M21 Sentinel Automated Infantry Unit

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The is a UNSC automated infantry unit, a mechanised android used for combat operations. They are used for multiple purposes across the entirety of the UNSC, including patrol, guarding, escort and reconnaissance. A tough, hardy, drone, it is lethal and surprisingly versatile, but only sees limited distribution due to concerns within the UNSC concerning it's use, especially close to civilian populations. 
==Development History==
The Sentinel entered development in late 2543. As various private firms began to invest in development of mechanical shock troops, the UNSC was left behind. While they certainly had advanced cybernetic and automation technology, many within the UNSC's high command, and the public opinion, were against drones with human-like capabilities, replacing human operators. Opinion began to sway in the wake of several high profile attacks, where Insurrectionist forces had struck major bases and even warships, and caused casualties to soldiers and marines guarding them. Various robotics companies began to market their particular models to the UNSC, but all were rejected. The UNSC instead approach Misriah directly. Misriah owned Jotun Heavy Industries, who specialised in automation, and Arcturan Medical, who specialised in cybernetics. Using specialists from their subsidiaries, and designers from their own talent pool, they began developing an automated infantryman for the UNSC.
While the design went as a leisurely pace, it was sped up following the Kirk Attack, when Insurrectionists fired mortars carrying chemical weapons into a UNSC base. As the marines inside suffered under the effects of the deadly weapons, the Insurrectionists marched in and took a vast amount of ammunition, explosives and weaponry from the armoury, then left without a trace. Now working on a limited time frame, they began taking on more staff, as well as off the shelf components in order to facilitate the development. 
The first prototype was ready for testing in early 2553. Initially shown in front of several of Misriah Armoury's own directors. Impressed by its aptitude and problem solving capabilities, they green lit it to be shown to the UNSC. Meeting with the Ordnance Committee, they too were impressed by its capabilities, and immediately ordered a large supply of them.
Their first combat deployment would prove to be a game changer. A separate Insurrectionist Force, emulating the Kirk Base attack, attempted a similar strike on a base on Arcturus. Causing a few initial casualties, as they advanced into the seemingly evacuated base, they instead met resistance from a squadron of Sentinels. Advancing through the haze of chemical smoke, their vision and sense unhindered, they gunned down the approaching terrorists with pinpoint ease. The returning fire proved to be near useless, as the munitions were either deflected by its heavy armour, or simply failed to damage it enough to cease operations. All in all, just ten Sentinels eliminated a larger force of thirty-six Insurrectionists, suffering six damaged units and two knocked off line in return. 
In light of this victory, requests for Sentinel deployments sky-rocketed. Soon, special forces operators began to look at Sentinels themselves, working with Misriah skunk works to develop a specialised drone for supporting their operations. This 'Warden' would cost nearly three times that of the standard drone, but were often lovingly tended and used with deadly accuracy. 
==Role==
The Sentinel is used for a variety of purposes throughout the UNSC. In order to deploy the Sentinel, most officers have to go through lengthy documentation, and their calibration must be done on site by specialised technicians. More often than not, their deployment is only precluded by the highest risk to human lives, or where automation is required. 
The most common role is that of guard duty. With their enhanced senses and their tireless nature, they make perfect guard dogs, whether at military installations or forward camps. At major installations, such as bases, facilities, laboratories, factories and head quarters, they are a common sight, holding vigil at entrances and patrolling it. In these circumstances, they are usually equipped with a non-lethal weapon, though may have a lethal secondary weapon. They are also used to guard forward operating bases, or other forward sights at risk. Here they'll often be equipped with a lethal weapon with orders to eliminate any threat. 
The Sentinel is commonly used to patrol in contested or dangerous areas, where it is ill advised to risk human lives. Usually a small team, directly monitored from their home base, will patrol along a predefined route, observing any potential threats and unusual disturbances, but are usually set to only return fire once fired upon. It is preferred that they patrol outside of populated zones, as civilian populations are often frightened of the drones, and there is a constant risk of collateral damage. 
Sentinels have been used with increasing for shipboard defence. Whether on operational warships or space stations, they enhanced senses, resistance to the harsh vacuum of space and zero reliance on oxygen allow them to to fight on the exterior hulls, or through hull breaches, and perform things like customs inspections and deep space salvage. When coordinated by the warship's A.I., they can respond faster to changing incidents within the ship, mounting a better defence, or even working with repair teams to repair damage of seal hull breaches. 
Sentinels are also used for reconnaissance in force. If it is suspected the enemy may have fortified a position, small units may be deployed to probe the area without risk to human life. Such operations are costly in regards to units though. 
Special Forces have been using singularly modified variants in increasing numbers, using them as expendable grunts to inflate their numbers, or as deadly shock troopers in close quarters. These variants often have their processing power boosted by close linked A.I.s, allowing them to perform increasingly complex manoeuvres and work with greater concert.
==Design==
===Sensors===
Each Sentinel has an impressive array of sensors. Its primary sensors consist of a pair of visual sensors. These are fitted on rotary gimbles with three options for sensors, including electro-optical, with four times zoom, dual phase thermal/nightvision with four times zoom, and a terahertz-wave camera that allows for short ranged vision through solid objects. These are positioned on each side of the triangular head and give a wide field of view, in a similar set up to 'prey' animals of Earth, such as deer. These allow the Sentinel to have excellent all round vision and the ability to locate targets to the front or each side of it. They are fitted with hardened lenses that are resistant to flares and flashbangs. 
For close ranged acquisition it is fitted with a short range, dual-phase motion detector, using ultrasonic and microwave waves to detect targets. This is useful for tracking targets in dense terrain, as well as detailing a variety of information on the target such as size and direction, and cross correlation with radio interrogation to determine if it is friend or foe. This sensor can also be used to map out the terrain ahead of it. 
For defence against terrorist attack or searching for explosive devices, they are equipped with a ARGUS sensor, allowing them to 'sniff' out explosives or other unusual chemical scents. They can turn this into visual data, which is uploaded as augmented reality data to the UNSC WarNet, allowing others to follow this chemical trail, using the Sentinel as a sniffer dog. 
===CPU and Electronics===
The central CPU of the Sentinel is contained inside the 'head', though secondary systems are mounted in the torso. This CPU uses a modern Liquid State Computer that has been ruggedized. Fitted with an advanced neural net that possesses a degree of problem solving and learning capabilities, this CPU can perform advanced tactics and actions. Fitted with visual recognition technology, they can identify whether a target is armed or not, if they are friendly and can even compare them to a list of known targets or enemies, and then choose appropriate action. They can also navigate difficult terrain, climbing structures or picking the most efficient path. It can be preprogrammed to react to various stimuli, allowing it to react preditably under certain conditions. In addition to this, they can also share data among one another, creating a gestalt sensor net. Each linked Sentinel allows them to boost their processing power, and when linked to a higher order of artificial mind, such as a automated or artificial intelligence, their processing power improves dramatically. 
For radio communication, it possesses a mid-ranged antennae on the right side of its head. Capable of sending and receiving vocal messages, these can be used to report to the human operators, or receive vocal commands to issue orders. It also functions as a device for interrogating Identify Friend/Foe (IFF) transponders. This antennae can be used as a wireless device as well, uploading or downloading data, or connecting to the extranet. It is fitted with a speaker allowing it to produce verbal speech, usually a collection of standardised messages to communicate with humans in proximity, whether friend or foe. It can also be set to autistic mode where it relies entirely on recognised vocal commands from human operators within proximity. 
It has other electronic packages, such as advanced gyroscopic sensors to maintain balance, weight distribution systems, pressure sensors, microsensors to report damage or maintenance issues and temperature sensitive heat regulation systems. 
For transferring data, it is fitted with two hardline parts, one on the back of its head, the other on its left hand, usually reserved for interacting with electronics in the field. It also has a motorised fibre optic probe, allowing it to investigate up two a metre away. 
===Armour===
The base frame of the robot is made of hollow tubes of high strength titanium. Resistant to immense shocks and gunfire, this chassis provides adequate strength for the built on actuators. Over the actuators themselves it has an external plating made up of titanium and titanium alluminide. On the torso, thighs and head this is reinforced by sandwiches of AEGIS tile inserts and liquid nanocrystals, all encased on a titanium shell. Its armour can further be reinforced by equipping it with plate carrying vests, being fabric vests carrying plates of AEGIS ceramic armour. It's combined armour is resistant to several repeated blows from 7.62mm calibre rifles, even those firing armour piercing munitions, but is susceptible to high calibre weapons, and has numerous weakpoints such as the neck and joints. 
===Weaponry===
The Sentinel features no inbuilt weaponry, although it does feature feature incredible strength, allowing it to lift several times its own weight. This translates to it being able to toss a human about with ease, or even kill with a single punch. Its true weapon is which every is planted into its hand. Since its hand possesses the range of motion a human one does, it can operate weaponry with ease, being capable of aiming, firing and reloading a wide range of UNSC weapons with ease. It is programmed to operate most, if not all of the firearms in the UNSC's arsenal, and has sufficient programming to use a back up sidearm, non-lethal weaponry and munitions and even hand grenades. Due to its ability to multi-task and link to electronic systems, it doesn't even need to be looking down the sights to fire accurately upon its target. 
===Powerplant and Motor Systems===
It is powered by a hydrogen battery planted in its chest. This high-energy battery can be recharged by plugging the Sentinel into a socket, or replaced entirely, and has sufficient power to keep it operating for upwards of 40 hours. Buried deep in its chest, it is difficult to hit, but reacts badly to explosive, thermite or plasma munitions, explode violently and vaporising the robot. 
The motor system consists of a series of electronic actuators, capable of impressive feats of strength and speed. By no means advanced, they are tough and reliable, and it is usually fitted with enough to give it redundancies in the event of a fire fight. These are backed up by magnetodynamic ball bearings and low profile joints to provide it with a huge range of motions, almost mirroring those of a human. 
===Maintenance and Repair===
The Sentinel not only features sophisticated diagnostic systems, allowing it to monitor the condition of its physical and electronic systems, but also sufficient programming to conduct maintenance and repair on its fellow units, allowing a unit to, theoretically, continue functioning indefinitely, with only minimum input. Despite this, it lacks the ability to solve complex issues, or repair severe damage, requiring a skilled technician. 
For ease of maintenance, the unit's head and limbs can be easily removed, and exterior plating removed to reveal the content beneath. It also features numerous off the shelf-components, allowing it to be repaired easily and cheaply, and even field repaired with little tools. 
==Variations==
While closely related to the M20 Badger Automated Multifunction Drone, the two have little commonality in terms of components or specs. The Sentinel only has two variations, both manufactured in limited amounts. 
===M21F1 Guardian Improved Automated Infantry Unit===
The Guardian was a attempt to improve the design of the Sentinel. Initially called the Sentinel II, it was designed with vast improvements to its CPU features, armour and motor system. However, attempts to make it more responsive in combat, especially short ranged conflict backfired, and they became much more aggressive in combat. While still limited by the robotic laws that govern the basic Sentinel, it's enhanced combat systems caused it to persecute targets with more prejudice. It became favoured by UNSC special forces as a means to boost their numbers during combat operations. 
===M21G Warden Special Automated Infantry Unit===
The Warden is a incredibly specialised variant designed by Misriah Skunk Works. The Warden is improved in every area, with increases to power systems, motors and armour. Its central processor is replaced with an Automated Intelligence, which while of a lower order than that of the the AQ-606 Wasp, it is several magnitudes superior to previous Sentinel models. To this end, it is capable of executing advance manoeuvres, moving deftly and with surprisingly agility, perform dexterous operations, grapple opponents in close quarters combat and solve complex problems. Like their predecessors, they were utterly fearless and walked willingly into doom, whereas their more advanced kin have some degree of self-preservation. To improve mobility they could perform short range jumps using a modified version of the jump kit designed for MJOLNIR, allowing it to leap up several stories, clear large gaps or evade enemy fire. To improve its survivability, it was fitted with a fusion reactor, which powered a energy shield, which in turn could be used to generate point defence shields or melee weapons. 
This rare variant was fielded by only a handful of users, including the Office of Naval Intelligence and Spartan Special Forces. Wardens would regularly undergo rapid field modification, to their design, armour, weaponry and central computer system, and were known to have customised paint jobs in the field. Operators were known to actively attempt to recover downed platforms during combat, such was their attachment to these machines. Most would be directed by a A.I., improving coordinate and enabling them and their human allies to work in near perfect concert.