1. Image Formation in Remote Sensing 
  
A possible definition for "Remote Sensing" is to obtain information 
about distant objects without coming into physical contact with them. 
Carrier of the information are force fields (electromagnetic energy, 
gravity, sound). The signals are received by sensors. Passive sensors 
record natural energy coming from the objects (solar radiation, tem- 
perature exchange). Active sensors are linked with artificial energy 
sources (Radar, Sonar), which radiate energy pulses to the objects; 
these are remitted there and received by the sensor. 
Many sensors are capable of one dimensional recording only (magneto- 
meter, airborne gravity meter, echo sounder, scintillometer). While 
the sensor platform is moved over the terrain, a profile record may 
be obtained. Of greater interest are sensors, which are not only ca- 
pable of recording the intensity of received signals but also permit 
to measure or determine their direction. Such informations may be com- 
bined into two-dimensional images. The study of geometrical relation- 
ships in these images is a task of photogrammetry. 
Imagery can be obtained in a direct manner only by means of the photo- 
graphic emulsion. For passive energy sources one is therefore limited 
to a sensitivity range between A= 0.3 and 1.0 um in the electromagnetic 
spectrum. The image must be created on the emulsion by optical projection 
through lens systems. Only when using active energy sources at close 
range, photgraphs may also be abtained with x-rays ( A= 60 to 1078 nm) 
as shadow-images in direct projection. Neglecting image motion during 
the time of the exposure (which for two-dimensional images only affects 
resolution, not geometry), direct imaging by frame cameras has the ad- 
vantages of a real-time system. 
When restricting the optical projection by diaphragms or slits to obtain