Radar

Radar, like sonar and seismology, uses a man-made pulse of radio energy to map distance based on the length of time it takes the pulse to return from the source. Radar (short for "Radio Detection and Ranging"), which can be airborne or spaceborne, has greatly changed the way we see the land and ocean surfaces. Radar is based on the principle of sending very long wavelength radiation (called microwaves) from an antenna, and then detecting that energy after it bounces off a remote target. The wavelength of the microwave, its polarization (vertical or horizontal orientation) and strength can be controlled at the source and measured when it returns. Many common land-cover types and materials affect the polarity and strength of the radar return differently, which helps in their identification.

Radar is something that is in use all around us, although it is normally invisible. Air traffic controluses radar to track planes both on the ground and in the air, and also to guide planes in for smooth landings. Police use radar to detect the speed of passing motorists. NASA uses radar to map the Earth and other planets, to track satellites and space debris and to help with things like docking and maneuvering. The military uses it to detect the enemy and to guide weapons. Meteorologists use radar to track storms, hurricanes and tornadoes. You even see a form of radar at many grocery stores when the doors open automatically! Obviously, radar is an extremely useful technology.

In 1927, while working for the Technical University in Darmstadt, Dr. Hans E. Hollmann developed and built the first ultra short wave transmitter and receiver for centimeter and decimeter waves. This work led to the development of the world's first micro wave tellecomunication system. In 1928 he graduated with a thesis on "Barkhausen" electron oscillations. At this time a scientist by the name of Hans-Karl von Willisen became good friends with Hollmann because he too was interested in ultra short waves. The two, with another man by the name of Guenther Erbsloeh, started a company called GEMA. Hollmann and von Willisen were the technical experts and Erbsloeh was the manager. GEMA built the first radar transmitter in the autumn of 1934 for detecting ships. The radar operated on 50 cm wave length and could find ships up to 10 km away. In the summer of 1935, a pulse radar was developed with which they could spot the ship the "Koenigsberg" 8 km away.

This radar unit used the "Braunschen" tubes and had an accuracy of 50 m. A magnetron had been tried but the frequency was not stable, and as such, tubes were used. A wave length of 60 - 80 cm was used so the sender and receiver could be spaced close together. An airplane at a height of 500 m and a distance of 28 km could be seen. By 1935, they had built the first successful radar unit. For ship born application, they used the same radar system, except that the ship born system operated on a wave length of 80 cm instead of 1.8-2.0 m. The land based radar was called "Freya" and the sea born radar was called "Seetakt."