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The airborne Remote Sensing technical system of the Chinese Academy of Sciences1

Tong Qingxi
The Joint Center for Remote Sinsing of CAS China


Abstract
During the last five years, by giving full play to the advantages of integrated technologies and multidiscipline research capabilities of the Chinese Academy of sciences, and Airborne Remote Sensing Research and Technical System has been developed. This system includes the airborne remote sensing information acquisition, information processing, information analysing and applications. In this paper the description of technical system was focused. The airborne remote sensing technical system consists of high-altitude platform, multi-sensors system, remote sensing pre-processing, the integrated monitoring, control and recording system, the air to ground information transmission system. The system is characterized by its integrity of sensors system, the extensive spectral coverage, the completeness of operation models and flexibility of data acquisition. In addition, it has the great potential both of remote sensing operation and technological development. This system has played important role in remote sensing of resources and environment, urban survey, surveying and mapping, engineering planning, geological and mineral exploration, castal survey, disaster monitoring, and other applications.

Introduction
Since the launch of the first Earth Resources Satellite of the United States, the remotes sensing technology has been gradually developed to practical use. The field of applications has been expended increasingly, its benefits have been remarkable. As an important component part of the remote sensing science and technology, the airborne remote sensing has obvious advantage and a indispensable role in recent years, when the satellite remote sensing has highly developed. The main advantages of its are the high flexibility, large load capability, multi-sensors combination and the ability of data acquisition with high resolution. It also can obtain images with large area coverage in case of flight over 10,000m. As an remote sensing system, airborne remote sensing is still in service for natural resource inventory, environmental monitoring, desaster monitoring and topographic mapping etc. in most countries in the world.

Recently, The development of high performance and high altitude light aircraft has caused the great attention in the world's remote sensing community. Such aircraft were equiped as special remote sensing aircraft in many countries due to its small in size, light in weight, excellence of take off, landing and flexibility of flight. The airborne remote sensing technical system of CAS is just based on two Cessna citation type aircraft to bring about the multi-sensors detection. It is suitable for multidiscipline and multiplications requirements. As technical intefrated system, the airborne system of CAS is much seldom outside China and its is just the contribution of CAS to the development of remote sensing in China.

The composition of system
The Airborne Remote Sensing System established in CAS consists of 3 parts:

  1. Airborne Based System
    Various optical, infrared, microwave, laser and other sensors are involved in this system. The sensors might be operated not only separately, but also in combination with each other to acquire the multi-information in accordance with the mission requirement and by control of on board Integrated Monitoring & Control System. In case of necessity the data can be transmitted real-timely and directly from aircraft to ground by on-board Data Transmission System.
  2. Ground-based System
    The main function of this System are the pre-processing, precision processing, analysing and mapping of information, acquired by remote sensing aircraft. The system consists of optical and photo-processing system, digital image processing system, information analysing and cartographic system and ground information receiving system.
  3. Research & Development and Technical Support System.
    Although, this system doesn't connect directly with the airborne information, but it is an un-separated important part of airborne remote sensing system. This system incorporated mainly the sensors calibration system, remote sensing spectral measurement and spectral data bank system and the test sites etc.
The complete constitution of the airborne remote sensing system established in the CAS is shown in Figure 1.


Fig 1. The block diagram of the Airborne Remote Sensing System of CAS


The important sub-systems
The construction of the airborne remote sensing system was conducted on the basis of technology of various institutes in the CAS. The most important subsystems are as follows:
  1. Platform Subsystem
    Two Cessna Citation S/II aircrafts with good high, middle and low altitude flight performance were selected as the airborne platform ( Fig. 2. ) Special modification of aircrafts make them to suit the need of operation of various remote sensing apparatus such as optical camera, scanners, microwave radar and others. The high altitude and speed performance of such platform improves the efficiency of airborne remote sensing in varying degrees.


    Fig 2. The View of Remote Sensing Aircraft of CAS


  2. The Sensors Sub-system
    Most of the sensors used in this system are developed recently by a member of institutions of the Joint Center for Remote Sensing of CAS. The spectral range of the sensors system covers all electromagnetic wavelength (Fig. 3.) Among the various sensors the great significance has achieved by the inclusion of the Airborne Synthetic Aperture Radar (SAR) and the supper-multispectral Scanner or the Imaging Spectrometer system (SMS). The SAR system has the performance of varying the polarization, while the SMS has 71 spectral channels in the spectral range from 0.4.12.5um. The main parameters of the SMS system are shown in Table 1.

    Table 1. The main technical parameters of SMS of CAS

    Spectral range
    (µm)    		 Numbers of bands Spectral Resolution
    (nm)    		 Spectral Resolution
    (mrad)
    0.4 ~1.1 32 20~25 3.0 ~1.5
    1.4 ~2.5 32 20~25 3.0~1.5
    8 ~12.5 7 400~800 3.0


  3. Real -time Data Transmission Subsystem
    This system consists of the on - board modulation and transmission system and field receiving, demodulation, display and processing system. The transmission distance can reach over 200 km. Over the recent years, the experiments of transmission of the flood and forest fire information in various places over the country have been carried out. Simultaneously with the transmission of images, also the geographical position (latitude and longitude), the altitude of platform and the time signal cab be transmitted to ground.
  4. Pre-processing and Processing System for Airborne Imagery
    This system has been designed and developed specially for processing the airborne multispectral Images. The main computer of this system is Micro VAXII. It has fast payback and pre-processing capabilities, also it has the general image processing capabilities such as radiometric and geometric correction, image enhancement and classification. The output production of this system is the standard CCT tapes, discs and photographs in format of standard original images , pre-processed images and precision processed images.

Fig 3. The View of Remote Sensing Aircraft of CAS


The pilot studies of system application
The airborne remote sensing system has great application capabilities. During the system development, a series of pilot application projects have been carried out, the capabilities of system have been examined and the application techniques and methods also have been developed.

  1. The Land Cover Studies
    During the past few years a number of projects connected with the land resources have been carried out in some regions of Heilongjiang province, Xinjiang, the Loess plateau, the North-China Plain, Tianjin, Hainan Island and other regions. According to the results of study in north part of Heibei province, the super small scale aerial photos of hight altitude color infrared photography (<1/100,000) can be enlarged 25 times for the land cover investigation.
    It is very efficient and economical
  2. Flood Monitoring
    The SAR system was flown above the large rivers area of China during the flood season. The flood monitoring was made in parts of the Valley of Yongdin river, Yellow river and , Yangze river. In each monitoring flight the area can be covered as large as tens of thousands km2. The monitoring experiment has proved that monitoring the flood by using active microwave system has excellent effect, the flood boundary has been shown very clearly. The advantage of such system is outstanding especially in cloudy and raining conditions.
  3. Monitoring the Forest Fire
    The system can not only be used for monitoring the forest and grassland fires, but also can determine the location of fire disaster. Experiments show that in case of flight altitude at 10,000m, the ground fire spots smaller than a few square meters can be detected, the detectable area can cover over 10,000km2/h.
  4. Sea Oil Pollution Monitoring
    The oil pollution is the main source, which led to server deterioration of ocean environment. The UV/IR dual channel scanner is very effective for monitoring the sea oil pollution and distinguishing the oil spill from ships. Due to the lower infrared emissivity of oil than the surrounding sea water, in the infrared band the oil is much higher than the sea water, thus, in this band, the oil displayed as black. On the contrary, in UV region, the reflectance of oil is much higher than the sea water, thus, in this band, the oil on the sea seems to be much brighter than the sea water. In addition to multispectral remote sensing, the microwave radiometer has also good results for sea oil pollution monitoring. In the last few years such monitoring has been done in Liudong bay, Bohai bay, Yellow and East China sea. The positive results have been obtained. This microwave techniques not only can reveal the existence of oil on the sea, but also show the thickness and the types of oil
  5. Geological and Mineral Exploration
    In the past few years, by making full use of the advantage of this system and by combination with other satellite remote sensing data, the scientists of CAS have carried out extensive studies on geological and mineral exploration in the vast land of Xinjiang. In Altay region of Nortern Xinjiang more than 10 prospective and target areas of gold and other metals have been discovered and a certain amount of anticipated ore deposits has been provided.
    In the Western Zhunger region of Xinjiang, by using the newly developed short-wavelength Infrared Multispectral Scanner (the Fine-Split Infrared Multispectral Scanner-FIMS), according to the absoprption features in spectral region of 2.0.2.5um of various rocks, minerals, alternations and mineralizations, the alteration and mineralization zones are determined. For this study the image processing and information extraction techniques are used. The ground verification proves, that this technology is very effective for mineral exploration at least in arid and semi-arid area such as Xinjiang.

    The studies in two regions mentioned above is an important practice for direct searching the minerals by remote sensing in our country, and laid the ground for such similar works in the future. In addition, an experiment for oil and gas exploration using this system has been conducted. The remote sensing detection of the "micro-seepage" caused by oil and gas deposit towards the earth surface provides a new nonseismic approach for the prospective assessment of petrolium exploration.
Discussion
The Airborne Remote Sensing System has integrated the main achievements in research and development of remote sensing technology of CAS, It not only has the characteristics of systematicness, advancement, integrity and practicality but also has the great potencial for R & D of remote sensing, which are reflected mainly in the following:
  • The multispectral and super-multispectral or imaging spectrometric technologies have important significance for the research of spectral signatures of earth objects and the mechanism of interaction between electromagnetic radiation and various objects.
  • Due to the all-weather, multi-polarization and multifrequencies characteristics the microwave remote sensing in this system have great significance for the development and application of next generation of microwave remote sensing.
  • The goals of development of remote sensing technologies are largely in application. The various technical combination has different effects: the combination of microwave remote sensing with information transmission, information processing and analysis has formed the airborne flood monitoring system, while the infrared remote sensing combined with information transmission, processing and analysing has formed the forest fire monitoring system. Consequently, this system enables to composed various professional applications technical system flexible according to the applications requirements and the characteristics of objects.
As the remote sensing technologies and the applications are constantly progressing, this system will be improved, enhanced and made even better benefits to serve all users in China and in the world.

1. This study started from 1986 is one of Scientific and Technological Development Program of National "Seventh-Give Year" plan. More than 10 Institutions were contributed in this joint project. There are: Institute of Remote Sensing Application, Institute of Electronics, Shanghai Institute of Technical Physics, the Center for Space Science and Application, the Ground Receiving Station,' Shanghai Institute of Optics and Fin Mechanics, Anhui Institute of Optics and Fine-Mechanics, Changchun Institute of Optics and Find-Mechanics, and Changchun Institute of Geography of Chinese scademy of Sciences and Shanghai Transportation University, Beijing University of Post and Communication Technologies, Qinghua University and Tainjin University.