Why satellite images are so useful?

What are the advantages of satellite images, when there are so many other sources of data on the Earth, notably from aerial photography, land surveys and geographical maps?

The answer is simply that satellite images can be obtained more quickly, are more accurate and more economical.  They are the most practical way of acquiring usable geographical information.

Earth observation by satellite is also known as remote sensing - how it works

A satellite image is not just a simple photograph taken by a camera fitted with a film.
Earth observation satellites acquire images using digital detectors similar to those of digital cameras.

The detector inside a satellite has thousands of sensors that measure the intensity of the electromagnetic radiation (the energy) coming from the Earth's surface and the objects that cover it: these are spectral measurements.

The spectrum reflected by the Earth is registered as a digital value, which is then relayed to the various receiving stations, where it is converted by computers into colours or light levels in the greyscale, creating an image resembling a photograph.

The reflectance measurements and the images created from them, give a highly accurate representation of the characteristics of the Earth's surface and the objects on it, just as they appear to the naked eye, in shape, size, colour and general appearance - this is called the "spatial content" of the satellite image.

More important still, the digital images show much more than simple spatial details: density, water content, chemical composition and other characteristics of the surface that are invisible to the human eye, emit specific energy that is reflected by the various wavelengths (or spectra).  Reflectance measurements can also reveal the mineral content of rocky outcrops, moisture in the soil, the state of health of vegetation, the structural composition of buildings and thousands of other details that are invisible to the naked eye - this is known as the "spectral content" of the satellite image.

The satellite image and the various sensors

The difference between spatial data and spectral data will determine the choice between the two types of optical satellite image: panchromatic or colour (multi-spectral).

Panchromatic images are acquired by a digital sensor that measures reflectance over a wide range of the electromagnetic spectrum (these ranges of the spectrum are often called "bands").  For most current panchromatic sensors, this single band generally covers the visible to near infrared parts of the spectrum.  Panchromatic data is represented as black-and-white images.

Multi-spectral images are acquired by a digital sensor that measures the reflectance in a number of bands.  For example, one set of detectors can measure the visible red energy that is reflected, while another set will measure the energy in the near infrared.  Two separate detection groups can even measure the energy in two different parts of the same wavelength.  These multiple reflectance values combine to make colour images.  Current multi-spectral, remote-sensing satellites measure the reflectance in three to seven different bands at once.

The panchromatic and multi-spectral imaging systems that we have just described use what are known as electro-optic sensors, which are the type most often used on satellites.  However, there is also another type of sensor, the Synthetic Aperture Radar (SAR), which is becoming more popular.

Electro-optic sensors are passive imaging instruments that measure the electromagnetic energy, basically coming from the Sun, which "bounces" off the Earth's surface.  These instruments are called passive because they do not transmit their own source of energy: they therefore only work in daylight.

SAR sensors, commonly called Radar, are active imaging systems.  They transmit a radar signal in the microwave range of the spectrum and measure the strength and other characteristics of the return signal reflected from the Earth's surface.  SAR sensors supply information that is slightly different from the spatial and spectral details obtained using electro-optic imaging. As SAR is active and operates at longer wavelengths than electro-optic systems, it can acquire images through clouds, fog, mist and darkness.

SAR is used in some of the same applications as electro-optic sensors.  It does, however, have specific uses for which it is excellent.  For obvious reasons, many SAR applications concern the cloudy equatorial regions, misty coastlines and polar regions that are often in total darkness.

For further information on Spot Image satellites, click on "Products & Services" on the Spot Image website: http://www.spotimage.fr/web/en/171-products-and-services.php

Images in the SPOT Gallery

The photos displayed on this site have all come from optical sensors.  Most of them come from Spot satellites (Spot 2, 4 or5) or from Formosat-2 or Kompsat-2 satellites.

Did you know?

SPOT stands for: Satellites Pour l'Observation de la Terre (Satellites For Earth Observation)

For further information on the technical aspects of satellites, click
on "technical information" on the Spot Image website: http://www.spotimage.fr/web/en/224-technical-information.php