and the theoretical cross-range resolution is

It is not practical to improve the resolution by using a smaller antenna because echoes would get weaker and various sources of error over a long section s would limit the resolution to a value higher than given by (12.21).

The reflectivity of the ground or sea surface depends on frequency, polarization, and the angle of incidence. At higher frequencies, echoes depend on the roughness of surface, whereas at lower frequencies, waves penetrate into ground and echoes are sensitive to the humidity of soil. By combining dual-polarization measurements made at different frequencies and angles of incidence, SLAR and SAR measurements provide multifaceted information for agriculture, forestry, geography, oceanography, and so on.

Scatterometers and altimeters are remote-sensing radar that do not produce images. A scatterometer is radar calibrated for the measurement of scattering. Scatterometers on satellites are used for the measurement of winds over the oceans. Sea waves correlate with the wind, and scattering in turn is sensitive to the height and shapes of the waves. Thus, by measuring scattering from different angles, the speed and direction of wind may be retrieved.

An altimeter is based on the measurement of the two-way propagation time of an echo. Altimeters provide information of the shape of the globe, ocean currents, ice coverage of glaciers, and so on.

12.8 Radio Astronomy

For a long time, only the optical window of the atmosphere covering visible light and the shortest infrared and longest ultraviolet waves was available for astronomers. In 1932, the American engineer Karl Jansky observed noise coming from the Milky Way as he was studying interference in communication produced by thunderstorms. In the late 1930s, the American amateur astronomer Grote Reber built a parabolic reflector and made the first rough map of the radio sky. After World War II, microwave technology became available to astronomers, and eventually radio astronomy developed into an important part of astronomy. A very important milestone was the observation of the interstellar neutral hydrogen at 1,420 MHz in 1953.

346 Radio Engineering for Wireless Communication and Sensor Applications

The atmosphere is nearly transparent (to zenith) from about 10 MHz to tens of GHz. Thus, the radio window covers about four decades of spectrum, whereas the width of the optical window is less than one decade. The reflection from the ionosphere sets the lower frequency limit. At millimeter and submillimeter waves the attenuation of gases becomes prohibitive, and telescopes must be placed on satellites.

The Sun, planets, gas and dust clouds of the Milky Way, pulsars, radio galaxies, quasars, and cosmic background radiation are subjects studied in radio astronomy [12]. Radio telescopes are also used for searching extraterrestrial intelligence (SETI).

12.8.1 Radio Telescopes and Receivers

Signals from radio astronomical sources are very weak. Thus, a large radio telescope and a sensitive receiver are crucial. A large telescope gathers waves from a large area and has a good angular resolution. The telescope should be situated at a high altitude and in a dry climate, especially if it is used at submillimeter wavelengths. The location should also be selected so that the level of man-made interference is low. The accuracy of the reflector surfaces should be better than l/10, which is a formidable requirement for a large telescope operating at high frequencies. Surface errors reduce the gain and deteriorate the radiation pattern of the antenna.

Radio telescopes are typically parabolic reflector antennas. The telescope of the National Radio Astronomy Observatory (NRAO) in Green Bank, West Virginia, which started its operation in 2000, is the largest fully steerable telescope. The size of this offset-fed reflector, shown in Figure 12.27, is 100m × 110m. It is usable even at millimeter wavelengths. Other large parabolic reflectors are the 100-m telescope of the Max Planck Institute in Effelsberg, Germany, the 76-m telescope of the University of Manchester in Jodrell Bank, England, the 64-m telescope of CSIRO near Parkes, Australia, and the 45-m telescope of the Nobeyama Radio Observatory, Japan, which operates up to 100 GHz. The 30-m telescope of the Institut de Radioastronomie Millime´trique at Pico Veleta, Spain, and the 15-m James Clerk Maxwell telescope at Mauna Kea, Hawaii, are so accurate that they can be used even at submillimeter wavelengths.

Due to gravitation, construction of steerable parabolic reflectors much larger than 100m is not practical on Earth. However, even fixed telescopes can cover large parts of the sky. The 305-m spherical reflector at Arecibo, Puerto Rico, is situated in a mountain valley. Moving the feed antenna allows observations up to 20° from the zenith. A large telescope can also be

Figure 12.27 Large steerable radio telescope of NRAO in Green Bank. (After: [13].)

made of a tilting flat reflector, which directs the waves to a fixed segment of a parabola.

Even the largest radio telescopes, as measured in wavelengths, are very small compared to optical telescopes and have correspondingly an inferior resolution. The angle resolution may be improved by an aperture synthesis, in which signals from several telescopes are combined. The Very Large Array (VLA) of the NRAO in New Mexico consists of 27 parabolic reflectors, each 25m in diameter. The antennas can be moved on three tracks 21 km long, so that a Y-shaped configuration is formed on the plain. By taking advantage of the Earth’s rotation and combining measurements made in 8 hours, an angle resolution equal to that of a continuous 40-km telescope may be obtained.