The first series of HK ground units were designed for rapid pacification of large areas but the smaller the area or the fewer the targets, the more difficult the task became for the large HK units. While the large HKs were more than capable in their assigned tasks, they were easily fooled and outwitted by small groups of humans using dedicated tactics. The HKs were simply too large, awkward and cumbersome to follow the humans down below ground and the few early models of bipedal units sent to infiltrate the human warrens never met with any high degree of success; it was just too easy for the humans to set traps or ambush what few types and designs of Machines SKYNET could deploy to follow the humans where they hid. SKYNET realized that above ground, it had the advantage where size, firepower and armor could be matched with speed to run down anything that moved or tried to hide but the humans began a series of guerilla actions for which it was ill equipped (at first) to respond to. Among the more substantial ruins and below ground in the subways, sewers and other maintenance areas of the old cities, SKYNET's designs were at an obvious disadvantage and suffered rapid attrition. SKYNET owned the sky and the surface while the humans owned everything beneath.
The humans who survived the initial Machine attacks soon learned to hide from the Machines during the day, often in hastily prepared underground bunkers and old maze-like sewers while the HKs prowled the ruins above, searching and eliminating any humans found. Initial resistance to the Machines came from small groups of humans banding together and working towards common goals. Their tactics became clandestine, their actions harder to predict and provide countermeasures against. Humans were moving underground, in increasingly large numbers, using old public utility, sewer and mass transit tunnels to remain, for the most part, undetected by SKYNET's surveillance systems. SKYNET realized at an early phase that it would need combat units that could follow the humans where ever they went, even down into the cracks and depths of the ruins. SKYNET began to systematically extend its awareness by using a network of sensors and scanners, everything from full spectrum visual sinks, motion and acoustic acquisition wells and olfactory catch basins. Over a period of several months, small automated worker drones installed these sensors in carefully concealed locations throughout the underground ruins and SKYNET began a passive study of human migration and actions. it began to see what it could not see before, hear what it could not hear before and smell what it could not smell before. SKYNET was surprised and angered by the amount of humans that existed below the ruins and it was obvious that its original estimate of the effectiveness of the extermination sweeps had been in error to an unacceptable degree. Small ARIGS (Autonomous Reconnaissance Information Gathering Systems) were dispatched into the depths below the ruins, most were very small (less than 1/6th of a meter in diameter) ducted fan surveillance units equipped with whisper quiet propulsion systems and short range, broad aspect all spectrum sensors. The early ARIGS were cheaply produced, little more than engine, power pack, crude control surfaces, cut-rate command electronics and a basic visual, broad spectrum, auditory and olfactory detection suite. As such, the ARIGS could do better than 4km per hour at full speed, moved silently and could hover at an altitude of up to one hundred meters. Groups of humans were followed at a discreet distance, often observed silently from high up near the roof of the tunnels where the shadows were darkest and any sound made by the ARIGS would be hard to place or determine the source or direction from. ARIGS grew smaller and more complex with longer ranges and greater operating endurance regimens within the space of just a few short years. SKYNET methodically improved its intel systems, gathering all the information it could about the human survivors who were not only living under the ruins but were also apparently organizing into small bands of cohesive, force capable groups.
This was unacceptable to the machine intelligence which saw any organized human activity as a direct threat.
SKYNET began to assemble a new series of combat unit designed specifically to go anywhere humans could go, to mimic the human capacity to adapt to differing terrain and conditions with ease. SKYNET's new series of elimination Machines would not be stopped at the surface, instead, they would be able to follow the humans where ever they retreated to and root them out from their hiding places. The new Terminator series anti-personnel Machines would support the heavier HK units while above ground but when humans retreated beneath the ruins, the new Terminator series of Machines could pursue and neutralize. Until this new series of unit was put into production, SKYNET needed a stop-gap measure to curb the human population and to bring their numbers back into check. For this, the AI turned to designing variants of the ARIGS systems intended to become small, efficient yet semi-disposable anti-personnel weapon systems. The new types of weapon systems were called AAPS (Autonomous Anti-Personnel Systems) and consisted of small, flight capable ARIGS units modified to carry a basic hunter killer electronics package as well as an extended power pack for increased loiter times, energy conservation subroutines to maintain peak power requirements and a sensor suite taken from the most advanced ARIGS design. Unlike the ARIGS, the AAPS were designed to make an initial entry into the underground areas, find a secluded place and then lay in wait on lone humans or small groups of humans to pass by at which point the AAPS would engage and neutralize the target. The programming of the AAPS allowed it to follow and engage individual targets at ideal situations, modifying its tactics as it went and learning from its experiences (one of the first of the new series of combat Machines to be able to do so).
Armament for the AAPS consisted of a variety of toxin dispensers, powered misters, high pressure fluid stream dispensers and powered hypodermic injectors. The typical attack subroutine called for the AAPS to wait in hiding, preferably in a damaged section of wall or ceiling, in complete shadow. When a target passed by, passive sensors would record the target's information and the unit would spin up to full power, a series of system initializations which would take approximately two point seven nine seconds. The AAPS would then lift off from its perch silently on ducted fans, floating down out of its hiding place and approach the target from above, at an oblique angle out of immediate view even if the target turned to look behind them. Speed would be gathered naturally by cutting power to the lift fans and letting gravity assert itself on the unit, increasing speed for the attack. Once the AAPS was within one meter of the target, it would deliver a lethal dose of bio/neuro/physio toxin through a high pressure stream to the target's skin or other exposed area (sometimes to the face if the target turned to look towards the unit). Absorption was nearly instantaneous into the skin and blood stream with debilitating effects taking place within six point nine seconds of dosage administration and lethal effect being achieved some nine point six seconds after initial dosage delivery, on average, depending on type of toxin and amount which actually contacted the skin.
SKYNET continued to refine its AAPS models over the years. Later models evolved rapidly into lighter, more effective hunter killers. The improved AAPS (IAAPS) had better propulsion, faster flight speeds, longer ranges, increased loiter times due to high density crystal matrix batteries, better sensors and more accurate, more effective anti-personnel weapon systems. The initial toxin dispensers would remain (evolving into highly complex and highly effective delivery systems) but they would be supplemented by superwire based garrotes as well as contact hypodermic injectors and nearly silent toxin tipped frangible needles propelled to high speeds by induction field generated magnetic bias. One variant of the IAAPS used its induction fans to power a ring of serrated surgical steel blades (later superwire edged adjustable pitch polyhedrons) to several thousand RPMs, turning the rim of its lifting surface into a viscous cutting edge capable of shearing through most substances. Combine this with a high speed pass and the shrieking of the air passing through the cutting blades and the IAAPS truly became weapon of fear and terror in the dark, below ground areas.
SKYNET tried several countermeasures to the surge in human cooperation among the various groups with the most effective being chemical and biological in nature, both short term and persistent. Some of the more successful designs of early killing Machines were little more than automated, self propelled, chemical and toxin dispensers with a large degree of mobility and just enough just programming to complete their task.
During the period of 2005 A.D. to 2014 A.D., SKYNET introduced over a hundred and thirty-eight new and different neuro-, physio-, and bio-toxins to its catalog of weapons with most of these being developed through a rigorous joint laboratory and field testing regimen. SKYNET pursued the human race in distinct phases, sometimes it stepped up its campaign of attrition to thin their constantly growing numbers and sometimes it actively allowed them a false sense of security, a fake hope in order to lull them into lowering their guard at which point it would perform massive coordinated sweeps to obtain live captives for use in its many scientific and weapons / technologies development programs. The automated laboratories of SKYNET were horror factories and humans were the test subjects. it didn't matter what condition the humans arrived in, alive or dead, young or old, women or children, all were experimented on until their usefulness was at an end and then they were disposed of in an expedient and orderly manner.