Earth observation satellites offer two types of images
Panchromatic images (PAN) – Black and white film, sensitive to all wavelengths of visible light.
Multi-spectral images (MS) – Images obtained not only in the visible, but also in the invisible range to the human eye, using sensors sensitive to different frequencies.
These satellites observe the earth from two main types of orbit:
Heliosynchronous quasi-polar orbit - Q uasi-polar orbit, quasi-circular of variable altitude. The movement of the satellite and the movement of the earth enable to acquire satellite data that depends on the altitude of the satellite. The altitude or inclination of the orbit may be combined so that the satellite is heliosynchronous (The satellite passes over a certain region always at the same solar time) in order to provide coverage every 12 hours.
Geosynchronous or stationary orbit – orbit where the satellite remains still in relation to the earth , its movement is synchronised with the rotation of the earth. This allows an almost continuous monitoring of the area facing the satellite.
Remote sensing
Remote sensing is based on the emission of electromagnetic radiation waves and their interaction with natural objects (clouds, landmasses and oceans, aerosols, etc.). The signals that reach the sensor on board the satellites are electromagnetic.
The electromagnetic spectrum comprises a wide range of wavelengths, classified according to the region: gamma rays, x-rays, ultraviolet, visible, infrared, microwaves and radio waves.
In order to understand the remote sensing technique, it is crucial to know the main characteristics of the emission spectra of both the Sun and the Earth: the main source of energy for the phenomena that occur on our planet is solar (or shortwave) radiation, which is concentrated for the most part in the visible region (between approximately 0.4 and 0.7µm). Upon interacting with the Earth-Atmosphere system, solar radiation undergoes a series of transformations and is emitted back to space in the form of terrestrial (or long-wave) radiation, which is mostly concentrated in the thermal infrared region (between approximately 4 and 100µm).
Visible region i mages - A re the result of the reflection of solar radiation by clouds and by the surface of the earth. The brightness in this type of image is an indication of the albedo of the bodies: light shades represent areas of high albedo and darker shades represent areas of low albedo.
Infrared radiation sensors - M easures the energy emitted by the surface and by the atmosphere of the earth. The quantity of energy emitted depends on the temperature of the radioactive source. Usually, in this type of image light shades represent cold areas and dark shades represent hot areas.
Image interpretation
Satellite data is usually indirect and requires interpretation. This is not a self-sufficient technique, i.e. it does not eliminate the need for other types of data. Image interpretation follows an intuitive process, based on three different aspects of interpretation:
Variations in the spectral field - Everything that can be distinguished in a visible or infrared image is the result of variations in the spectral field of the various surfaces present (clouds, ground, water). The analysis is made by associating any surface with the energy coming from it.
Variations of spatial field - Variations in the spatial field of a visible or infrared image are related to the different geometric characteristics which help to identify the various surfaces such as shape, size, texture, characteristic pattern and geographic location.
Analysis of the spatial field is done in conjunction with the spectral analysis.
Variations of temporal field - This feature of interpretation takes into account variations that occur with the passing of time.
Image manipulation
Image manipulation is a fast-developing field, in which new methods are being introduced every day to assist with interpreting and manipulating remote sensing data.
Animation: This technique consists of producing a temporal image sequence, frame by frame, through the usual animation process. The time interval is dictated by the spatial resolution and by the scale of the phenomena under analysis.
Zoom (amplification): This technique, which consists of amplifying certain specific sectors of the image, is used whenever greater detail of the phenomenon in question is required. This is particularly important whenever the complete image cannot be visualised with full spatial resolution. In such cases, the image is divided into sectors and smaller areas are selected in order to maximise spatial resolution.
Highlighting: This technique is used to increase the contrast and resolution of an image in order to make its interpretation easier. Each element in a digital image has a number (count), to which a different greyscale intensity level or colour can be attributed depending on the measured radiance. Highlighting consists of adjusting the greyscale (or colour) according to pre-determined rules in order to produce a new image. Highlighting techniques can be used to identify severe convective activity.
|
