The Z-1500 Automated Systems Drone - also commonly referred to as the Sentinel or Aggressor Sentinel - is a type of robot from the Halo game series. Designed by the ancient Forerunner race, these machines are found aboard Halo Installations and fulfill a variety of roles such as maintenance, construction and fabrication, mineral retrieval and defence.
Following the construction of the Halos, Sentinels were built to serve as the first line of defence against the Flood, a race of parasitic creatures that preyed upon sentient beings. The inorganic nature of the Sentinels made them ideal units to battle the parasite without risking infection. Controlled by their own autonomous AI and their Installation's Monitor, Sentinels patrol the Halo rings and destroy anything deemed to be a threat.
Sentinels are grayish-white in colour and have two arm-like appendages with two grapple fixtures at the end of each arm, along with a central "head" and a curved undercarriage containing a Sentinel Beam. The form of a Sentinel may differ depending on the situation of their Installation. Their powerful offensive beam weapons and defensive energy shield make them very effective in combat against intruders or foreign objects that present themselves as threats. Their smaller lasers are used for both repairing damaged Forerunner constructs, and self-repair should the Sentinel be damaged in combat. Sentinels are propelled by both an anti-gravity unit located on the undercarriage and small engines on the back of the unit. They are capable of traveling at high speeds but cannot sustain long range travel.
Sentinels are created at Sentinel Production Facilities, massive floating platforms that are located on every Halo Installation. These Facilities produce all types of Sentinels when facing either a Flood outbreak or an external threat to a Halo Installation or Construct. These massive assembly lines at maximum performance can produce Sentinels at a very fast rate to either supplement existing Forerunner defenses or to replace those destroyed for whatever reason. Sentinel Production Facilities are not limited to just the Halo Installations, Production Facilities are located on every Forerunner Construct that relies on Sentinels for guarding, although these facilities are different in appearance and may be either ground or mobile based platforms.
The Forerunners refer to the Sentinels as "our very basic countermeasures", and they are the first automated defence systems employed to assist with a situation and generate a proper response protocol. While Sentinels are strong enough to handle a situation, they are more of a supplement rather than a reliable combat system. The Sentinels' weapons system are limited and not as powerful as some of the other Forerunner weapons employed against the Flood, and as such can only hold them at bay for a limited amount of time.
Because of their limited A.I, they are only able to handle situations on a basic scale, which makes them vulnerable to strategic Flood attacks. Because of their limited intelligence they are not often able to perform evasive maneuvers when facing imminent attack and are usually destroyed. In large overwhelming numbers Sentinels are more than capable of handling a situation until more permanent measures can be applied.
The Sentinel's energy shield is its primary defense mechanism - once taken down, the Sentinel is susceptible to any type of small arms fire, especially plasma weaponry. When the Sentinel has been damaged beyond repair, the Sentinel explodes violently and releases a powerful EMP blast. This is a strategic last resort, as the blast will disable any nearby electronics, including any weapons the enemy may be using.