Image Processing Reference
In-Depth Information
High-resolution radio telescopes have to be enormously large for two reasons:
the phenomenon of wavediffractionmeans that as an antenna gets smaller, its
spatial resolution or beamwidth grows larger, which effectively blurs the resulting
image. This effect is proportional to wavelength, and in the case of microwaves
with wavelengths of10 cm, antennae have to be tens of meters in diameter
to achieve sufficient resolution to study many compact radio objects in the sky.
Radio telescopes must also have large collection areas in order to be able to detect
distant sources, since more electromagnetic energy is concentrated on the detector.
Figure 3.13 shows the Parkes Observatory radio telescope in Australia. This 64-m
diameter dish weighs 1000 metric tons, and it images in the microwave band.
This chapter has thus far presented applications for radio imaging that are passive,
i.e., relying on radio emissions from the source or scene, rather than on active
illumination. Passive radio imaging of terrestrial objects such as buildings, people,
and mountains is possible in the millimeter-wave band, but becomes much more
problematic in the microwave band because the intensity of thermally generated
radio emissions is extremely low. We need active imaging technology to see
terrestrial objects in the microwave band (unless they happen to be microwave
transmitters). Active imaging of terrestrial objects with energy from the radio
band of the spectrum dates from the 1940s, whenradarwas invented to detect
enemy aircraft. Radar stands as one of the most significant advances in imaging
technology, and the inventors of radar are often credited with being the people
who won World War II for the Allies. Indeed, radar is an extremely powerful
imaging tool that makes it possible to see objects at long range, day or night, and
in any weather. Radar systems use multiple pulses of radio energy that reflect off
of a target and back to a receiver. The amplitude and frequency of the returning
echoes contain information about both the geometry and reflecting properties of
the reflecting object or material, as shown in Fig. 3.14. Radar on an aircraft or
Figure3.13 Parkes Observatory. (CourtesyofDavidMcClenaghan,CSIRO)
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