| 
point the reception of the remitted pulse will be at a different ® o 
time, and it will occur with varying intensity. The time varying 
sequence of signals may be fed to a line deflected cathode ray tube, 
which will then carry a line image in distance representation (slant 
range or ground distance depending on deflection circuitry). The 
image is projected onto a film, which is moved forward proportional 
to the platform motion while new pulses continue to be emitted. Thus 
the image of an active sensor is generated. 
Early active sensors (the Plan Position Indicator Radar) use the 
mechanical rotation of an antenna in a horizontal plane. The image 
received is a distance representation in polar coordinates. 
Since the active sensor image formation for any system occurs over 
a finite time interval dynamic errors are also present. 
The classic development of photogrammetry started with the invention 
of photography in 1839 (Niepce and Daguerre). Aerial photographic e © 
surveys began in 1858 (Laussedat), but it was not until 1888 (first 
stereoplotter by Deville) and 1901 (first stereocomparator by Pulfrich) 
that photogrammetric instrumentation began to be developed which to- | 
gether with the invention of the airplane (Brothers Wright 1903) and 
the first modern aerial camera (Messter 1915) started the widespread 
use of photogrammetric mapping and the development of a refined tech- 
nology. 
Remote sensing outside the visible light range, however, compared to 
photogrammetric techniques, is only in the beginning of its develop- 
ment. While the physical concepts for measuring and producing radiation 
had been developed (Bolometer by Langley in 1880, first radar design 
in 1933, first PPI radar 1940 in Britain), it was not until the first 
University of Michigan symposium on Remote Sensing in 1964 that the 
general use of newly developed hardware (the thermal infrared scanner, 
the radiometer,the sidelooking radar)for sensing was recognized. = &