EO Basics
The number of Earth Observing satellites are growing rapidly for both scientific research and operational application within marine meteorology and oceanography including sea ice covered regions. Whereas in-situ measurements are highly needed for obtaining 2- and 3 dimensional observations of the world oceans surface and subsurface layers, satellite observations provide an inherent wide area unique capability to obtain regular quantitative information of surface variables and upper layer phenomena at global, regional and local scales. The combination of these observational fields are highly needed for ocean model development, validation and assimilation, and form an essential part for systematic ocean monitoring and modeling.
Satellite oceanography is primarily using three domains within the electromagnetic (EM) spectrum, notably radiation in: - the visible/near-infrared (VNIR); - the thermal infrared (TIR); - and the microwave bands of the EM spectrum (Figure 1). The visible and infrared channels utilize intervals of the EM spectrum with high atmospheric transmission, such as in the bands from 0.4-2.5µm, 3.5-4.0µm and 10-13µm (Figure 1, top). The EM waves in these bands do not generally penetrate clouds, so remote sensing observations of the Earth's surface in these bands can only be done satisfactorily under cloud free conditions. As such this is posing severe limitations in regions where clouds are frequently present. In the microwave area, on the other hand, at wavelengths above 0.3cm, EM waves generally penetrate clouds which makes it feasible to obtain regular, daily observations of ocean and sea ice surfaces (Figure 1, bottom).
Figure 1. The electromagnetic (EM) spectrum showing the bands used in remote sensing together with the operating area for some sensors (upper graph). The atmospheric transmission of the EM spectrum is shown in the lower graph. Note that the operating areas for satellite oceanography are located in parts of the spectrum where atmospheric transmission is high. |
Spatial coverage
Most SAR sensors launched up to the 1990s could only cover a 100km wide strip on the Earth's surface during one overpass. As a result, ERS (1 or 2) images are only 100km wide which is insufficient for the monitoring of many oceanographic coastal processes like fronts, eddies or algae blooms.
This shortcoming has been solved for most recent SAR sensors like Radarsat (1/2) and ASAR aboard Envisat, which now have a swath width of 400-500km.
Even better spatial coverage is provided by most sea surface temperature and ocean colour sensors, whose swath width covers several thousands of kilometers. This means that many of these sensors provide daily coverage of the entire surface of the Earth.
The following table gives an overview of the swath widths of some of the recent satellite sensors.
| Sensor | Type of measurement | Max swath width | Revisit time |
| ERS-1 SAR | roughness | 100km | 35 days |
| ERS-2 SAR | roughness | 100km | 35 days |
| RADARSAT | roughness | 500km | 3-5 days |
| ASAR | roughness | 400km | 3-5 days |
| ATSR-2 | temperature | 512km | 3 days |
| AATSR | temperature | 512km | 3 days |
| AVHRR | temperature | 2399km | 0.25 days |
| SeaWiFS LAC | colour | 2801km | 1 day |
| MODIS | colour | 2330km | 2 days |
| MODIS | colour | 2330km | 2 days |
Temporal coverage
The spatial extent of a region that a satellite sensor can monitor in a given time is limited by the orbit of the satellite and the width of the sensor's field of view (swath width).
Many SAR sensors have a relatively narrow swath of about 100km (ERS 1 and 2) which means that a measurement of the same segment of the Earth's surface is carried out every 35 days. This temporal resolution can be insufficient for the monitoring of many coastal processes.
Many sea surface temperature and ocean colour sensors have a much wider swath, thus covering a larger ground segment in one overpass. This means that most sensors provide daily coverage of the entire surface of the Earth.
The table below gives an overview of the revisit times of some of the recent satellite sensors.
| Sensor | Type of measurement | Max swath width | Revisit time |
| ERS-1 SAR | roughness | 100km | 35 days |
| ERS-2 SAR | roughness | 100km | 35 days |
| RADARSAT | roughness | 500km | 3-5 days |
| ASAR | roughness | 400km | 3-5 days |
| ATSR-2 | temperature | 512km | 3 days |
| AATSR | temperature | 512km | 3 days |
| AVHRR | temperature | 2399km | 0.25 days |
| SeaWiFS LAC | colour | 2801km | 1 day |
| MODIS (2 sensors) | colour | 2330km | 1 day |
| MODIS | colour | 2330km | 2 days |
Glossary:
| (A)ATSR | (Advanced) Along Track Scanning Radiometer. A British sensor flow in various forms on ERS-1, ERS-2 and ENVISAT (as AATSR). |
| ASAR | Advanced SAR, sensor flown on ESA's ENVISAT satellite. |
| AVHRR | Advanced very-high resolution radiometer |
| ERS | European Remote Sensing satellite: two dedicated EO satellites flow by ESA (European Space Agency). ERS-1 flew from 1991 and was decomissioned in 1996. ERS-2 was launched in 1994 and is still operational. |
| GAC | Global Area Coverage. Denotes the lower resolution data which is recorded for the whole planet and sent once a day to a dedicated reciever). |
| LAC | Local Area Coverage. Typically used to denote the higher resolution images which are lost unless a recieving station in within view. |
| MERIS | Medium Resolution Imaging Spectrometer, ESA ocean colour sensor. |
| MODIS | Moderate Resolution Imaging Spectrophotometer, NASA ocean colour sensor. |
| RADARSAT | Canadian Space Agency SAR sensor, commercially based operation |
| SeaWiFS | Sea viewing Wide Field of view Sensor: A commercial satellite which the scientific community has access to for data from Sep 1997 to Dec 2003 through a NASA agreement |