Remote Sensing

Remote sensing is the science and art of obtaining and interpreting information about an object, area, or phenomenon through the analysis of data acquired by a sensor that is not in contact with the object, area, or phenomenon being observed. There are four major characteristics of a remote sensing system, namely, an electromagnetic energy source, transmission path, target, and sensor.

The Sun is a common source of electromagnetic energy. It radiates solar energy in all directions. Earth reflects the energy from the sun and emits some energy in the form of heat.

Based on the energy source, remote sensing systems can be grouped into two types, passive and active systems. Passive remote sensing systems detect radiation that is reflected and/or emitted from the surface features of Earth. Examples are the Landsat and European SPOT satellite systems. Active remote sensing systems provide their own energy source. For example, the Radarsat-1 synthetic aperture radar (SAR) system has an antenna that beams pulses of electromagnetic energy towards the target.

The transmission path is the space between the electromagnetic energy source and the target, and back to the sensor. In the case of Earth observation, the transmission path is usually the atmosphere of Earth. While passing through Earth's atmosphere, the electromagnetic energy can be scattered by minute particles or absorbed by gases such that its strength and spectral characteristics are modified before being detected by the sensor.

The target could be a particular object, an area, or phenomenon. For example, it could be a ship, city, forest cover, mineralized zone, and water body contaminated by oil slick, a forest fire, or a combination thereof.

Electromagnetic energy that hits a target, called incident radiation, interacts with matter or the target in several ways. The energy could be reflected, absorbed, or transmitted. When incident radiation hits a smooth surface, it is reflected or bounced in the opposite direction like a light bouncing off a mirror. If it hits a relatively rough surface, it could be scattered in all directions in a diffuse manner. When incident radiation is absorbed, it loses its energy largely to heating the matter. Portion of the energy may be emitted by the heated substance, usually at longer wavelengths. When incident radiation is transmitted, it passes through the substance such as from air into water.

The sensor is a device that detects reflected and/or emitted energy. Passive remote sensing systems carry optical sensors that detect energy in the visible, infrared, and thermal infrared regions of the electromagnetic spectrum. Common sensors used are cameras and charge-coupled detectors (CCD) mounted on either airborne or space-borne platforms. In active remote sensing systems, the same antenna that sends out energy pulses detects the return pulse.

Present applications of remote sensing are numerous and varied. They include land cover mapping and analysis, land use mapping, agricultural plant health monitoring and harvest forecast, water resources, wildlife ecology, archeological investigations, snow and ice monitoring, disaster management, geologic and soil mapping, mineral exploration, coastal resource management, military surveillance, and many more.

One main advantage of a remote sensing system is its ability to provide a synoptic view of a wide area in a single frame. The width of a single frame, or swath width, could be 37 mi x 37 mi (60 km x 60 km) in the case of the European SPOT satellite, or as wide as 115 mi x 115 mi (185 km x 185 km) in the case of Landsat. Remote sensing systems can provide data and information in areas where access is difficult as rendered by terrain, weather, or military security. The towering Himalayas and the bitterly cold Antarctic regions provide good examples of these harsh environments. Active remote sensing systems provide cloud-free images that are available in all weather conditions, day or night. Such systems are particularly useful in tropical countries where constant cloud cover may obscure the target area. In 2002, the United States military initiatives in Afghanistan used remote sensing systems to monitor troops and vehicle convoy movements at spatial resolutions of less than one meter to a few meters. Spatial resolution or ground resolution is a measure of how small an object on Earth's surface can be measured by a sensor as separate from its surroundings.

The greater advantage of remote sensing systems is the capability of integrating multiple, interrelated data sources and analysis procedures. This could be a multistage sensing wherein data on a particular site is collected from the multiple sources at different altitudes like from a low altitude aircraft, a high altitude craft, a space shuttle and a satellite. It could also be a multispectral sensing wherein data on the same site are acquired in different spectral bands. Landsat-5, for example, acquires data simultaneously in seven wavelength ranges of the electromagnetic spectrum. Or, it could be a multitemporal sensing whereby data are collected on the same site at different dates. For example, data may be collected on rice-growing land at various stages of the crop's growth, or on a volcano before and after a volcanic eruption.

Two satellite systems in use today are the Landsat and Radarsat remote sensing systems. Landsat is the series of Earth observation satellites launched by the U.S. National Aeronautics and Space Administration (NASA) under the Landsat Program in 1972 to the present. The first satellite, originally named Earth Resources Technology Satellite-1 (ERTS-1), was launched on July 22, 1972. In 1975, NASA renamed the "ERTS" Program the "Landsat" Program and the name ERTS-1 was changed to Landsat-1. All following satellites carried the appellation of Landsat. As of 2003, there are seven Landsat satellites launched. The latest, Landsat-7 was launched on July 15, 1999.

Landsat-7 carries the Enhanced Thematic Mapper Plus (ETM+) sensor. The primary features of Landsat-7 include a panchromatic band with 49 ft (15 m) spatial resolution and a thermal infrared channel (Band 6) with 197 ft (60 m) spatial resolution. Like its predecessors, the Landsat-4 and -5, Landsat-7 ETM+ includes the spectral bands 1,2,3,4,5,6 and 7. The spatial resolution remains at 98 ft (30 m), except for band 6 in which the resolution is increased from 394 ft (120 m) to 197 ft (60 m). Landsat-7 orbits Earth at an altitude of 438 mi (705 km). It has a repeat cycle of 16 days, meaning it returns to the same location every 16 days.

Radarsat is the series of space-borne SAR systems developed by Canada. Radarsat-1, launched on November 4, 1995 by NASA, carries a C-band 0.022 in (5.6 cm wavelength) antenna that looks to the right side of the platform. The antenna transmits at 5.3 GHz with an HH polarization (Horizontally transmitted, Horizontally received). It can be steered from 10-59 degrees. The swath width can be varied to cover an area from 31 mi (50 km) in fine mode to 311 mi (500 km) in ScanSAR Wide mode. Radarsat-1 orbits Earth at an altitude of 496 mi (798 km) and has a repeat cycle of 24 days.

Several space-borne remote sensing systems planned for launch in the near future include the Radarsat-2 and the Advanced Land Observing Satellite (ALOS) in 2003, and the Landsat-8 in 2005.