Sensor Type

A is a shinobi that is capable of detecting others, typically by sensing their chakra.

Overview
Anyone can feel the presence of chakra when it is especially powerful or released in large enough quantities. However, only sensor type shinobi have the ability to sense chakra at their own discretion, allowing them to detect it in non-combat situations, as well as tell individuals apart by their unique chakra signatures. Skilled sensor types can sense any chakra within a radius of several dozen kilometres, as well as hone in on a specific chakra signature to perceive its movements in greater detail. It is also possible for them to tell if a person is lying by detecting any fluctuations in their chakra, sense when other nearby sensor types are using their abilities, and determine whether they or other individuals are under the effects of genjutsu. Nagato was able to determine the exact location of the person controlling him through their chakra, despite said person erecting a barrier around themselves. Members of the Yamanaka clan can use a technique to transmit their sensory perception to others.

Even for sensor types, detecting chakra is not an automatic process; in order to sense chakra, the shinobi must actively be kneading chakra. They must also change their chakra to, making it possible for them to miss important events if they are preoccupied with an ongoing battle. Since concentration is required, it can be difficult to participate in battle while focusing on sensing. Picking out specific individuals from within especially large crowds can also prove challenging, requiring the aid of an experienced sensor type.

As chakra signatures are similar among relatives, sensor types can tell members of different species, populations, and bloodlines apart by the feel of their chakra. For example, Karin was able to tell the chakra signatures of dogs apart from those of humans, while Mū commented that Gaara and his father, Rasa, share similar chakra, and also noticed that chakra signatures from several different villages were gathered together in one place. Likewise, Tobirama Senju could tell that Orochimaru's body was almost entirely made up of his brother's cells, as well as discern that Karin was descended from the Uzumaki clan.

Sensor types often search for chakra while forming the Ram hand seal and closing their eyes, though this is not always the case. Tobirama Senju and Minato Namikaze are able to detect the presence and number of nearby enemies by simply touching the ground with their fingers. In Karin's case, she uses a technique called Mind's Eye of the Kagura to sense chakra; it is unknown if other sensor types use specific techniques, but in the anime, the general sensing ability is referred to as the Sensing Technique. The means by which one becomes a sensor type are also unclear. It is known that some shinobi are inherently better at sensing than others, and that some individuals appear to possess sensory abilities prior to receiving any shinobi training. Obito Uchiha is an example of a shinobi who displays sensory abilities in adulthood, despite seemingly not having any as a child.

There are a number of methods that can hinder a sensor type's ability to detect individuals. When transmitting chakra signals, the user can prevent their location from being discovered by constantly changing the frequency of their chakra. Karin is able to hide her chakra to avoid being found by other sensor types, though this prevents her from using her own sensing abilities. Members of the Aburame clan can use their insects to create false chakra signals in order to confuse sensor types. Mū is able to render his chakra completely untraceable while invisible, earning him the title of Non-Person due to having neither detectable form nor chakra. Gengetsu Hōzuki and his summon use a wide-range genjutsu that prevents them from being sensed while it is in effect. Black Zetsu is also able to render itself undetectable through unknown means.

Aside from those who can sense chakra, there are others who qualify as sensor types due to the nature of their abilities, such as members of the Hyūga clan with their Byakugan. Sensor types have different specialties depending on their village of origin, with Konohagakure shinobi often using ninken for tracking. There are also that can be used by those who lack sensory abilities of their own. Examples include Nagato's Rain Tiger at Will Technique and Gaara's Concealed Sand Picture Cat, which is a sensory technique.

As stated by Jiraiya, no matter how skilled a sensor is, they still cannot sense attacks and need to physically see an attack in order to counter or dodge it. The only way to enhance one's sensory abilities enough to sense attacks is with Sage Mode.

Trivia

 * The term "sensor type" first appears in Naruto chapter 335. Though the concept of sensing chakra is not fully explored until Karin's introduction in chapter 347, there are several earlier instances in which characters who are later revealed to be sensor types demonstrate the ability to sense chakra.
 * Sages, who utilise natural energy, are also capable of sensing chakra: in fact, the word uses the same hiragana (せんじゅつ) as the Japanese art of, which may allude to a sage's ability to sense chakra while in Sage Mode.
 * Kurama and some of its jinchūriki have the ability to detect others by sensing their negative emotions, which is impossible for even the most skilled sensor types.
 * The Greatsword Samehada is attracted to powerful chakra, which it can track downs via its "scent". When Samehada's wielder fuses with the blade, they also gain the ability to detect chakra.
 * Much like humans, animals are capable of sensing chakra when it is especially powerful or foul.
 * Chūkichi was praised for having the greatest sensing ability in Kirigakure.
 * In Naruto: Shippūden episode 226, an unnamed sensor type demonstrates the ability to detect things that would usually fall outside the realm of sensing, such as depth charges, underwater air currents, and a large subterranean hot spring beneath him.
 * In Naruto: Shippūden episode 396, Suiren demonstrates the ability to remotely view the exact position of a person's body using the Sensing Technique, rather than simply perceiving their chakra.