From a distance things become clearer. In 1858, French author and artist Felix Tournachon was the first, who shot an aerial photography from a crewed balloon. This was the beginning of the story of remote sensing – the acquisition of information of terrestrial objects from air or space. Today hundreds of civil and military satellites map our planet in various scales and send their records to ground stations. Latest satellite sensors are able to depict objects smaller than one meter. Airborne sensors supplement this information by high-resolution aerial photos (Figure 1).

Figure 1: Remote sensing in different scales. Left: NOAA-AVHRR satellite Image of the Alps (1996); smallest resolution: 1km. Middle: LANDSAT 7 satellite image of Adige Valley with Bolzano in the centre (2002); smallest resolution: 30m. Right: aerial photography of Bolzano (2000). Confluence of Talfera and Isarco with the EURAC building under reconstruction; smallest resolution: 1m

Remote sensing makes use of the fact that objects (plants, houses, water surfaces, air masses ...) reflect or emit radiation in different wavelength and in different intensity according to their current condition.
While early satellites and airborne sensors recorded only the reflection of visible light (like we do when we take a photo of something), modern sensors have "channels" for visible, near and far infrared radiation. Furthermore, satellite and airborne radar sensors actively emit radar rays that penetrate clouds and can provide information even in nighttimes. Beside images, elevation information can be delineated from pairs of radar sensors. Airborne high precision laser scanners measure terrain height even within a centimetre limit. Remote sensing data is usually recorded in digital format and is processed, enhanced and interpreted with the help of computers.

But what is remote sensing good for?

- Remote sensing can provide area-wide, up-to-date information at relatively low costs. This is important for tasks like weather forecast, mapping of the consequences of natural hazards (e.g. the 2004 tsunami, forest fires, landslides) and the update of thematic maps (e.g. land-use maps).

- Image analysis of remote sensing data can provide information, which is not deducible from visual interpretation alone. The so-called Normalised Vegetation Index (NDVI) for instance, which is based on a calculation of the reflection in the red and the near infrared spectrum, can indicate the condition and vitality of vegetation (Figure 2).

 
Figure 2: Image Analyses. Right: real-colour Image. Left: Normalised Vegetation Index (NDVI), June 2002, Adige Valley. Vitality of plants increases from red (in higher elevations) to green (in the valley bottoms).

- Remote sensing allows monitoring changes in the earth system. Satellite images have been recorded for more than 30 years and most sensors have a repetition rate (time between reoccurrence over the same spot) of less than 10 days. Meanwhile large remote sensing archives exist that document the changes, which our globe underwent in the last decades. Remote sensing data helped for instance, to demonstrate the expanding of Africa's desserts, the "greening" of the northern latitudes and the shrinking of the Antarctic ice-shield as a consequence of global climate change.

At EURAC's Institute for Alpine Environment, remote sensing is, amongst others, used for tracking historical land-use changes in the Alpine Space (see here).

What has started mainly as method of military reconnaissance (which it still is!) has become a powerful tool for detecting environmental changes and ecological problems. From a distance things become clearer. Solving the problems remains "ground work".

Before I flew I was already aware of how small and vulnerable our planet is; but only when I saw it from space, in all its ineffable beauty and fragility, did I realize that human kind's most urgent task is to cherish and preserve it for future generations.
Sigmund Jähn, first German Astronaut, German Democratic Republic.

Marc Zebisch

18.07.05