Prowler

"If you saw a prowler, it probably wasn't a prowler."

- Anonymous Prowler Corps officer

A prowler is a line of warship, represented by the hull classification symbol PRO. Originally, the term properly referred to the slipspace-capable reconnaissance ships used by the Office of Naval Intelligence. However, it has been adopted by many human organisations as a moniker for any ships that are similar in size to a corvette which are designed for covert operations.

History
Up until the late 25th century, the notion of a truly invisible spacecraft was considered a pipe dream. Although hiding the ship itself from visual or radar detection would be viable, any ship would be blatantly visible on infrared sensors a system away the moment it fired its thrusters. Sustained, highly energetic drive burns are required to move about a planetary system in reasonable spans of time, and these are impossible to hide via conventional means. Once an enemy has detected a burn, it is relatively straightforward to calculate the ship's subsequent trajectory and track it over long distances; while minor course alterations may be possible on the way, any significant burn will also be detected by the enemy.

Although trickery involving decoys and electronic countermeasures could confuse or disable enemy sensors over short ranges, no ship (or at least any human-crewed, decently fast ship) could be made truly undetectable to opponents with access to appreciable telescope equipment. High-resolution sensors are near-ubiquitous across the Human Sphere, as they are relatively cheap and widely commercially available due to their importance to space traffic monitoring; this also means all but the most poorly-equipped insurgent groups have access to such systems. Given that sensor satellites are much easier to camouflage than ships, most systems with insurrectionist activity had numerous hidden sensor platforms (coupled with occasional insurrectionist collusion with local space traffic control), which made it impossible for the UNSC to deploy into such systems undetected. Instead, early generations of reconnaissance spacecraft relied more on deception than true stealth, often being designed to mimic civilian craft in drive signature and physical makeup along with hiding among high volumes of civilian traffic. Passive observation with minimal maneuvering was still somewhat viable, but only applicable in specific cases.

In the early years of the Insurrection, prowlers mostly relied on appearing as something else than warships by using baffles to alter their drive signature; the near-complete emissions masking that would characterize later prowlers is a later innovation. Since letting some emissions escape also improves drive efficiency and allows prowlers to move faster, many prowlers still rely on disguising themselves as civilian or logistics craft. Such deception is complemented by building the prowlers into hulls that mimic the designs of civilian yachts, for example. This tactic was effective against most Insurrectionist groups, particularly those working with limited technological resources, but against the Covenant it could at most cause them to de-prioritize the ship. Most Covenant War-era prowler types forgo such deception and instead optimize their hull configuration for more complete stealth via the use of angled surfaces and radar-resistant materials. Metamaterials that inverted the gradient for infrared radiation quickly became standard in ablative armor.

The heightened tensions of the UNSC-CMA Cold War, coupled with advances in spacecraft thermal management, fusioneering and slipspace theory, saw the UNSC reopening research on thermal emissions masking. The solution arose from a novel application of slipspace. As early experiments with slipspace-based communication had shown, moving energy into slipspace (or wavespace) is not particularly difficult; the trouble is keeping that energy coherent and controlling its trajectory past entry. Any radiation transmitted into slipspace (including radio waves or tightbeam) rapidly decoheres, then radiates back into normal space predominantly as random patterns of microwave radiation scattered across wide swathes of space in what is known as the Muratovski effect. This phenomenon could be utilized for concealing highly energetic fusion engine burns, with field-based emissions baffles coupled into the prowler's fusion engines shunting the resulting emissions into non-spatial dimensions. These then scatter the energy widely enough to fully or partially obfuscate the ship's drive flame.

UNSC stealth proved surprisingly effective against the Covenant; despite their superior sensor technology, the Covenant's lack of AI-based detection means they rely mostly on organic personnel to monitor their sensor feeds, meaning Covenant sensors are only as good as their operators. Having realized this, the UNSC began to exploit this weakness from early on with an emphasis on prowler warfare. However, the Covenant caught on to this relatively quickly. The devastation of Zhoist in 2526's Operation: SILENT STORM was a major watershed moment that prompted the Covenant to put a far greater emphasis on detecting human stealth ships, with improved detection protocols and sensor operators specifically trained to look out for tell-tale signs of prowler activity. This forced the UNSC to continually improve their stealth systems and prowler tactics throughout the war for prowler warfare to remain effective.

Technology
Prowler emissions baffles take the form of a wavespace field generator coupled into a drive's magnetic nozzle. They project low-energy micro-wavespace fields behind the nozzle, shunting most of the drive flame into shallow layers of wavespace microseconds after they leave the ship, thereby still allowing the drive to impart thrust. In modern systems, these fields are arranged in a multi-layered stack, as no individual field is powerful enough to capture all of the high-energy particles in the drive flame. When the baffles are operating properly, the radiation backscatter is virtually indistinguishable from cosmic background radiation.

Early emissions baffles were finicky and had to be constantly tuned, with even a minor miscalibration or failure to account for a drive flare exposing the ship's presence. Modern baffling arrays are coupled with a highly specialized "dumb" AI, which calculates the precise output of the engines at any given time and synchronizes it with the baffle field generator. This is in addition to a system of hull cooling, metamaterial cloaking, insulation and internal heat sinks for other onboard heat sources. By the Human-Covenant War, even the prowler's slipspace exit can be convincingly masked or obfuscated, though this means fissile materials cannot be carried onboard as their Cherenkov radiation aura interacts with slipspace in ways too unpredictable to be accounted for by the baffles. While this was possible early in the war, the Covenant quickly caught on to the UNSC's liberal use of nuclear weapons, and began to tune their sensors to look specifically for telltales of radioactive decay. The use of spin-polarized fuels in modern fusion reactors allows for most of the reaction's secondary neutron emissions to be eliminated, though prowlers' reactors and engines must still be designed and constantly tuned to burn much "cleaner" than those on normal ships.

For these reasons, prowlers and their engines have to be constructed to far stricter tolerances than normal ships, which increases their costs and makes manufacturing a specialized and expensive affair. To this day, the partially state-owned Deimos Special Projects Division remains by far the largest and most experienced source of prowler designs.

Prowler stealth is not completely efficient and cannot be used indefinitely. By the end of the Covenant War, baffled engines could only be run at up to 30% power before a geometric increase in the likelihood of detection. As a rule, any burn of the main engines is regarded as a costly maneuver, as this contributes to the heat buildup that will eventually compromise stealth. Although modern baffling systems combined with internal heat sinks are very efficient at removing heat, they can only be used for a limited time before the prowler must radiate out its stored heat. Due to this, prowlers use cold-gas thrusters and gravity-assist maneuvers over sustained fusion engine burns as much as they can.

These challenges also multiply as the prowler's size increases, meaning that prowlers should ideally be as small as possible. First conceptualized in the later years of the Insurrection, the Human-Covenant War saw the introduction of destroyer-sized prowlers termed stealth cruisers, albeit in extremely limited numbers due to their great expense and diminishing returns in most fields of performance.

Although secondary to their emissions masking and only relevant on short ranges, modern prowlers also make use of visual camouflage. This provided by a relatively simple chameleoflage stealth coating, though advances during the Human-Covenant War have seen prowlers shift increasingly toward photoreactive metamaterials.

Tactics
Prowlers are support vessels that see use in various roles, including intelligence gathering, surveillance, covert insertion/exfiltration, and minelaying. They may operate in conjunction with fleet or battle group, individually, or in flights comprosed entirely of prowlers. Prowler missions range from short-term, such as conducting surveillance for the imminent arrival of a fleet for several hours to long-term passive observation missions lasting weeks or months. In general, operating a prowler is always a balance between stealth and speed. The ideal prowler mission is one where the prowler can afford to take its time by using minimum-energy Hohmann transfers or by not maneuvering at all. Many prowler missions do not have the luxury of time, however, and crews must weigh stealth against the schedule they are working on in a given operation.

Although stealthy compared to conventional spacecraft, even modern prowlers are not completely invisible. A fraction of a drive burn's emissions always escape and persistent active scans will reveal the presence of an object. However, they are very difficult to detect or track, especially over long distances and to an unsuspecting enemy. Ideally, the goal is not to be detected in the first place. While a "soft" detection will not directly reveal the size or type of ship to long-range sensors, it will alert the enemy to the presence of an unknown ship. This, in turn, will typically lead to more detailed scans that will expose the prowler. Over long distances, however, an unconfirmed detection may not yet fully compromise a mission; for example, it is still impossible to gauge a prowler's speed or heading based on one or two scattered puffs of drive plasma. In the time it takes a sensor operator to register the anomaly and decide to follow up on it, the prowler may already be dozens or hundreds of kilometers from that point, depending on its vector. Further course corrections and decoys may be used to confuse and distract the enemy, allowing the prowler to continue operating in the theater. With the Covenant reliant on non-AI means of detection and tracking, this is a common occurrence; the Covenant identify a likely prowler, but fail to track its subsequent movements. However, if the Covenant do manage to get a fix on the prowler's location and sweep that region with a precision Luminary scan, detection is inevitable.

Mimics
Mimics, also interchangeably called semi-prowlers, q-ships, or proto-prowlers when retroactively applied to pre-25th century craft, are a venerable class by human standards. The first examples had actually seen service before the Interplanetary Wars, when a variety of Earth's nations desired a surveillance ship that was both inconspicuous and able to be used as a support platform for special forces operations. Such a vessel could effectively hide in plain sight without drawing attention, gathering intelligence on bases that otherwise could not be approached with conventional military craft. And their relatively low cost means that it is easy to create a fleet large enough to maintain ears throughout most of their polity's territory.

All mimics start their lives as a civilian shipping vessel, and are rarely built for the task. Instead, they are brought in for modification at a later date, where they receive highly-attuned sensors, communications-tapping equipment, and false-transponders and registries as standard. These all tend to be craft that have reason to be far-ranging, with those charged with ferrying messages or news articles being the most popular; that said, civilian yachts are also used if the mission requires loitering in a given system. Some are called upon to support prowlers in the field, whenever that be to drop off supplies or, in the case of larger container ships, conduct actual repairs and maintenance.

Most mimics boasted little in the way of armament or defensive systems outside of what is normally afforded to civilians, to better allow their operating navies to deny any involvement. The only exception is the installation of weak pulse lasers for destroying satellites. As a result, mimics that are detected must immediately either scuttle their ship or return to base for redeployment and alteration of its disguise. However, during the Insurrection the UNSC secretly armed a handful of mimics with powerful missiles and naval coilguns to bait out and destroy insurgent raiders. These saw huge momentary success until organized Insurrectionist movements adapted to the threat.

Although their ubiquity means that their crews tend to be drawn from the most junior of an intelligence service's naval personnel, as they and their ships are considered disposable, ONI has done much to cultivate enduring professional traditions in their mimic sailors. The organization has emphasized tight-knitted bonds and sharing of unofficial knowledge that is, nevertheless, vital to the success of their mission. ONI's attempts to encourage this has seen servicemen that all speak the same non-English language being assigned to the same vessel, and keeping crews together as much as possible. To an extent, this worked: senior sailors often pass on their unique habits and etiquette to new recruits, giving each ship an enduring identity. It is hoped that this makes crews more comfortable in order to pass even thorough inspections from insurgent-sympathetic officials, although history has showed that they have somewhat-higher levels of defecting. Nowadays, such a culture has died out. The Covenant's indiscriminate destruction of all human ships culled mimics and their experienced crews, and the rapid expansion of the Prowler Corps meant that anyone with skill was quickly reassigned there.