ESA has developed an off-line SAR browse processor capable of generating low-resolution (around 200m) images corresponding to the totality of an acquired SAR segment (up to 4000 km). The main purpose of the browse images is to promote the use of high-resolution data (by identification of scenes to be processed at high resolution). These images are also of use for the observation of large-scale features, in particular with oceanography and sea ice studies.
The SAR instruments on board ERS-1 and ERS-2 satellites have generated an impressive amount of data. Most of the results achieved so far have been obtained with high-resolution products because the original ESA list did not contain medium- or low-resolution SAR products (i.e. with pixel spacing between 50 and 500 m). However this family of products would be beneficial for several reasons:
In order to meet these needs, two groups of products can be foreseen:
Since 1996, ESA generates off-line browse images with an ERS SAR Browse Processor currently installed at the UK-PAF and ESRIN. The browse images mainly correspond to the Kiruna acquisition station (for Europe and the Arctic region).
The image generated by the ERS SAR Browse Processor corresponds to the full acquired stripe, has a pixel spacing of 200m, is not calibrated and is Jpeg-compressed (one stripe of 2000 km corresponds to about 1.5 Mbytes, one scene of 1 00x100 km to about 70 Kbytes).
The browse image is used as basic SAR product to populate image servers such as the experimental Image Browser of 'Earthnet online', the information service for Earth Observation users provided by ESA and currently accessible on the Internet address: http://services.esrin.esa.it.
In order to show the potentialities of browse images over land, we produced a mosaic of Europe (limited to Kiruna acquisition visibility). This mosaic (Fig.1) is created by searching the ESA Earth Observation image browser and by stitching the SAR browse stripes generated by the SAR Browse Processor.
ERS mosaic of Europe composed of Jpeg browse images and covering an area of about 2200x2200 km. The acquisition segments were selected according to the wind speed in order to have a mosaic with relatively similar oceanographic conditions. The ERS stripes were merged together with a contrast matching algorithm provided by the ERDAS Imagine software. The mosaic is composed of 41 orbit tracks. If the SAR instrument was always switched on, the mosaic would correspond to a time range of slightly more than one month. In reality this mosaic is composed of data from various cycles spanning over one year.
Figure 2 shows a mosaic extract over the Alps and gives an idea of the achievable spatial resolution.
Extract of the mosaic over the Alps covering an area of about 820x600 km.
Over the ocean, the SAR images show the capillary waves which are formed primarily in response to the wind speed. Oceanographic phenomena may change these capillary waves and thereby the radar backscatter. The oceanographic phenomenon detectable with SAR ranges from scales of a few metres to several kilometres. As the typical ERS SAR scene covers an area of 1 00x100 km, the browse images (1 00x4000 km maximum) enlarge the visibility of the oceanographic phenomena.
Several events have confirmed the excellent ERS oil-slick detection capabilities in suitable wind conditions (Johannessen et al., 1994). Despite their pixel size of 200m and their low radiometric resolution, browse images allow to easily detect oil-slicks. For example, oil-slicks are visible around oil-ridges in the North Sea (Fig.3). Even when the wind speed is rather strong for oil-slick detection, raw-oil spills could still be observed. If they would be distributed in near real time to pollution control authorities, browse images would definitely be a valuable tool for oil-slick detection and monitoring.
SAR browse image (200 m pixel spacing, Jpeg-compressed) showing an oil drilling field
with several oil-slicks (black patches) in the North Sea. The white pixels are the oil rigs.
The oil seems to drift away from the platforms.
Many examples of internal waves are seen in the mosaic of Europe (Fig.1), for example North of Cap Arkona, Germany, and in the southeastern part of the Norwegian Trench. It is interesting to follow the evolution of wave trains from their generation point, but this requires good spatial coverage. Tidal currents generate internal waves in a 1 2-hour cycle, and in the North Sea internal waves propagate with an average speed of 0.5 m/s (Wahl et al., 1996). Because of this low speed, it is possible to compute their evolution and history from SAR images. Train sets have been observed over more than 100 km, which make the use of browse images valuable. However, in the North Sea, the resolution in these images (200 m) limits the average number of wave crests visible to a maximum of three crests (Wahl et al., 1996).
The SAR browse images (or the future medium-resolution products) are the natural complement of low-resolution optical images such as those provided by the radiometer instruments, particularly in oceanography. As an example of this complementarity, we can use the instrument synergy allowed by the ERS-1/2 tandem mission by overlapping one ERS-1 SAR browse image to an ERS-2 ATSR image (Fig.4). As the images were acquired within a time separation of 30 minutes, we can assume that they describe the same weather and sea-state conditions.
SAR browse stripe overlapped with an ATSR image. The SAR image was acquired by ERS-1
on 5 May 1995 about 30 minutes before the ATSR-2 nadir view image was acquired by
ERS-2. In the North Sea we can notice the large circular cloud in the ATSR-2 image and the
corresponding high sea surface backscattering in the SAR browse image. (Courtesy M.
Mangolini).
Other examples of ocean feature detection with SAR browse images (e.g. atmospheric waves, ocean eddies, bathymetric signatures) are available in Dokken & Laur. 1997.
A key objective defined for the ERS missions was to improve sea-ice and iceberg monitoring for offshore activities and ship routing in polar regions. For that purpose, fast delivery of the data to users operating near the ice-edge or in thick pack ice is necessary. However, the limitation of the transmission system is cited as the primary constraint (ESA, 1996). Another constraint might be the difficulties in identifying ice type and the border between open water and ice. The ERS SAR browse image size (in bytes) is 2000 times smaller than the corresponding PRI image (i.e. maximum of 3 Mbytes for a 4000 km stripe), and, therefore, its transmission performances are no longer an obstacle. The capability of the browse images for ice-edge movement and static ice properties should be further examined.
A mosaic of the Arctic ice-cap (Fig.5) is used to observe the ice-edge location on the Yermak Plateau in the Svalbard region during March 1996. The ice-edge is determined in the browse images in most weather conditions. The area north of Spitsbergen (top left of the image) shows an ice-edge easily distinguishable from the open water, while in the area north of Nordaustlandet (top right of the image) it is possible to see two ice-edges. This is due to the movement within the 10 days separating the two acquisitions (7 and 17 March). This area also contains some slush (thin) ice and ice-floes connecting the ice pack to the islands. Several automatic algorithms are developed to derive sea-ice motion by detecting the displacement of features in pairs of satellite images. By mosaicking browse stripes, we can easily obtain a view of the ice-edge movement for the period of interest. As an example, the 120 km large area north of Spitsbergen also has a distinct ice-edge in January 1996 (Fig. 6). The movement of the ice-pack was 15 km during the ten weeks between January and March 1996.
Mosaic of ERS SAR browse images covering the north-west part of Svalbard (Norway). The mosaic is formed of six ERS-1 stripes within 10 days in March 1996(16, 13, 10, 7, 17, 14March). The north-east part of the ice-edge is unclear because of the 10-day time interval separating the merged ERS stripes.
Two ERS browse stripes from ERS- 1 show clearly the ice-edge in January 1996.
The new ESA SAR browse image is a valuable supplement to the series of ERS products already available. Comparison with the high-resolution PRI image corroborates the usefulness of the browse image for ocean and ice feature detection. Most of the browse images are available on the ESA Earth Observation information service 'Earthnet Online' http://services.esrm.esa.it).
This demonstrates the potential value of real-time browse images if they could be made available over Europe and in particular over the Mediterranean area. If coupled with real-time information extraction such as oilslicks detection, this would certainly strengthen the use of ERS SAR data.
The Envisat satellite will be equipped with a more advanced C-band SAR. Browse and medium-resolution products (resolution around 150 m) will be systematically generated and will certainly strengthen the scientific studies and surveillance applications of large-scale phenomena, in particular over the oceans and the polar regions.
Johannessen J, Digranes G, Espedal H, Johannessen O, Samuel P, Browne D & Vachon P, 1994: SAR Ocean Feature Cataloaue. ESA SP-1174.
Wahl T, Dokken ST & Vinje MK, 1996: Statistical characterisation of ocean internal wave trains observed by ERS-1 on the Norwegian Continental Shelf, submitted to Int Jl of Remote Sensing, Remote Sensing Letters.
Dokken ST & Laur H,1997: Ocean and ice features detection using the ERS SAR browse images, Proc. 3rd ERS Symp. [Internet: httD://florence97.erssymposium org/]
ESA,1996: Applications Achievements of ERS-1, ESA SP-1176/ll.
The authors would like to thank Phillipe Bally for providing valuable information in the starting phase of this project.
ESA EOQ Nr. 55