The NERC Remote Sensing Data Analysis Service

Steve Groom, RSDAS, Plymouth

Satellite images of sea-surface temperature have provided an excellent way of viewing ocean features such as currents, eddies and fronts (see figure 1) and have inspired much oceanographic research. However, the exploitation of SST data by the wider, non-specialist scientific community has been partly restricted because images are provided in form that requires significant further processing. Even for remote sensing specialists, the effort required to develop software and process large numbers of images is considerable. Also there is often a lengthy delay between reception of the data at the receiving station and the images appearing on a scientist's desk or computer. It is these problems which a new NERC service is planning to address.

Thumbnail of Fig 1. Figure 1 . AVHRR Sea surface temperature image for 24 June 1995 03:26 GMT. Features of interest include: cool tidal mixed waters off Brittany, in the Irish Sea, and central English channel; a cooler band along the continental shelf edge (around the 200m isobath) caused by internal waves generated at the shelf edge mixing deeper (cooler) water into the surface layer. Also note the cool eddy almost detached from the Irish Sea/Celtic Sea front.

From Summer 1996 the Remote Sensing Data Analysis Service (RSDAS) based in NERC's Plymouth Marine Laboratory, will be able to provide scientists with processed satellite images, received at the NERC Satellite Receiving Station, Dundee in a form that can be used immediately for oceanographic research, or compared with in situ data. The images will also be available in near-real time, that is within hours of capture at the receiving station so that scientists can follow the development of phenomena, possibly allowing modification to planned ship sampling.

The service will use data from the Advanced Very High Resolution Radiometer sensors carried on-board the NOAA (USA) meteorological satellites. Because there are two satellites operational at any time it is possible to see the same point on the Earth's surface about four times each day, and more frequently at latitudes around the UK. As well as observing SST the AVHRR can be used to detect high reflectance features in the visible such as high suspended particulate concentrations or blooms of the phytoplankton Emiliania huxleyi (see figure 2 and box).

The way the system works is that data are received at the NERC station at Dundee, and immediately transferred over the UK computer network (SuperJANET) at speeds of around 10Mbit/s. Images are then processed using highly automated software: this involves assigning a latitude and longitude to each point making up the image, calculating SST and mapping the data into a standard map projection, such as Mercator which is used for nautical charts. Finally, images are annotated and optionally assigned a colour palette. The resulting images can be placed on the group's World-Wide Web server for collection, or alternatively can be automatically sent via the internet to the scientists' host computer. For scientists at sea the images can be particularly valuable to enable choice of the best places to sample before reaching the area, or for locating transient features like eddies (see box).

Most requests for data, however, are likely to arrive some time after the data have been obtained, when, for example, a cruise has taken place with good weather. So for each satellite overpass data covering a number of 'key areas', such as the Celtic Sea, or Bay of Biscay, are extracted processed and stored irrespective of whether there is a cruise underway or any interest expressed. In addition a large area image is produced which can be used to synthesise new areas of interest. These images are stored on the same lattice as data produced by international projects, such as the NASA SST Pathfinder experiment: this means that if the NERC and NASA data are compared the boundaries of each Pathfinder SST point will exactly overlap a number of NERC processed SST points.

The continuous production of images for standard areas results in long time-sequences that can be used to monitor sea-surface variability (eg figure 2). Also, in regions with frequent cloud cover, a number of images can be added together so that cloudy areas on one image may be filled with cloud-free data from another image. Similarly a week or month of images can be added together to obtain a weekly or monthly mean SST, or variability over the same period (eg figure 3).

Future plans include analysis of ocean colour data from the NASA Sea-viewing Wide Field-of-view Sensor (SeaWiFS) due to be launched in 1997. SeaWiFS will measure the colour of the sea in several wavebands to provide information on surface distributions of phytoplankton, the base of the marine food chain (see figure 4), and on suspended particulates in coastal waters related to coastal erosion, or riverine outflow. It is also hoped to extend the range of products from the AVHRR particularly for terrestrial applications.

For further information contact:

Steve Groom
Remote Sensing Data Analysis Service
c/o Plymouth Marine Laboratory
Prospect Place, West Hoe
Plymouth, Devon, PL1 3DH, UK.

Tel: 01752 - 633150
Fax: 01752 - 633101