In SERE and evasion, signature means the detectable features of a person, their equipment and their activity. These include optical features such as outline, movement and light, acoustic features such as footsteps and rattling equipment, and thermal features on skin, clothing and other surfaces. Such features may unintentionally reveal an isolated person or be used deliberately for signalling. Their detectability results from the interaction between source, propagation path, background and observer.
From a personal feature to detection
A feature must reach an observer and stand out sufficiently from the surroundings. Light needs a usable optical path, a sound must still be distinguishable at the ear or microphone, and a camera receives only radiation within its sensitive band. An exposed body part, a brief flash or a single rattle can attract attention before anyone recognizes a human shape. Detection, recognizing a person and checking their identity are different steps: a patch in a thermal image can mark an area for further observation without revealing a face, affiliation or intention.
Detectability therefore cannot be judged solely by looking at one's clothing. The observer may be closer, higher or off to one side; a different background may make the same outline more distinct. Changing the device also changes the image. Someone hidden behind an obstacle from a ground observer may remain partly visible from the air. When choosing a position, the person needs to consider which parts of their body and equipment are actually exposed from possible directions.
Optical signature: outline, contrast, movement and light
Optical signature includes reflected light and personal light sources. A head and shoulders against a bright sky, a straight equipment edge among irregular plants, light skin against a dark background and a shiny surface can stand out. Movement changes these features: a previously hidden arm appears in a gap, a swinging bag creates a recurring stimulus, and turning a shiny surface produces a brief reflection. A switched-on lamp, screen or flame adds a light source that can attract attention even without a recognizable body outline.
The person can switch off or cover unneeded light sources, cover shiny objects and keep equipment within their concealment. Before making an additional movement, they check whether their head, hand or baggage would extend beyond an edge or into a gap in vegetation. Shadows move with the sun; a shaded halt may later be illuminated. A change of background from dense vegetation to open sky calls for a fresh assessment of body height and outline. An object's colour helps only in relation to its actual background.
Night vision and thermal imaging use different signals. Image-intensifying night vision devices use available visible light and, depending on the device, near-infrared light. Cameras with near-infrared illumination emit light of their own and receive the part reflected by the object. A thermal camera uses other infrared bands. A place that looks dark to the naked eye may therefore still produce a distinct image in a device. A material that appears well matched in visible light can also reflect near-infrared differently; its visible colour does not determine that behaviour.
Acoustic signature: limit noise at its source
Acoustic signature includes speech, coughing, equipment alarms, items knocking together, friction and contact with the ground. A snapping twig produces a brief impulse; a loosely suspended object can strike again with every step. Such sounds can draw a listener's attention to a location even before they see the person. Noise discipline in US evasion procedures therefore starts at the source: turn off unneeded alarms, secure rattling items and place each foot deliberately, instead of discovering the behaviour of equipment and ground with the next step. Securing equipment must preserve access to needed items.
Audibility depends on the strength, pitch and duration of a sound and on the local background. Wind can mask other sounds; air temperature and humidity affect their propagation. The person therefore assesses the current noise level and stops for their own listening. What they fail to hear while making rustling movements may be distinguishable to a stationary observer. A drop in the wind changes conditions again. Continued rattling creates repeated opportunities for perception; a seemingly favourable background does not replace securing equipment.
Thermal signature: surfaces and radiation
Thermal cameras receive radiation that reaches their sensors in the relevant infrared band. The exposed surfaces emit radiation of their own and may reflect radiation from the surroundings. On a person, exposed skin and the outer side of clothing affected by body heat contribute to the image. Hot equipment or a fire site can form additional conspicuous surfaces. What matters is the difference from the background, rather than simply the idea of a “warm person”. Colour on the camera display represents the received signal and does not directly measure core body temperature.
A substantial barrier of earth, rock or a tree trunk interrupts the direct view of the body parts behind it. An observer may nevertheless see over its upper edge or around its sides. Vegetation leaves openings between leaves and branches; movement can bring further body parts into those openings. A covering's outer surface becomes an observed object in its own right. A warmer part of the body can warm a cooler surface in contact with it. Materials also differ in transmission: some thin plastics pass thermal infrared radiation even though they appear opaque in visible light.
A reflective rescue blanket makes the distinction between an obscured body shape and a vanished signature particularly clear. It can change the recognizable human outline while its surface stands out as an unusually cold or geometric patch and flashes in sunlight. Solar heating, cooling, wind and moisture also change the image of the surroundings and a covering. Darkness does not remove the thermal channel; light fog can weaken visible observation while a thermal camera retains useful contrast. Without a suitable camera of their own, the person cannot confirm this effect by looking at the surface. They can, however, assess actual sightlines, gaps and unnecessary movement.
Survival, concealment and necessary signals
An activity can create several signatures at once. A fire supplies warmth but also produces light, hot surfaces and, depending on fuel and combustion, smoke. Obtaining water, caring for an injury or preparing a signal may require movement and noise. The person's decision therefore has to weigh the immediate benefit against the resulting detectability. Removing needed insulation and protection from wind or cold ground to appear less warm can impair the person's ability to act. Useful approaches include limiting additional movement, retaining actual visual barriers and covering unneeded sources of light or glare.
Deliberate signalling makes a selected channel detectable. A mirror flash needs a suitable light path to the observer; speech or a whistle must reach the listener. An infrared marker requires the other party to have a sensor that receives its radiation. An IR filter designed for night vision goggles does not automatically turn a light into a thermal-imaging signal. Nor does it automatically restrict the direction of emission: on the military MS2000(M2) marker light, the IR mode radiates in all directions, while the blue filter has a directional function. An enemy observer with a compatible device can also use the IR channel. Attracting attention with a signal does not replace the subsequent identity check.
Before an activity, the person can check their signature in a short sequence: what source will be created on the body or equipment, what path leads to a possible observer, and what function must still be preserved? For a necessary contact, they check the means actually available and the usable signal path. They direct a signal towards a recipient they can actually observe and intend to contact, or use a signalling procedure or contact direction agreed beforehand, if such an arrangement exists. They prepare the device while concealed and transmit the required signal deliberately, instead of using every signalling device continuously and at the same time. After a change of location, a longer halt or changed light and weather conditions, they reassess the same sources. A single heard sound can prompt someone to examine that part of the terrain with optical or thermal equipment.