Here is an interview with one of the leading experts in satellite ocean monitoring, Andrey Kostyanoy, PhD in physics and mathematics, Chief Researcher of Shirshov Oceanology Institute, professor of the University of Liège. The topic of conversation is ocean remote sensing.
Which are the methods of studying the ocean from space?
Satellites have been long, profoundly and actively utilized for ocean monitoring and nowadays make an essential part of the Global Ocean Observing System. This system includes nearly 10,000 earth-based stations; 1,000 upper-air stations; over 1,000 ships; 1,200 drifting buoys; 200 anchored buoys; 3,000 diving buoys “Argo” and nearly 3,000 commercial airplanes. The system’s space group consists of 6 synchronistic satellites, 5 polar orbiting satellites, 5 active satellites for environmental monitoring and about 50 other satellites. The most informative method of Earth remote sensing is utilization and thematic analysis of the images made by the space-vehicle facilities of different frequency range. Spacecrafts equipped with remote sensing devices (radio detectors, scatterometers, radiometers and optical devices) are specially placed into orbit to gather comprehensive geophysical information necessary to estimate environmental situation and natural resources research.
Various active and passive sensors operating in visible, infrared and microwave electromagnetic spectrum regions are utilized for measuring the four principal characteristics of the ocean: colour, temperature, ocean surface height and roughness. Knowing these characteristics allows to address different tasks:
- Colour scanners define the spectral property of water-surface radiation, which provides information on various optical characteristics of the ocean surface: water transparency, suspended materials concentration, chlorophyll concentration, green scum, etc. The optical spectrum also allows to observe ice accumulation and ice edge, icebergs and – under certain conditions – oil pollution.
- Infrared and microwave sensors are used to measure ocean surface temperature. Unlike infrared radiometers and optical scanners, passive microwave sensors allow to measure ocean surface temperature in overcast sky (though with less precision and spatial resolution). Infrared radiometers also allow to monitor ice accumulation and ice edge, while microwave radiometers can measure surface water salt content (these measurements are still not precise enough to be utilized for most oceanography needs).
- Active microwave sensors (altimeters, scatterometers, radars with synthetic aperture) are utilized for measuring sea surface altitude, oceans and seas level, wave height, near-water wind speed, ice and oil pollution.
Which are application areas of the ocean remote sensing?
The application scope of the remote sensing data provided by different satellites monitoring seas and oceans is very wide and is not limited to the following list of particular tasks:
- real-time mapping and studying ocean temperature behaviour;
- real-time mapping and monitoring suspended material habitats;
- phytoplankton concentration and distribution analysis to determine bioproductivity of a sea and green scum;
- monitoring ice and snow situation on land and on the water:
- monitoring water level and dynamics of various ocean portions.
- monitoring big-river level, flow and delta;
- environmental control;
- monitoring ecological condition of land and water areas in the regions of extraction, processing and transport of oil, gas and other mineral resources;
- coastal zones control, observing ships, detecting and tracking oil pollution;
- ongoing monitoring of oil and gas facilities construction and real-time observation of their current state;
- monitoring cloud cover, dangerous atmospheric phenomena, wind speed and wave height;
- monitoring natural and man-made disasters, forest fires, floods.
What are the advantages and limitations of satellite monitoring?
Ocean satellite monitoring has a variety of advantages over marine or aerial observation methods, including global planet coverage, real-time mapping of huge water areas, mapping of neighbouring states water zones, the highest data acquisition speed, possibilities for repeating observations on a day-to-day basis, high spatial resolution (1-60 km), complex and multisensory data, real-time complex monitoring in any part of the World Ocean, using the same satellite information to address subtasks and complementary problems overland (fires, floods, desert advancing), considerably low cost of satellite monitoring in comparison to marine observation. Satellite monitoring also has certain disadvantages and limitations, and specialists are aware of them.
Which satellites are used for monitoring coastal areas?
Satellite monitoring of ocean and closed seas coastal areas is the essential method of their ecological state control. Based on receiving digital data from radiometers, scanners, spectrometers, radars, altimeters, scatterometers installed on various satellites (NOAA, Terra, Aqua, TOPEX/Poseidon, Jason-1, Jason-2, GFO, Envisat, Radarsat-1, Radarsat-2, ERS-2, QuikSCAT, Landsat-1–7, IRS, Kompsat-2, EROS-A, IKONOS, SPOT-1–5, QuickBird, Formosat-2 and many others), they allow to get information on sea surface temperature field, suspended materials, chlorophyll concentration and water surface optical features, oil pollution, as well as sea level anomalies, flow and wind speed changes – with high spatial and time resolution.
For more detailed information on active satellites and those planned to be launched, you may visit websites of NASA, ESA, CSA, as well as some other foreign and Russian organizations, such as IOCCG, SPUTNIK, SOVZOND, etc.
In view of the Gulf of Mexico oil platform accident in April 2010, please tell us about satellite monitoring of oil pollution.
Radiolocators with synthetic aperture are essential for monitoring ocean oil pollution. The information they provide allows real-time tracking of endangered water zones ecological condition, estimating pollution area and level, as well as studing physical processes that determine wastes flow to the monitored water zones and sometimes even detecting pollution origin. Possibility to monitor huge water areas as well to repeat observations of the same region within intervals up to 12 hours makes satellite monitoring one of the most cost-effective, rapid and objective methods of the ocean ecological observation.
Oil films suppress short capillary-gravity waves and provoke local changes in water surface roughness. The difference between signals intensity from film-covered and unpolluted areas allows radiolocators to determine oil patches with high spatial resolution (25-75 m). Radiolocators with synthetic aperture have advantages over optical devices used in airplanes as they can cover larger water areas, independent of cloudiness and illumination (day/night). This equipment is installed on Envisat and ERS-2 satellites of the European Space Agency and Radarsat-1 and Radarsat-2 of Canadian Space Agency.
In March 2002 Envisat satellite was launched that has 10 devices for real-time monitoring of the ocean, ice, land and atmosphere. The satellite has a 35-day flight recycle, but the wide field of view of the equipment’s better part allow it to observe any location on our planet with intervals from several hours to several days. Advanced Synthetic-Aperture Radar (ASAR) is used for monitoring oil patches and ice on water surface, measuring various ocean phenomena (currents, fronts, whirlwind, internal waves), determine ship location, oil and gas fields, etc. This information is actively utilized by coast guard, national environmental control agencies, oil, ship, building and insurance companies, as well as scientific organizations.
Shirshov Oceanology Institute has a unique diversified experience in complex satellite monitoring of the Baltic Sea, Black Sea, Azov Sea, Caspian Sea, Aral Sea, Mediterranean Sea, Barents Sea, Kara Sea, as well as the four oceans. For instance, the researchers of Shirshov Oceanology Institute together with the scientists of Space Research Institute, Geophysics Centre, as well as Marine Hydrophysics Institute (Sevastopol) have developed an effective complex (multisensory and multidisciplinary) approach to real-time monitoring of Russian seas oil pollution.
For the first time, this approach was implemented in 2004—2005 in the south-west Baltic region, when we (under the contract with Lukoil – KaliningradMorNeft) have actually organized the oil pollution monitoring service that worked 24/7 during 18 months. Later on, this complex approach was also used in Azov and Black Sea, Caspian Sea and Gulf of Finland. The results we received through 2004—2011 proved the efficiency of complex satellite monitoring of the Baltic Sea, Black Sea, Azov Sea and Caspian Sea. This technology and the experience we acquired can be easily transferred onto other seas of the Russian Federation and the World Ocean regions.
Oil pollution, south-west portion of the Baltic Sea, 25 August 2005.
What are other sources of sea pollution?
Besides oil pollution, there is a danger of suspended materials entering into seas as a result of industrial activities on the water area and coastal zones, such as pipeline construction (e.g. Nord Stream in the Baltic Sea), cabling, dumping, explosions at the sea-bed, etc. Huge amount of suspended materials also comes with river flows and gulf waters. Increased roughness of shallow waters also provokes increase in suspended materials concentration. All this leads to secondary pollution, increased turbidity, decrease of photoactive radiation, bioproductivity, changes in population structure, kill of sea-bed inhabitants.
Suspended materials concentration in the eastern portion of the Gulf of Finland, according to MERIS-Envisat, 12 July 2010. Roiled waters of Finland coastal zone crossing the route of Nord Stream pipeline (black line).
Eutrophication (excessive increase of biogenic elements) of surface waters provoked mainly by abundance of nutrient substances (phosphorus and nitrogen) is an important problem that is getting more and more acute. The consequence of eutrophication is rapid blooming of blue-green algae (many of them are toxic) which are spreading over the Baltic Sea and has appeared in the Black Sea and Caspian Sea as well.
Abnormal water blooming in the north-east of the Black Sea, MERIS-Envisat, 16 July 2010.
Chlorophyll concentration, yellow substance absorption and substance material backscattering are the characteristics that can be determined by satellites and allow to study time and spatial changes of the three principal sea water substances: phytoplankton, suspended materials and coloured organic substances, as well as detect the origin sources of these substances, observe their distribution and transformation.
Nowadays, monitoring of suspended materials distribution and water blooming is done with MODIS scanners installed on the satellites Terra and Aqua, as well as MERIS, with which Envisat satellite is equipped. Since 1999, Terra has become a leader of the Earth Observation System, allowing to gather comprehensive information on the atmosphere (aerosol and cloud properties, temperature and water steam profiles), land (changes in natural landscape, vegetation, snow cover and land temperature) and ocean (surface temperature, suspended materials and chlorophyll concentration). Multifunctional 36-channel spectral radiometers MODIS allow to get information on the underlying surface optical properties with spatial resolution 250, 500 and 1000 m in nadir, as well as thermal images in the infrared range with resolution 1 km. The swath that reaches 2,330 km in width allows to observe any place of our planet at 1 or 2-day intervals. Combining the data on temperature field, sea colour and other water surface optical properties allows to determine suspended materials distribution, water blooming, sea surface temperature, as well as flow fields with high resolution by means of sequential animation of satellite images.
Many satellite devices with high visible spectrum range spatial resolution (0.5 – 2.5 m) can be successfully utilized for marine research, first and foremost in coastal zones, where high spatial resolution is essential due to high process variability or their small scale, such as small-size whirlwinds or currents. Satellites EROS-B, Ikonos and QuickBird equipped with such devices provide panchromatic images of water or land surface with high resolution less than 1 m. The information is provided on paying basis, depending on its type.
Grand Canal in Venice, Ikonos satellite image.
What can you say about temperature monitoring and ocean water dynamics?
Monitoring of mesoscale structure and water dynamics can be carried out, for instance, by AVHRR radiometers installed in various NOAA-KLM satellites or the above-mentioned MODIS spectral radiometers. The satellites of NOAA (National Oceanic and Atmospheric Administration, US) series are quasi polar satellites. They are equipped with various tools, including AVHRR radiometer with 5 spectrum channels with spatial resolution 1 km and temperature resolution 0,1оС. The radiometer is used to address a wide range of environmental control needs. It allows scientists to analyze and forecast weather, climate changes, study ocean surface temperature fields, as well as atmospheric temperature and humidity, monitor ocean water dynamics, volcano eruptions, forest fires, dust storms and vegetation level. Twice a day, each satellite covers any location on the planet.
The Black Sea surface temperature, 7 August 2011, satellite METOR-2
What other ocean characteristics can be monitored from space?
Satellites TOPEX/Poseidon , Jason-1 and Jason-2 are equipped with altimeters used for monitoring sea and ocean level, wind waves height and wind speed. These systems are a common project of NASA and CNES (French National Space Agency). The joint ocean surface topography monitoring program of the USA and France was specially developed to address oceanology tasks, including monitoring ocean and sea meso- and large-scale circulation, sea and ocean level synoptic and climate changes, etc. In June 2008, another satellite Jason-2 was launched under this program, it main objective is ongoing World Ocean level monitoring, initiated by TOPEX/Poseidon and Jason-1 satellites. Every 10 days, the satellite repeats pre-tracked measurements with spatial resolution 7.5 km. Altimeter’s precision is approximately 2 cm, wave height – 0.4 m or 10% and wind speed 1.5 m/s.
Caspian Sea level measurements 1993—2009, TOPEX/Poseidon, Jason-1 и Jason-2
The method of scatterometry operates as follows: a sensing microwave impulse transmitted to the sea surface is scattered due to its roughness caused by wind waves. Thus, a part of the reflected signal captured by the radar correlates well with the near-water wind speed. The backscattering rate allows to determine the wind speed, and its relation to azimuth angle – wind direction. The spatial resolution of SeaWind scatterometer is 25 km. The wind speed from 3 to 20 m/s can be calculated accurate to 2 m/s and 10% starting with 20 m/s; wind direction – 20° for 3-20 m/s. The results of QuikSCAT scatterometry allow to analyze near-water wind fields, for instance, in the Baltic Sea area, twice a day.
Are there any satellite data archives, and how can one use them?
Nowadays, when global regular satellite data and reanalysis information on sea surface temperature field, sea level, chlorophyll concentration, ice cover, atmospheric pressure, wind, precipitation, humidity, heat fluxes and other hydrometeorological characteristics (PODAAC JPL, UT/CSR, NCEP, GSFC NASA, DAAC GSFC) are widely available, we have an opportunity to study not only seasonal, but also annual changes of ocean and land condition. This is especially important for regional and global climate change research.
Professional database require special preparation from users, including specific software and ability to work with different data formats. There are plenty of satellite data and Earth images archives, however, which can be useful for anyone. Some satellite data archives you may download here.
What are the prospects of ocean satellite monitoring?
There are ample plans of ocean remote sensing and satellite monitoring systems development. USA, Canada, Europe, India, Japan and other countries launch satellites annually, designed for gathering diverse information on the land, ocean and atmosphere. Those satellites which are out of use are substituted by new ones, equipped by improved devices. Their precision and resolution capacity is constantly growing, together with a set of characteristics that can be measured from space. USA and European Space Agency make satellite data available for free use or significantly reduce prices for paid data. More and more specialists are involved in development and implementation of new international Earth remote sensing programmes.
Satellite Sentinel-3 that will be launched by ESA in 2013, designed for measuring ocean level, temperature and monitoring ocean and land colour under the program Global Monitoring for Environment and Security (GMES)).
Despite the rapid increase of satellite date consumption, the most effective Earth monitoring system should be based on complex satellite, aircraft and overland (marine) measurements, as well as numerical simulations of various environmental processes. Thus, improving Earth remote sensing methods should be accompanied by development and expansion of overland (marine) measurement tools and improvement of numerical simulations.