Remote sensing for resources development and environmental management: Remote sensing for resources development and environmental management

Damen, M. C. J.

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Multivolume work

Persistent identifier:: 856342815

Title:: Remote sensing for resources development and environmental management

Sub title:: proceedings of the 7th international Symposium, Enschede, 25 - 29 August 1986

Year of publication:: 1986

Place of publication:: Rotterdam; Boston

Publisher of the original:: A. A. Balkema

Identifier (digital):: 856342815

Language:: English

Additional Notes:: Volume 1-3 erschienen von 1986-1988

Editor:: Damen, M. C. J.

Document type:: Multivolume work

Volume

Persistent identifier:: 856343064

Title:: Remote sensing for resources development and environmental management

Sub title:: proceedings of the 7th international Symposium, Enschede, 25 - 29 August 1986

Scope:: XV, 547 Seiten

Year of publication:: 1986

Place of publication:: Rotterdam; Boston

Publisher of the original:: A. A. Balkema

Identifier (digital):: 856343064

Illustration:: Illustrationen, Diagramme

Signature of the source:: ZS 312(26,7,1)

Language:: English

Usage licence:: Attribution 4.0 International (CC BY 4.0)

Editor:: Damen, M. C. J.

Publisher of the digital copy:: Technische Informationsbibliothek Hannover

Place of publication of the digital copy:: Hannover

Year of publication of the original:: 2016

Document type:: Volume

Collection:: Earth sciences

Chapter

Title:: 2 Microwave data. Chairman: N. Lannelongue, Liaison: L. Krul

Document type:: Multivolume work

Structure type:: Chapter

Chapter

Title:: SLAR as a research tool. G. P. de Loor & P. Hoogeboom

Document type:: Multivolume work

Structure type:: Chapter

115 Symposium on Remote Sensing for Resources Development and Environmental Management / Enschede / August 1986 SLAR as a research tool G.P.de Loor & P.Hoogeboom Physics and Electronics Laboratory TNO, The Hague, Netherlands ABSTRACT: In the early seventies for seme time an EME real aperture X-band SLAR with imaging on film was flcwn in the Netherlands. It shewed many new and unexpected phenomena, in particular over the sea. It soon became clear that for a closer investigation of these phenomena an absolute and digital system is necessary. Being simple and straightforward and still giving sufficient coverage and resolution for the research purposes under consideration, a real aperture system was chosen. Its final construction and implementation required the efforts of several institutes. Radiometric and creometric errors caused by unwanted aircraft motions can new be corrected during the data processing and resampling, resulting in a presentation of the data in a map corrected format. The radicmetric accuracy of the system and its internal and external calibration are discussed. 1. INTRODUCTION Already since the early sixties side-looking radars have been flcwn over the Netherlands with as a result the detection of many new and interesting phenomena (de Loor 1981). They delivered images on film of which the respective grey tones had only a vague undefined relation with the radar backscatter of the observed phenomena. Therefore, when the Dutch remote sensing program proceeded, it soon became clear that such simple imaging systems were not sufficient. The major advantage of radar - apart from the fact that it can be used day and night and through clouds - is the fact that it can produce absolute figures for the backscatter. Good examples in this respect are the windscatterometers in satellites as SEASAT and the ERS-1. Ihe Dutch remote sensing program emphasizes the various aspects of the interaction of the sensor (the radar in this case) and the objects observed in order to design optimum sensors and data handling procedures which require a minimum of signal transmission bandwidth and -time. Therefore groundbased measurements have been carried out on agricultural crops (de Loor et al 1982) and the sea surface (de Loor and Hoogeboom 1982) and a large data base is new available in the Netherlands. In this way an insight was gained in the use of radar for e.g. classification, monitoring, oil detection, etc. It was also found that the good use of radar asks for special procedures which require accurate observation systems. Therefore the choice was made for a SLAR system with digital data recording, internal calibration, and accurate absolute data handling. A real aperture system is the least costly, relatively simple and straightforward, and still gives sufficient coverage for research purposes. 2. DYNAMIC VERSUS GEOMETRIC RESOLUTION When speaking about resolution one usually means geometric resolution. Dynamic resolution ("contrast" in photography), hewever, is just as important. Measurements (de Loor et al 1982; de Loor and Hoogeboom 1982) have shewn that in the microwave window the variation in radar backscatter between areas of interest (e.g. due to waves at sea, due to different crop species on land, etc.) are small: a 10 to 20 dB at maximum. To differentiate between these different targets (e.g. between crops) the radar must have a high dynamic resolution: better than J. dB, This is a seyepe requirement in itself b-’h there is another problem due to the coherence of the illumination we use in radar. The targets mentioned are all compound targets with dimensions larger than the pixel size. Within a pixel they all contain many scatterers as e.g. the stems and leaves in a vegetation canopy. The reflection measured by a radar will be the vector sum of the reflections at the individual scatterers. Because of their movement this vector sum will vary with time. The-return signal to the radar so fluctuates with time and its strength varies according to a Rayleigh distribution. This means that single independent observations can vary considerably. This is the well knewn speckle in many radar images. To obtain an accurate value for the backscatter coefficient averaging over a sufficient number of independent observations is necessary. The more the number of independent observations, N, the better and smoother the grey- tone. See figure 1. The eye does better in this respect (Moore 1979) than a machine, since unconsciously the eye averaaps over neighbouring pixels. An image of pixels with №=5 to 10 looks nice but is difficult to handle for a machine (variation between pixels: +2.5 dB). So speckle is inherent to radar images and must be taken into account. Among others it can obscure texture or - wrongly - be seen as texture (Churchill and Wright 1984). Three approaches are possible to deal with it: averaging within a pixel, averaging per area, or a combination of both. In the Dutch SLAR averaging within a pixel is used. This averaging is either over 8 independent samples (geometric resolution 7.5 x 7.52m ) or over 30 (geometric resolution 15 x 15 in ) which gives an accuracy per pixel for a compound target of respectively + 2.5 dB or + 1 dB. This is insufficient for many applications, among others in crop classification, and therefore Hoogeboom (1986) also uses averaging per field. 3. MEASURES TAKEN IN THE DUTCH DIGITAL SLAR The above assumed that the independent observations taken by the radar are absolute values (are radiometrically correct). This requires a series of measures in the system, which indeed were taken in the Dutch digital SLAR. They will now be described shortly. This can best be done with the aid of the radar equation:

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