Today, the use of GIS technology is of growing importance for governmental and private economy bodies. A GIS is the main tool for collecting, managing, manipulating and visualizing data, referenced to a certain place on Earth. Already now most of monitoring and decision making processes are not feasible without modern GIS technologies.
However, the power of GIS technologies is limited by the unavailability of geo-data. While in larger scale applications for local governments or for limited areas land survey techniques are preferable, for larger areas (smaller scales), regions or even countries, remote sensing technologies are most appropriate.
RapidEye AG, Germany, launched its own satellite system August 29th, 2008. The RapidEye satellite constellation is the first, and currently the only, operational system that is able to repeatedly image larger areas in short intervals up to 24 hours. With its capability to produce orthorectified imagery with 5m resolution in 5 spectral bands (including a red-edge band), it is an ideal data source for monitoring tasks in the fields of environment, agriculture, and forestry.
The advantage provided by RapidEye is data availability and quality. The Company provides a wide spectrum of services for clients in our core markets: Agriculture, Forestry, Energy and Infrastructure, Spatial Solutions, Emergency Analysis, and Environment. A few solutions will be presented in detail: crop typing, field boundary extraction, crop condition assessment, forest monitoring, and change detection for infrastructure and environmental purposes.
Bring On EO
The development of Earth observation (EO) satellites allowed for high and very high spatial resolutions. Currently, systems with 0.6m resolution are available and others with as much as 0.4m resolutions are being manufactured. The use of satellite remote sensing applications in a number of application areas is still somewhat limited, especially in cases where information is needed at a predefined moment in time, where imagery is needed with a high repetition rate, or where image availability must be guaranteed. High repetition rates, for instance, are necessary for multi-temporal investigations, based on plant growth models, in agriculture and forestry, for monitoring applications, for ecological tasks, as well as for land cover classifications. As the image availability for optical systems is highly dependent on a lack of cloud coverage, only a high repetition rate can increase the probability of image acquisition. The temporal aspect of data acquisition, and therefore the probability of image availability in a given time frame, has an increasing impact on the acceptance and usage of remote sensing technologies.
The RapidEye System + Approach
Based on the limitations discussed above, the concept for the RapidEye satellite system was developed as a solution. The core capability of the system had to be a high temporal resolution in combination with high spatial resolution and large, daily area coverage. The relationship between ground resolution and area coverage was optimized to 6.5ms ground sampling distance, resampled to 5m pixel size, and a 77 km swath width for the sensor. With a system comprised of five satellites in a nearly polar orbit, any point on the Earths surface can be accessed every 24 hours. Each satellite performs 15 orbits per day, and the whole system can cover as much as 4 million square kilometers per day (within the mentioned specifications). Each of the satellites is equipped with a multispectral imager, designed as pushbroom-scanner with 5 CCD lines, one for each spectral channel. The integrated image memory is capable of storing an overall length of 1,500 km of image stripes in 5 spectral bands. During image sessions, the camera can be switched on and off to spread the image capacity over the entire orbit. Further, every satellite is equipped with a tilting system this enables RapidEye to point the satellite to an area located to the left or to the right from the nadir-looking footprint. Large continuous areas, such as the sample of Eastern Europe, can be fully covered in a few days.
In addition, the RapidEye satellites contain a sophisticated infrastructure for order handling, satellite control, data downloading and image processing, all located in the company headquarters in Brandenburg, Germany. A customer request, entered by electronic order system into, or by an operator, will be sent to the satellites via S-band connection to the satellites twice a day. During each session, the imaging program for the next seven days is uploaded into the satellites memory. This ensures image acquisition even in the event a satellite cannot be contacted for some time. Together with the area to be imaged and other special options, an up-to-date cloud forecast is factored into the acquisition planning.
After successful image acquisition, the data is downloaded to Svalbord, Norway, and subsequential transferred by land-line to the Brandenburg headquarter. Here, the data is subjected to the standard processing of as many as three levels. The highest is level 3, an orthorectified image tile of 25 x 25 km with 5m pixel size, ready to be loaded into any GIS. Optional atmospheric correction can also be applied. The tile size is an internal storing solution. Customers can order data for individual areas of interest and will be charged for the ordered area only.
RapidEye also provides a wide spectrum of value added products and services to its customers, each service being an individual solution for that customer.
The RapidEye system was designed to meet agricultural demands, especially the need for up-to-date information for precision farming technologies. RapidEye imagery with its high spatial, temporal and spectral resolutions is an excellent data source for this group. Thematic information still must be extracted from the data and can be prepared for immediate use in agriculture GIS.
An initial product is crop area detection. With the help of multi-temporal land cover classification approaches RapidEye specialists can detect and map arable land, distinguish between crop types, and determine field boundaries. This provides the base information for an agriculture GIS setup. Currently, the Company develops a web-based GIS this enables customers to visualize, edit, and store field data for further thematic investigations.
In addition, RapidEye is currently defining and operationalizing detection methods for further crop types, applicable for certain environmental conditions.
The RapidEye system with its red-edge spectral band is especially sensitive to chlorophyll in plant tissue, which is an important indicator for plant vitality. A repeated visit of plant chlorophyll content allows for the monitoring of the nitrogen nutrition status of plants.
RapidEye specialists use vegetation indices for the derivation of growth curves, which can be used to distinguish crop types and to assess crop condition and growth stages. Once the normal growing behavior of a certain crop type is known, repeated data acquisition enables the specialist to assess the plants status in comparison to what is standard for this particular area. Farmers receive helpful information in order to apply the right measures for optimal growing.
In this context, RapidEye can provide customers with ground cover maps. Such maps reveal what percentage of the ground is covered with green vegetation in various parts of a field. Areas with a lower percentage of ground cover may need special treatment, such as fertilizer application, irrigation measures, and pesticide application.
Ground cover maps offer an overview about plant status, but do not indicate reasons for either poor or good plant growth. The chlorophyll content highly correlates with the nitrogen content of the plant, which can be observed with the help of the red-edge band of RapidEye imagery. If the detectable relative content can be scaled by means of ground-based nitrogen measurements, an absolute value can be calculated, visualized as map layer, and later used for precision fertilizer application. The knowledge of plant status at different moments in the growing cycle and their comparison to standard values gives an indication about expected yields.
Of course, the results in the earlier stages of a growing season are less reliable than those acquired a few weeks prior to harvest. The accuracy of yield estimations further depends on the knowledge of other factors, such as meteorologic conditions, solar radiation, and so on. The RapidEye spatial resolution allows such investigations at field level. With the help of field size values, determined during creation of agriculture base information collection, real yield estimation values can be calculated.
An accurate estimation of yield will help to plan harvest, warehouse and food processors logistics as well as predicting the food supply and/or overhead, thats ready to export. In greater scale, this is an indicator for food prices in the world market.
Forestry is the another huge vegetation class. Frequent image acquisition for monitoring purposes doesnt actually make a great deal of sense for forests because of their low growing activity. Nevertheless, customers ask us for annual status images of their forests. Such imagery is needed, especially if damage appears, such as after a storm event. The content of an image, acquired after such an occurrence, will be compared to the previous status image to determine the area affected and wood volume loss.
For boreal forests, RapidEye can determine the stem volume of coniferous trees with 70 to 80 percent accuracy. This technology is of interest for damage assessments and for plantation evaluation, wood supply management for paper mills and so on.
Insect infestations detection is another area of focus imagery that is of great value, as it gives an indication of trees vitality. Infestation centers, distribution directions, and speed, can be assessed. This is valuable information for responsible and cost effective pesticide application.
For Monitoring + Change Detection Services...
Another strength of the RapidEye system is the frequent image availability, which makes the data very suitable for monitoring applications. With the help of repeated status images over a specified time period, changes between the different image dates can be detected. This helps identify trends, describes ongoing processes, and is the base for mathematical modeling with the goal of predicting future status of this process.
Changes result from different causes. If changes are related to spatial movements, the objects of interest need to be identified with high confidence in the data. The difference of coordinates describes the dynamics of location changes within a certain time period.
Changes in objects features, expressed in different spectral values, require a normalization of auxiliary conditions. Only in this case, the relevant changes can be detected. Due to the inhomogeneity of objects, a prior classification and masking will be necessary. This step results in change detection for whole classes of image content.
Appearing or disappearing objects can be detected, as well. This requires an initial detection of objects. Then, a comparison between identified objects can be completed. As a result, positive changes, or the appearance of new objects, and negative changes, and disappearing objects can be visualized at a map layer.
The RapidEye images pixel size allows detection of changes in objects greater 20x20m, or of linear objects wider than 10m. Infrastructural changes are often smaller. RapidEye imagery can be helpful in detecting areas of change, even if such is not possible to visualize exactly what has changed. This can be further investigated by ground teams, or with the help of other information sources.
Requests for environmental monitoring can also be fulfilled by RapidEye satellites. The vegetation cover is a good indicator of environmental impact. A regular image of an area of interest serves as a reference. Whenever a new image is acquired, changes to the last image can be detected. Areas of noticeable change point to abnormal impact to growing conditions. A highly productive vegetation area in the vicinity of a water pipeline in the desert indicates leakage.
A decreasing vitality of arable crops indicates plant stress, possibly caused by low soil moisture level. This is a first indicator for identifying progressive desertification activity. Abrupt changes, especially with sharp borders, speaks to accidental changes, such as land slides.
The RapidEye satellite constellation is a long awaited system and is now able to reliably provide up-to-date imagery in 5 spectral bands with 5m image resolution for a wide range of customers. The constellation overcomes the hinderance of using satellite imagery for frequent Earth observation — the uncertain image availability. Cloud cover remains an obstacle, but the system can attempt to acquire the requested imagery at frequent intervals. Simultaneously, large areas can be imaged.
The information content, represented by 5 spectral bands, each with 12 bit radiometric resolution, is tremendous. With its high standard processing level Rapid Eye imagery is a preferable data source to be used in GIS, especially for agriculture, forestry, environmental and monitoring applications, as well as for topographic mapping and other spatial applications.