Remote sensing depends on the interaction of a source of energy with a target, and energy measured from the target. In the "Remote Sensing" diagram, Source 1a is an independent natural source such as the Sun. Source 1b is a source, perhaps manmade, that illuminates the target, such as a searchlight or ground radar transmitter. Source 1c is a natural source, such as the heat of the Earth, with which the Target interferes. The Target itself may produce
emitted radiation, such as the glow of a red-hot object, which Sensor 1 measures. Alternatively, Sensor 2 might measure, as
reflected radiation, the interaction of the Target with Source 1a, as in conventional sunlit photography. If the energy comes from Source 1b, Sensor 2 is doing the equivalent of photography by flash. Source 3a is under the observer's control, such as a radar transmitter, and Sensor 3b can be tightly coupled to Source 3. An example of coupling might be that Sensor 3 will only look for
backscatter radiation after the speed-of-light delay from Source 3a to the target and back to the position of Sensor 3b. Such waiting for a signal at a certain time, with radar, would be an example of
electronic counter-countermeasures (ECCM), so that a signal jamming aircraft closer to Sensor 3b would be ignored. A
bistatic remote sensing system would separate source 3a from sensor 3b; a multistatic system could have multiple pairs of coupled sources and sensors, or an uneven ratio of sources and sensors as long as all are correlated. It is well known that bistatic and multistatic radar are a potential means of defeating low-radar-observability aircraft. It is also a requirement, from operations personnel concerned with shallow water operations. Techniques such as synthetic aperture have source 3a and sensor 3b colocated, but the source-
sensor array takes multiple measurements over time, giving the effect of physical separation of source and sensor. Any of the illuminations of the target (i.e., Source 1a, 1b, or 3a), and the returning radiation, can be affected by the atmosphere, or other natural phenomena such as the ocean, between source and target, or between target and sensor. Observe that the atmosphere comes between the radiation source and the target, and between the target and the sensor. Depending on the type of radiation and sensor in use, the atmosphere can have little interfering effect, or have a tremendous effect requiring extensive engineering to overcome. First, the atmosphere may absorb part of the energy passing through it. This is bad enough for sensing if all wavelengths are affected evenly, but it becomes much more complex when the radiation is of multiple wavelengths, and the attenuation differs among wavelengths. Second, the atmosphere may cause an otherwise tightly collimated energy beam to spread.
Classes of sensor Sensing systems have five major subcomponents: :*Signal collectors, which concentrate the energy, as with a telescope lens, or a radar antenna that focuses the energy at a detector :*Signal detectors, such as
charge-coupled devices for light or a radar receiver :*Signal processing, which may remove artifacts from single images, or compute a synthetic image from multiple views :*Recording mechanism :*Recording return mechanisms, such as digital telemetry from satellites or aircraft, ejection systems for recorded media, or physical return of a sensor carrier with the recordings aboard. MASINT sensors may be framing or scanning or synthetic. A framing sensor, such as a conventional camera, records the received radiation as a single object. Scanning systems use a detector that moves across the field of radiation to create a
raster or more complex object. Synthetic systems combine multiple objects into a single one. Sensors may be passive or coupled to an active source (i.e., "active sensor"). Passive sensors receive radiation from the target, either from the energy the target emits, or from other sources not synchronized with the sensor. Most MASINT sensors will create digital recordings or transmissions, but specific cases might use film recording, analog recording or transmissions, or even more specialized means of capturing information.
Passive sensing Figure "Remote Sensing Geometry" illustrates several key aspects of a scanning sensor. The
instantaneous field of view (IFOV) is the area from which radiation currently impinges on the detector. The
swath width is the distance, centered on the sensor path, from which signal will be captured in a single scan. Swath width is a function of the
angular field of view (AFOV) of the scanning system. Most scanning sensors have an array of detectors such that the IFOV is the angle subtended by each detector and the AFOV is the total angle subtended by the array.
Push broom sensors either have a sufficiently large IFOV, or the scan moves fast enough with respect to the forward speed of the sensor platform, that an entire swath width is recorded without movement artifacts. These sensors are also known as
survey or
wide field devices, comparable to wide angle lenses on conventional cameras.
Whisk broom or
spotlight sensors have the effect of stopping the scan, and focusing the detector on one part of the swath, typically capturing greater detail in that area. This is also called a
close look scanner, comparable to a telephoto lens on a camera. Passive sensors can capture information for which there is no way to generate man-made radiation, such as gravity. Geodetic passive sensors can provide detailed information on the geology or hydrology of the earth.
Active sensors Active sensors are conceptually of two types, imaging and non-imaging. Especially when combining classes of sensor, such as MASINT and IMINT, it can be hard to define if a given MASINT sensor is imaging or not. In general, however, MASINT measurements are mapped to pixels of a clearly imaging system, or to geospatial coordinates known precisely to the MASINT sensor-carrying platform. In MASINT, the active signal source can be anywhere in the electromagnetic spectrum, from radio waves to X-rays, limited only by the propagation of the signal from the source. X-ray sources, for example, must be in very close proximity to the target, while lasers can illuminate a target from a high satellite orbit. While this discussion has emphasized the electromagnetic spectrum, there are also both active (e.g., sonar) and passive (e.g.,
hydrophone and
microbarograph) acoustic sensors.
Quality of sensing Several factors make up the quality of a given sensor's information acquisition, but assessing quality can become quite complex when the end product combines the data from multiple sensors. Several factors, however, are commonly used to characterize the basic quality of a single sensing system. •
Spatial resolution defines the correspondence between each recorded pixel and the square real-world area that the pixel covers. •
Spectral resolution is the number of discrete frequency (or equivalent) bands recorded in an individual pixel. (Relatively coarse spectral resolution from one sensor, such as the spectroscopic analyzer that reveals a "bush" is painted plaster, may greatly enhance the ultimate value of a different sensor with finer spectral resolution.) •
Radiometric resolution is the number of levels of energy recorded, per pixel, in each spectral band. •
Temporal resolution describes the intervals at which the target is sensed. This is meaningful only in synthetic imaging, comparison over a longer time base, or in producing full-motion imagery.
Cueing Cross-cueing is the passing of detection, geolocation and targeting information to another sensor without human intervention. In a system of sensors, each sensor must understand which other sensors complement it. Typically, some sensors are
sensitive (i.e., with a low incidence of false negatives) while others have a low incidence of false positives. A fast sensitive sensor that covers a large area, such as
SIGINT or acoustic, can pass coordinates of a target of interest to a sensitive narrowband RF spectrum analyzer for ELINT or a hyperspectral electro-optical sensor. Putting sensitive and selective, or otherwise complementary sensors, into the same reconnaissance or surveillance system enhances the capabilities of the entire system, as in the
Rocket Launch Spotter. When combining sensors, however, even a quite coarse sensor of one type can cause a huge increase in the value of another, more fine-grained sensor. For example, a highly precise visible-light camera can create an accurate representation of a tree and its foliage. A coarse spectral analyzer in the visible light spectrum, however, can reveal that the green leaves are painted plastic, and the "tree" is camouflaging something else. Once the fact of camouflage is determined, a next step might be to use imaging radar or some other sensing system that will not be confused by the paint. Cueing, however, is a step before
automatic target recognition, which requires both extensive signature libraries and reliable matching to it. ==See also==