Detection of energy by sensors Source of EMR Sensor data output Transmission through atmosphere Transmission of sensor data to ground station Data processing and analysis Interaction of EMR with earth surface Fig.1: Stages of Remote Sensing History of Remote Sensing The knowledge about the history of Remote Sensing is necessary for better understanding of the subject and its scope, and also for future development, particularly for the welfare of human society. The development of remote sensing over time can be broadly divided into following six phases. Phase I (Up till 1920): Initial Phase Man always inquisitive about the things across a forest or a mountain, which he can not see directly. So, since time immemorial man has always tried to reach greater heights, such as tree tops, mountains etc. to observe phenomena of his interest on the earth surface, viz. to decide habitat places, farming and other day-to-day activities. This inquisitiveness to get a bird’s eye view prompted man to take photographs of earth from elevated platforms. So, the initial photographs of earth were captured from elevated platforms on the surface of the earth. However, the actual beginning of Remote Sensing can be traced back in 1958, when free balloons were used for photography by the French Gaspard Felix Tournachon (known as Nadar) to photograph the village of Petil Becetre near Paris. In 1860, a part of Boston and Massachusetts were photographed from a captive balloon at 1200 feet height in USA. In 1909,
the first aerial photograph was taken from an aero plane, piloted by Wilbur Write over Centocelli, Italy. World War I provided a boost in the use of aerial photography. During that time, aerial photographs were used for two purposes – spying and mapping. Phase II (1920-1945): Development of Platforms and Sensors Improvement in aero planes, cameras, films etc. resulted in the development of aerial photography during this period. The proper planning of flight for photographic purpose was also started. Topographical mapping was the main thrust of the aerial photography. However, a number of scientists like geologists, botanists, soil scientists, geographers began interpreting the photographs to get information of their interest, especially for development of natural resources. During this period photographic coverage were increased both on the large and medium scale. World War II gave a real boost to photo interpretation technique, which was widely used for military intelligence purposes. The mapping of strategic location, military targets and assessments of damages could be done accurately. Phase III (1945-50): Development of Teaching and Training After World War II, much emphasis on teaching and training of this technique was given due to previous experience of its wide use in different spheres. Many courses on Remote Sensing were started in reputed universities of United States and Western European countries. A commission on the utilization of aerial photographs was set up by International Geographical Union (IGU) in 1949. The members of the commission emphasized the need of knowledge of those parts of world which were not earlier photographed and also attention was given to cover more area by aerial photographs and techniques essential for interpretation. Phase IV (1950-60): Development of Instruments for Interpretation In this phase, the techniques of photo interpretation became much more an applied technique. A number of instruments was developed and introduced for interpretation during this period, which may be termed as a landmark in the progress of these techniques. It opened a new horizon for accurate and fast analysis and also for monitoring the changes. Hence a considerable advanced
interpretation was made in many disciplines such as Geography, Geology, Geophysics, Agriculture and Archaeology. Phase V (1960-1985): Significant Phase This phase is very significant in the history of Remote Sensing as artificial satellites were launched in the space for acquiring information of earth surface. Though two American satellites, i.e. Explorer I and II were launched in 1958 and 1959 respectively under Explorer and Discover Programme, they were not important from Remote Sensing point of view. On 1st April, 1960, one satellite of eight members of TIROS (Television and Infrared Observation Satellites) family was launched as a research and development project. As TIROS’s name suggested, the satellite carried two types of sensing devices – firstly, television, camera etc. which took picture of the visible spectrum; and secondly, infrared detectors which measured the non visible part of spectrum and provided information of local and regional temperature of earth’s surface. The supply of remotely sensed data of earth surface was greatly increased with the launching of ERTS-I (Earth Resources Technology Satellite) on 23rd July, 1972. It was placed in a sunsynchronous polar orbit about 600 miles above the earth surface. It makes 14 revolutions in a day around the earth and its sensors were covering a series 160 kms. wide strip. Then it was followed by ERTS-2 in 1975. With the launch of this satellite, the name of these satellites has been changed from ERTS-1, 2 to LANDSAT-1, 2 respectively. Four other satellites in these series were launched one after another in this phase, with improved cameras and sensors. Beside this, many other satellites were launched in the space by European and Asian Countries during this period. Phase VI (1985 onwards): Recent Development Phase In this period, Remote Sensing technique has been improved in two ways. Firstly, there have been developments of sensors which can use infrared and microwave spectrum other than visible spectrum to get information about earth’s surface. Secondly, there have been very important advances with respect to the platforms in which sensors are mounted. Besides, satellites have been launched for specific purposes and with specific capability. The ground resolution is continuously increasing till today. Hence, interpretation and mapping is becoming very easy, accurate and purposive. The European Radar satellite (ERS-I) launched in 1991 opened the
avenue for systematic global observation in the microwave region. The French Satellite ‘SPOT’ is producing the imagery to provide the three dimensional view under stereoscope. The satellite – IKONOS, launched on 24th September, 1999 has 1 m. resolution in panchromatic and 4 m. resolution in multi-spectral cameras. USA, France and India have planned a series of satellites, with improved capability, so that the users are assured continuity of data. Technical Components of Remote Sensing Platforms The base, on which remote sensors are placed to acquire information about the Earth’s surface, is called platform. Platforms can be stationary like a tripod (for field observation) and stationary balloons or mobile like aircrafts and spacecrafts. The types of platforms depend upon the needs as well as constraints of the observation mission. There are three main types of platforms, namely 1) Ground borne, 2) Air borne and 3) Space borne. 1. Ground borne platforms: These platforms are used on the surface of the Earth. Cherry arm configuration of Remote Sensing van and tripod are the two commonly used ground borne platforms. They have the capability of viewing the object from different angles and are mainly used for collecting the ground truth or for laboratory simulation studies. 2. Air borne Platforms: These platforms are placed within the atmosphere of the Earth and can be further classified into balloons and aircrafts. a. Balloons: Balloons as platforms are not very expensive like aircrafts. They have a great variety of shapes, sizes and performance capabilities. The balloons have low acceleration, require no power and exhibit low vibrations. There are three main types of balloon systems, viz. free balloons, Tethered balloons and Powered Balloons. Free balloons can reach almost the top of the atmosphere; hence they can provide a platform at intermediate altitude between those of aircraft and space craft. kilograms of scientific payloads can be lifted by free balloons. Thousands of Unless a mobile launching system is developed, the flights can be carried out only from a fixed launching station. The free balloons are dependent on meteorological conditions, particularly winds. The flight trajectory cannot be controlled. All these make extremely difficult to predict whether the balloons will fly over the specific area of interest or not.