Remote Sensing - a tool for environmental observation

Penetration depth tends to increase with longer

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Penetration depth tends to increase with longer wavelengths. A X-band radar (3 cm) will only penetrate the first leaves on top of the trees of a forest. Hence, such an image will provide information on the canopy structure of the forest. A L-band radar (23 cm) will penetrate much further and in that case the image contains information on branches and tree trunks. Figure 3.10 illustrates the effect of wavelength on different natural surfaces. Polarization describes the direction or orientation in which the electromagnetic radiation vibrates (e.g. in the vertical or horizontal plane). When radar radiation strikes the terrain, most of the energy returns to the antenna with the same polarization as the emitted pulse. This energy is recorded as like polarized or parallel polarized. However, terrain surfaces also tend to change the polarization of the electromagnetic energy. Most radar systems transmit polarized pulses and can register vertical and/or horizontal polarization. Radar systems carry often an indication of the type of polarization transmitted and the type of polarization that they register : HH, VH, HV, VV. The first term refers to the polarization of the emitted radiation, the second term to the received radiation. Hence, X HV refers to the X-band, H transmit and V receive. Figure 3.10 Backscatter of natural targets in the microwave spectrum (FAO, 1993).
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50 3.8 Radar Satellites Although earth resources satellites are in orbit since 1972 and provide many images of the earth’s surface, the data availability for applications in agriculture and renewable resources has been a problem. The main cause for the limited data availability is poor atmospheric conditions: cloud cover and/or rain or other bad weather conditions. Especially, the tropical zones with an almost continuous cloud coverage and the temperate climate zones with very frequent cloud coverage lack sufficient optical images for e.g. land cover surveys. Therefore, radar techniques which are independent of solar illumination and atmospheric conditions has been an issue of research and several radar satellites are planned or already available. Seasat Seasat was an experimental satellite designed by NASA to establish the utility of microwave sensors for remote sensing of the oceans, but Seasat images have also proven to valuable for terrain observations. Seasat was launched in June 1978 and unfortunately it ceased operation already in October 1978 because of a major electrical failure. Seasat had a circular, non sun- synchronous orbit at an altitude of 800 km with two interesting sensors aboard 1) a radiometer in two wavebands: visible (0.47-0.94 μm) at a spatial resolution of 2 km and thermal infrared (10.5-12.5 μm) at a spatial resolution of 4 k, 2) a Synthetic Aperture Radar or SAR with an L- band (23.5 cm with an HH polarisation and a nominal spatial resolution of 25 m and a depres- sion angle between 67 ° and 73 ° .
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