Airborne remote sensing has come a long way since its beginnings as photography from a balloon. Military demands for covert information first spurred advances in imaging technology. Increased public concern over environmental issues has heightened the need to incorporate these remote sensing methods into observing and monitoring global environmental changes. Today products from many military developments are finding their way into commercial markets because of peace initiatives and demand for more technology transfer from government sponsored programs.
       Along with the advances in instrumentation, continued improvements in communications technology, data management and geographical information systems (GIS) are building a remote sensing version of the information highway across interdisciplinary and international boundaries. Environmental concerns are no longer limited by geography. Everyday operational needs and demands by industry and government are making remote sensing a cost effective option for providing a variety of products on global, regional and local scales.
      Remote sensing is divided into disciplines based on where the data are collected: from the ground, air or space. Collecting these data require different platform and/or sensor types. The type of platform depends on the requirements for spectral and radiometric resolution, extent of coverage, periodicity, repeatability, dynamic nature of the phenomenon to be observed, availability and cost factors.
      Each type of platform has its place in environmental monitoring. In this article, airborne and satellite systems are compared because these appear to best satisfy the needs of broad environmental demands.
      Satellite platforms were first touted for their potential use in environmental monitoring. They offer advantages of broad coverage, synoptic viewing and a stable/predictable orbit from which to view the Earth. Viewpoints vary on how costly the imagery products are, but satellite imagery is available to any individual or organization with the computing capability and expertise to handle the processing. Platform limitations include the inability of satellites to provide continuous areal coverage or to collect imagery during poor weather conditions. Satellites are also expensive to build and complex to launch and reorient in orbit.
      By comparison, airborne platforms generally have lower operating costs, are more easily maintained, provide higher resolution and allow greater flexibility to conduct routine surveys, special missions or respond to emergency events. Airborne surveillance systems include general purpose platforms such as a single engine airplane with a large format survey camera to high altitude ER-2 planes or even the space shuttle with highly sophisticated sensors. The particular cost effectiveness of airborne systems is best exemplified in conventional fixed wing or rotary airframes. Many possible examples have proven themselves to be useful smaller aircraft for remote sensing, including the Cessna 404, Islander, Convair 580, etc.
      Airborne remote sensing platforms are often purpose built. Instrumentation is frequently selected with a specific customer in mind. Optimizing a sensor system for a particular job usually maximizes the performance for that single task. However, purpose built systems can become costly. Their specific purpose only requires occasional use and they are not flown in more regular operations or in response to more diverse market needs. Aircraft can be more efficiently utilized, if for example, they serve as platforms for instrumentation testing and evaluation surveys or other contract work between their special purpose missions. It is possible to conduct multiple projects with one platform, share flight time and reduce the cost to any one user. This takes administrative effort to coordinate but is especially valuable if the overall quality of imagery information is enhanced or system use increases because of it.
      The Baseline Remote Sensing System (BRSS), developed in 1994 for the Marine Spill Response Corporation (MSRC) by Atlantic Reconnaissance Ltd., Coventry, UK and TerraMar Resource Information Services Inc., Mountain View, Calif. is an example of a system primarily designed for oil pollution monitoring. In addition to oil detection, the sensor and processing systems would be suitable for a variety of environmental monitoring applications on regional and local scales.
      The BRSS, housed in a Shorts SD 360-300, is a research and development "test bed" designed for performance evaluation of hardware such as new sensors, for developing the technology needed to better integrate surveillance system components or software such as new imaging algorithms or for atmospheric analyses. Research on how reconnaissance aircraft are best used to support response operations concentrate on how to increase daylight cleanup efficiencies and how to extend operations into the night and during periods of reduced visibility. It is generally agreed that the availability of such technology can make a significant difference in the effectiveness and efficiency of spill response. Cost studies indicate that cleanup costs could potentially be reduced by two-30 times through more precise targeting of concentrated oil patches.
      The aircraft for the BRSS R&D program was purposely selected to be larger than what would be used for operational missions because its R&D mission required a flexibility which allowed it to easily accommodate additional sensor packages. The Shorts 360 fuselage was modified for maximum adaptability and extended range to accomodate both its R&D and operational missions.
      The BRSS sensor suite consists of the newest proven, commercially available, high performance sensors. They include a Terma Model 4001 X-band side looking airborne radar (SLAR), Rank Taylor Hobson Talytherm 890 infrared (IR) 8-13um sensor, Photonic Sciences ultraviolet (UV) 300-400nm sensor, high resolution Panasonic S-VHS color video and a Kodak Model DCS 420 digital still camera.
      The types of sensors in the BRSS system are similar to those used in Europe and Canada for oil spill surveillance. The innovation incorporated in the American BRSS design, lies principally in the development of a common processor which uniquely manages the way images are collected and combined with aircraft position data. It integrates those data with sensor models, image processing algorithms and even external databases to automate and streamline image interpretation and decision support. After processing the imagery, extracted information can be rapidly transmitted to a ground station.
      The processor system consists of a Sensor Integration Processor (SIP) in the aircraft and a portable Ground Support Station (GSS). An additional ground system is used for training system users. The two ground systems are equivalent in their basic software and hardware components, but are individually optimized for their separate functions. The software used for the system is known as IDIMS (Interactive Digital Image Manipulation System). It was originally designed for image analysis of Landsat and other satellite data. The Global Data Catalog (GDC) is a spatial database management software which stores, locates, and retrieves imagery.
      By selection and design, the processor is an extremely powerful integrator of airborne imaging data for the detection of oil on water and other applications. As with most technology, with power, comes complexity. To maintain system performance and yet support oil spill response needs, a system within a system was developed. The "inner" system is transparent to a user, but is designed to help manage an operator's workload by automating many of the system functions associated with data collection, processing, analyses and use. The "inner" system also prioritizes data acquisition and facilitates data management based upon their importance during the spill response. This helps reduce the pressure to perform many tasks similtaneously as usually required during an emergency response.
      All sensor, position and flight data are stored by the aircraft scientific processor. The operator generally integrates the background, sensor and aircraft data onboard to produce preliminary annotated images. This processed information is transmitted via a Harris HF radio to a ground support station where it provides useful aerial views of land or sea operations. To reduce transmission time, background data are not usually sent with the new imagery data. They are referenced to similar data resident in the ground station. Additionally, when the aircraft is positioned to assist a particular vessel operation, interpreted sensor and position data can be communicated directly to vessels at sea.
      The BRSS participated in a number of experiments and exercises in 1994, to demonstrate its utility for oil spills and other purposes. These include: 1) an aerial dispersant spray test involving the United States Air Force Reserve (USAFR), United States Coast Guard (USCG) and Texas General Land Office (TGLO); 2) a surface current radar experiment, TexSonde '94 with Imperial Oil and TGLO; 3) a study of natural oil seeps with the Naval Research Laboratory (NRL), Amoco and TGLO; 4) performance testing of IR sensors with USCG and TGLO; 5) an aquatic vegetation detection feasibility study; 6) landfill fire surveillance; 7) resolution of features within urban environments; 8) oblique color video surveys of the U.S. east and west coasts; and 9) an experimental oil spill test in the North Sea with the UK Marine Pollution Control Unit (MPCU) and Exxon.
      Integration technology has made airborne remote sensing a more cost effective spatial data acquisition and management method. The pace of development has accelerated so much that marine airborne remote sensing as a field of study is now large enough that it warranted its own conference last year. The First International Airborne Remote Sensing Conference and Exhibition, in Strasbourg, France (September 1994) was attended by about 600 practitioners.
      The use of remote sensing technology and need for ongoing development can be summarized for oil spill response with the following observations: 1) All sensors have shortcomings. Multiple sensors validate and add more detail to images than single sensors. Cost effectiveness and value added must be considered in choosing sensor combinations; 2) Each application is unique. It is not feasible in terms of cost, resources and capability to be prepared for all contingencies; 3) Even with inherent limitations, remote sensing offers a cost effective way to satisfy information needs; and 4) Critical considerations in successfully applying surveillance technology must include both tactical responses at a spill site and strategic planning support at an incident command center.
      The direction for the remainder of this decade looks toward advances in more affordable and more portable sensors and computer hardware. The development of image processing and analysis techniques to interpret and understand existing and new data requires even further expansion to keep pace with technology. Major applications for both airborne and satellite systems will be directed toward understanding the many diverse effects humans are having on the environment. Actual applications of this technology are currently emphasized much less than scientific experiments and need to be increased. The usage of these new integrated sensor and processor systems in the future will depend upon continued practical demonstrations of their capabilities, utility and cost effectiveness such as those now being conducted with the BRSS.

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