combat harness. The tracker, located on the heads-up display's lower left corner, indicates individual allies, enemies and vehicles in relation to the location of the armor's operator.
The motion tracker is a circle with a central dot, which represents the user's location. The tracker shows individual allies as yellow dots and enemies as red dots. The size of the dot is depended on the size of the individual, with normal infantry represented by a normal dot, smaller life forms, such as rats, fishes, or even Pod infectors, are represented by smaller dots, and large dots representing vehicles or large infantry, notably the Mgalekgolo. Unmanned vehicles appear as large gray dots.
The motion tracker allows the user to locate enemies or allies behind cover, and is proved to be important for predicting enemy activities behind obstacles.
The motion tracker is apparently vulnerable to sea water, as indicated when Spartan-II Frederic-104's motion tracker froze due to a crusting of sea salt. Equipments like Covenant radar jammer can be used to scramble the motion tracker's signal, effectively obliterating its use. The tracker is also shown to be susceptible to slow moving objects, which will not shown up on the motion tracker if within its range.
The first known UNSC motion tracker was integrated in the MJOLNIR Mark I exoskeleton. These exoskeletons never saw use in battle. The HRUNTING/YGGDRASIL Mark I ADS, which was developed in Algolis, included a 1400m+ tracker and a map mode. Unlike other iterations, enemies were indicated by short yellow lines.
The prototype MJOLNIR Mark V includes a motion tracker with a radius of 25 meters. This 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 radius of the motion tracker in the MJOLNIR Mark V's final iteration has been reduced by 10 meters, at only 15 meters.
The MJOLNIR Mark VI features motion trackers with the range of 20 or 25 meters, possibly depending on the operator's choice. Navigation points appear on the sides of the motion tracker as small white triangles. John-117's MJOLNIR Mark VI HUD motion tracker was improved by Cortana; this upgrade allows his motion tracker to identify many vehicles with distinct overhead outlines, replacing the indistinct circles displayed by earlier motion trackers. The sticky detonator also implements the motion tracker in relation to the projectile, telling when to detonate the projectile to ensure maximum damage.
The Sangheili seems to be the only Covenant species that included motion tracker into their armor. The Sangheili combat harness' motion tracker operates exactly like that of the MJOLNIR armor, having the inclusion of navigation points, verticality, and varying range.
In Halo: Combat Evolved multiplayer, the player will not appear on an opponents motion tracker while crouched, or while crouched and moving, or while standing still. In MCC, if you have toggle crouch on, you won't be able to move at full crouched speed without standing up again. Be gentle with the thumbstick because of this, or avoid the problem completely and toggle crouch off. This is not an issue for MCC Steam.
In Halo 2, if a player were to look up or down, the motion tracker "plane" will adjust to be parallel to the player view, allowing them to see how far someone is above or below them. The Blind skull in campaign removes the entire HUD, including the motion tracker. In multiplayer, when the motion tracker is disabled through game settings, the tracker will remain on the HUD, but with an "X" over it.
In Halo 3 campaign, the Blind skull once again removes the motion tracker with the rest of the HUD, but the Fog skull removes only the motion tracker. In multiplayer, the motion tracker'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. During the game through Xbox LIVE, when an opposing team player speaks, their red blip is surrounded by a white glow and it becomes bigger.
In Halo: Reach, the Blind skull returns again and when activated, removes the entire HUD, including the motion tracker. The Cloud skull, when activated, wields the same effect as Halo 3's Fog skull, removing the motion tracker. The armor ability icon is then relocated from the top left of the motion tracker to the lower left of the HUD. In multiplayer, the motion tracker's lower right corner displays the name of the area the player is located in. This feature is absent in campaign, unless the player leaves the playable area, then the label is shown as "OUT OF BOUNDS".
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