Satellite based remote sensing emerged in the last decades from individual scientific applications to commercially available operational services. Recently, this tendency got a new momentum: Individual sensors with a certain technology and specification are more and more challenged by so‐called “constellations” comprising a number of sensors. They operate on the same orbit with frequent and regular revisits of any point on Earth.
Constellations offer new opportunities regarding information content: Frequent revisits allow frequent information updates or acquisition of information over large areas in a very short time. Radar constellations offer the possibility to provide information independently of any cloud and weather conditions. An integrated use of radar and optical sensors constellations can be used for change identification through radar and subsequent identification of changes through optical sensors. The integrated approach is also useful for maritime applications like oil spill, ship and ice monitoring for increasing revisit frequency as well as the time of information, as radar satellites are passing early morning or during the evening, while optical sensors image around noon.
Onshore Change Detection and Identification
Satellite sensor constellations are used to frequently monitor an area of interest onshore. The need for this application could come from the need to monitor land and sea borders, construction/infrastructure activities or any other kind of activity on the ground.
Figure: Construction Site Monitoring of Arena Cidade da Copa Stadions in Recife, Brazil
Pre‐ and post‐disaster damage assessment provides valuable, timely and cost effective geo-information for recovery plans as well as economical loss calculations. Pre‐disaster geo-information strongly depends on archive imagery available over the area affected by the disaster. Post‐ disaster images are dependent on quick and frequent satellite sensor acquisitions to get an overview on damages but also to plan and monitor the progress of any reconstruction activities. In both aspects, an operational constellation of optical and radar sensors strongly supports due to the ability to quickly build‐up data archives as well as quickly acquire new imagery even on a daily basis or independently of cloud when leveraging the use of radar technology.
Offshore Oil Spill Monitoring and Characterisation
The oil and gas industry operating oil production offshore requires intense oil spill monitoring in order to detect potential oil slicks very early to limit and control possible environmental consequences. In case of emergency, frequent monitoring with satellite imagery is the key in order to determine the temporal evolution of the oil spill for planning, execution and controlling of oil spill response measures. Furthermore, it is necessary to leverage different coverage and resolutions as quickly as possible. Moreover, the characterisation of the oil spill is important, e.g. nature, age of the oil spill as well as the quantification of some parameters like thickness and volume is requested for selecting the most appropriate oil spill response measures.
A full constellation of optical and radar sensors effectively fulfills all three requirements. First, the radar constellation serves to effectively detect and monitor an oil spill. The use of radar sensors is the most common remote sensing technique for this application: The oil spill is identified very effectively due to sea waves “flattened” by the oil slick on the sea surface and thus creating stronger reflection of the radar waves. The area affected by an oil spill is visible as a darker area, while the area around appears brighter with some structures in alignment with any kind of existing wave pattern.
The approach could be restricted, if there is either too flat sea or if there are too strong winds prevailing causing also waves in the area of the oil spill. A full constellation of up‐to‐date radar sensors, i.e. with right and lefts looking options could allow a frequent revisit of the same site.
Second, optical satellite imagery is useful for two reasons: Imagery acquired by a fully operational and commercially available optical satellite constellation could effectively be used to either have an independent data source for oil spill monitoring or to close any gaps within the radar imagery acquisition scheme, whenever cloud conditions are suitable. Optical imagery, additionally, can be used for the characterisation of the oil spill (e.g. concentration of the oil), especially if multiple bands exist.
Offshore Ship Detection and Identification
Maritime security is an important application for many countries with a sea‐borderline especially for those countries with a sea‐transportation and commercial trade interest. Piracy activities and illegal fishing is still a threat to our modern society.
Astrium offers an integrated optical and radar satellite based ship detection and tracking service, covering all steps from satellite tasking, data processing and AIS matching to the delivery of ship detection information in near‐real‐time down to 15 minutes after image acquisition.
Again, a two‐fold monitoring sequence is proposed: First, there is the need for ship detection. This can on the one hand be done by using the optical constellation (e.g. SPOT 5, SPOT 6, SPOT 7, FORMOSAT‐2, Pléiades 1A and 1B).
All sensors are available with high temporal revisit potential resulting in a dense temporal screening of the area of interest regarding any kind of ships. Information can be delivered as quickly as 1 hour after image acquisition, By combining results from subsequent, optical imagery even the bearing as well as the speed of the ships can be given. The other alternative for ship detection is to use radar constellations and its potential to automatically analyse radar datasets regarding ships. Delivery is possible in near‐real time up to 15 minutes after acquisition.
Second, there is the need for ship identification, whenever no other sources of information are available. Again, this can be done either by exploiting optical constellations or by using radar datasets of highest resolution like the new “Staring Spotlight Mode” from TerraSAR‐X.
The detection as well as the potential to identify specific targets with a combination of high resolution and very high resolution sensors is the perfect space borne technology responding to maritime security challenges.
As a consequence, if really optical and radar satellite constellations are exploited, a high revisit of any area of interest can be envisaged ‐ by using radar, even cloud independent.
Quick large Area Data Acquisition and Mapping
The use of a full constellation is also the answer whenever a large area needs to be mapped quickly. Due to the high revisit rate, e.g. with the constellation consisting currently of Pléiades 1A/b and SPOT 6 (SPOT 7 to come in December 2013) twice per day (once with VHR sensor Pléiades 1A or Pléiades 1B, once with SPOT 6 or SPOT 7) there is a high chance of getting cloud free imagery of an area of interest for further mosaicking. This is especially valid if sophisticated sensor systems with frequent uplink of meteorological information are used like in the case of Pléiades 1A/b and SPOT 6/7.
With the near future availability of SPOT 7 it can be expected that the whole coverage could even have been achieved in approx. half the time. As the optical constellation from Astrium Services also contains VHR satellites Pléiades 1A and 1B, also large scale image acquisition with very high resolution on smaller spots can be offered.
Integrated Use of Optical and Radar Constellations
The availability of constellations comprising optical and radar satellites opens new possibilities with regards to remote sensing and monitoring. First, detection and identification of objects, features or structures could be improved by an integrated use of optical and radar constellations: Optical sensors depend as passive instruments e.g. on solar radiation, sensor geometry and the reflecting properties of objects / features or structure on the ground. Detection and identification is possible depending on the resolution. Both can be done based on the spectral signature of reflection. Radar sensors are active instruments, i.e. electromagnetic energy is transmitted actively by the sensor and reflected by any kind of object / feature or structure on the surface. The reflectance depends e.g. on the type of the radar source, sensor geometry as well as dielectric properties of the reflecting object, feature or structure on the ground and has different spectral information. As a consequence, optical and radar sensors are sensitive regarding different surface characteristics. Thus, the capability to identify or to discriminate between different objects could is improved when both sources ‐ optical and radar ‐ are used in an integrated manner through an analysis of both spectral signatures.
The discrimination is even improved, if also the potential to frequently observe a certain area of interest is used: This allows also deriving a “dynamic signature” of a specific object or features. It helps as such also to detect and to discriminate between different objects as some objects are changing in their reflectivity and others are not. Radar with its potential to exploit also phase information of the reflectance especially has some advantages here: The identification of changes can be done through amplitude as well as coherence change detection. Furthermore, also subtle movements of the surface can be derived interferometrically with millimetric precision. Second, the constellation is useful if there is a need for frequent updates, e.g. when highly dynamic processes need to be monitored (e.g. construction or illegal cross‐border activities). Depending on weather conditions as well as monitoring demands, different sensors in either the same constellation or in the other constellation can substitute the other e.g. in case of any acquisition failures, cloud cover, acquisition conflicts. These can even be applied crosstechnological, i.e. radar complementing optical acquisitions or vice versa.
Third, constellations are useful to get reliably either optical or radar imagery from an area of interest, through their frequent revisit. This is important for larger mapping tasks: If image acquisitions are foreseen to be the base of a topographic mapping project, very often a certain acquisition window is requested in order to guarantee the same surface status within the map to be generated ‐ a constellation of optical sensors could help to stay within the requested acquisition window. A combination of optical and radar sensors could also support in this sense: The acquisition window could even be guaranteed, if also cloud independent radar imagery can be considered, e.g. to close gaps resulting from stronger cloud coverage.
Fourth, constellations offer the possibility to very quickly acquire up‐to‐date imagery again due to the high frequent revisit of any site of the earth.