Basic Interaction of Energy Sources With Targets
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 2 measures. Alternatively, Sensor 1 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 1 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.
Read more about this topic: Measurement And Signature Intelligence
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