Satellite Imagery Printable Version
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Satellite Orbits • Image Characteristics • Marine and Coastal Applications • Trends

Satellite Orbits

Satellites are launched from rockets which carry their payload beyond the influence of the Earth’s gravity. For those satellites which observe the Earth’s surface there are two main types of orbit.

Geostationary

Geostationary orbit, image courtesy NASA
Geostationary orbit, (image courtesy NASA).
In this orbit the satellite is placed 36,000 km from the Earth directly above the Equator. From this position the satellite orbits the Earth at the same speed at which the Earth rotates. If the satellite were observed from a fixed point on the Equator it would appear not to move. Most meteorology satellites (e.g. the European Meteosat series) fly in this orbit configuration. In this configuration the satellite always sees one part of the Earth only.


Near-polar

Near-polar orbit, image courtesy NASA
Near-polar orbit, (image courtesy NASA).
In this case the satellite is placed between approximately 600 km and 1000 km above the Earth and it orbits in an almost North-South direction near to the poles. Taking about 100 minutes to complete one orbit, the satellite can perform 14 orbits a day. On each orbit it views a different part of the Earth as it rotates beneath the satellite. Most of the satellites used for environmental monitoring purposes fly in this configuration.

If you observe the sky on a clear night a couple of hours after sunset, within five or ten minutes you will usually be able to see a few satellites as they fly north-south or south-north. They look like small moving stars.


Image Characteristics

Quickbird 1m resolution, London
One metre resolution image of central London from Quickbird
Most images are collected by custom built digital cameras on board the satellites. These images are then sent electronically to a receiving station located on the Earth. The highest resolution images, in which objects of less than 1 metre in size can be seen, are in black and white, the different shades corresponding to the different levels of reflectance from the objects being imaged. The satellites with the highest resolution currently available for civilian applications are IKONOS and Quickbird and are operated by commercial companies.

Some satellites carry an array of cameras or sensors that can capture images in different parts of the electromagnetic spectrum simultaneously. Also separate cameras can be designed to be sensitive to red, green and blue light. Therefore three images are collected. But a powerful feature of these cameras is that they can also capture an image outside the visible part of the spectrum to which the human eye is sensitive. For example cameras which are sensitive to infrared energy can be built. It is also possible to measure thermal energy and therefore collect images showing the temperature of the Earth. Images can also be collected in the microwave or radar part of the spectrum. Sensors that collect images in a number of different bands or different parts of the spectrum simultaneously are called multi-spectral sensors.

These images, known as colour composites, show part of the Cork Harbour area and were captured by the LANDSAT TM sensors in 2001. The composite on the top shows three images from the red, green and blue cameras combined. The one on the bottom shows three images from the near-infrared, red and green cameras combined.
Real Colour Composite
False Colour Composite

The amount of detail that can be seen in a satellite image is known as the spatial resolution. Satellites such as IKONOS have a high spatial resolution (~ 1m) and a lot of detail can be seen. The LANDSAT TM sensors have a resolution of 25 m while sensors such as MODIS have resolutions of ~1 km. Sensors with a lower spatial resolution can capture an image of a large area however the level of detail is much reduced. Usually the sensors which capture lower resolution imagery are able to acquire data on the same ground area more frequently, so in many applications of satellite data a compromise has to be reached between the level of detail required and the frequency with which an image is required..


Click here to view a MODIS image.

Click here to view a SPOT mosaic.

Click here to view a colour composite of LANDSAT images.

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Marine and Coastal Applications

Some of the more common uses of satellite image data include sensing of sea surface temperature, detection of algal bloom and pollution events and the study of ocean currents. Satellite imagery is also used to study coastal erosion and land use change in the coastal zone.

Sea Surface Temperature
El Niño
Quickbird 1m resolution, London
Average Sea Surface Temperature in December 1997
El Niño is a natural phenomenon, which occurs on average every three to seven years in the Pacific ocean. Generally the waters of the eastern Pacific near South America are much cooler than those in the west, near Indonesia. However, during an El Niño event, the waters around the Equator, off the coast of Ecuador and Peru, warm up significantly. The consequences can be quite severe, leading to flooding in South America, droughts in East Asia and upsets to the regular weather patterns over many parts of the Globe. In the graphic above, the red and yellow colours off the west coast of South America indicate abnormally warm waters during the El Niño event of 1997-1998. (Image, ESA).
Sea surface temperature (SST) has been measured for many decades, by both ocean going ships and buoys. In the past, ship measurements often involved simply throwing a bucket over the side and then measuring the temperature of the water collected. Fixed buoys can measure temperature on an almost constant basis, but only in specific locations. On the other hand, satellite measurements allow high quality monitoring of all ocean areas on a regular basis.

However, drawbacks of the method are that cloud cover hinders measurements and that the measurements are averaged over areas from hundreds to thousands of square meters.

One of the primary applications of SST is in the monitoring of climate and climate change. Study of time series of global SST can help scientists to see trends or changes in the temperature at regional or global scales. The climate phenomenon known as El Niño, which causes dramatic weather changes in the Pacific region, if not globally, has been successfully forecast and tracked using SST measurement.

Knowledge of SST can also be of use to the fishing community. Different fish favour waters of different temperatures. Sometime convergence zones between warmer and colder waters can be the location of nutrients and therefore fish.

Click here to see how SST varies around Ireland during the year.

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Algal Blooms

Phytoplankton are microscopic plants which grow in water and are a vital part of the food chain. In the same way as land plants bud and leaf in the Spring and Summer, plankton is sensitive to amounts of light and water temperature. Therefore, as days grow longer phytoplankton respond by initiating heightened photosynthetic activity, leading to the explosive growth of phytoplankton populations called a "bloom" or eutrophication.

These blooms can be seen by satellite sensors, because the presence of the plankton changes the reflectivity characteristics of the water and hence the image captured by, typically, multi-spectral sensors. Excessive plankton growth can produce dioxins or block sunlight to the ocean floor therefore causing the death of other plants and animals including shellfish and fish.



In the summer of 2005 a bloom of a planktonic species called Karenia mikimotoi took place off the west coast of Ireland. It was first seen in northwestern areas, but also appeared in the southwest later in the summer. This bloom led to the death of significant numbers of wild and farmed fish and shell fish. During the 2005 event, images from the SeaWIFS sensor were used by the Marine Institute to track development of the bloom.

Algal Bloom

These images from the SeaWIFS sensor have been processed to show ocean chlorophyll concentrations. The dark red colours (left) indicate an intense Karenia bloom off the coast of Donegal on June 25th 2005. Two days later (right) the extent of the bloom was reduced although a number of patches along the west coast are still visible. (Images: Marine Institute)

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Pollution

ENVISAT, oil slick
A huge oil slick from the Prestige tanker, which sank off the coast of Spain in 2002, appears in black in this ENVISAT radar image.
Satellites can also be used to detect and monitor pollution at sea. Sewage from outfalls and nutrient runoff can cause changes in the optical reflective properties of water or can lead to increased plankton growth and can be detected in a similar way to algal blooms as discussed above.

Images captured by radar sensors such as those flown on the European ENVISAT and the Canadian Radarsat platforms are useful for detecting oil slicks. Radar sensors can see through clouds and can capture images during both the day and the night, therefore they are particularly useful for applications in cloudy areas.
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Coastal Change

With over 50% of the world's population living within 200km of the coast, change in land use is an ongoing concern in these areas and with predictions that sea levels will rise by many centimetres over the coming century, there is a need to map and monitor coastal change. One European project, called LACOAST, looked at changes in coastal areas between 1975 and 1990 using a series of LANDSAT images, while the update to the CORINE land-cover maps in 2000 can also be used to study coastal land use change.
Click here to view a map of coastal change.

Erosion is a concern along soft shore lines, and a sequence of high resolution images over time can help to monitor erosion and accretion. One of the most dramatic recent uses of satellite imagery in coastal environments was in mapping the areas devastated in the December 2004 Asian tsunami.

Tsunami, Banda Aceh

These images from the IKONOS satellite show the devastation caused by the December 2004 Tsunami in the Banda Aceh region of Indonesia (Images: DLR, Germany).

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Trends

Satellite imagery was once the preserve of governments and military establishments. However, with the increase in the number of satellites, the falling costs of launch and the entry of commercial companies, satellite imagery is becoming ubiquitous. For many years TV viewers have been used to seeing satellite images as part of weather forecasts. One of the more recent developments has been that of products such as Google Earth or NASA World Wind. These allow Internet users to zoom into any part of the Earth using their web browser. Satellite images at varying spatial resolutions are provided, the most detailed ones allowing viewers to see individual buildings, streets and cars in their hometown. With the expansion of Geographical Positioning System (GPS) related technologies companies and individuals can add additional information at specific locations to these images.

There is now a move away from large, multi-functional satellite platforms to smaller and cheaper ones with a small number of specific sensors. Also, they are no longer the preserve of space agencies. Private companies, schools, universities and other organisations are building micro-satellites which are cheaper to launch and run and can return images or data for specific themes or areas.
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