Science and technology of imaging from space

Imaging techniques from space, started mainly as a military reconnaissance tool, have come a long way from its early concepts to meet the present day needs of providing a precise metrology of the Earth processes and its features. The first part of the paper deals with the basic concepts of imaging from space. In this, the energy source available and the effect of atmosphere on the radiation are elaborated. Also the signatures of objects for identification and their characteristics in spatial and spectral domain are briefly touched upon. The second part of the paper deals with the challenges in realizing a satellite with limited volume and weight constraints by giving examples of very innovative approaches in the design of the optical systems for telescopes. The challenges associated with high resolution imaging and resulting high amount of data handling through appropriate data compression techniques are highlighted. The work presented here has been primarily carried out at the Indian Space Research Organization (ISRO). The paper concludes with an assessment of the futuristic direction that include improved algorithms for parameter retrieval and improved classification accuracies, improved models for atmospheric correction, new approach in artificial intelligence and expert systems for feature based classification, improved data compression techniques, ultra-light weight mirrors and adaptive optics, optical materials and detector arrays with built-in read out in the infrared region.