In its four years of operation, the ERS-1 ground segment has delivered almost 26 000 SAR scenes and a few million low-bit-rate fast-delivery products that have allowed hundreds of scientific investigations, and pilot and demonstration projects as well as many commercial and operational applications to be carried out.
Given that the ERS-1 and ERS-2 satellites will now be operated in tandem until at least mid-1996, the same ground segment must be able to manage a much greater workload. It must perform the exploitation of the ERS-2 payload data, including the data from new instruments such as GOME, PRARE and the ATSR visible channels, in parallel to the handling of newly acquired ERS-1 data and the retrieval of ERS-1 archived data. Most of the facilities have been modified to cope with the higher requirement for data and services and, where applicable, for the handling of data from the new sensors.
ESRIN, via its ERS Exploitation Division, is responsible for the exploitation of the ERS Payload Data Ground Segment and for user services.
The ERS Payload Data Ground Segment is composed of the following facilities:
Figure 1 shows the interfaces between the facilities and their relationship to the user community.
Figure 1. The ERS ground segment
The EECF, located in Frascati, Italy, includes User Services, the Product Control Service (PCS), and the Payload Reference System. It provides:
The User Services unit is responsible for planning the ERS-1 and ERS-2 missions in line with the user requests and for scheduling the worldwide data acquisition accordingly. In addition, it supports the end users, maintains the centralised catalogue of acquisitions and products, and handles user requests and product orders.
The Product Control Service's operational tasks include the monitoring and control of ERS data-product quality and assessment of the compliance of system performance with the system specifications. Another of its main roles is to assess instrument behaviour and the related margins. This information provides vital feedback that will be used in the development of future programmes, including the analysis and development of algorithms for calibration and validation activities. The Product Control Service uses a range of systems, including the so-called 'Reference System' for the High- and Low-Rate
Fast-Delivery Processing chains, which also supports the maintenance of the operational software installed at the ESA ground stations. For ERS-2, the Reference System has been upgraded with a dedicated GOME QA and performance monitoring system and with a tool for the continuous long-term monitoring of the newly added low-bit-rate (LBR) trans-cription system at the stations.
The EECF also provides support for the monitoring of the progress of the pilot and demonstration projects, for the training programmes given in developing countries on the application and exploitation of ERS data, and for promotional activities in conjunction with the industrial consortium for the ERS commercial applications development, including the preparation of materials for symposia and conferences and for public-relations purposes.
The ESA ground station network has been set up to allow the maximum coverage over the European area for the Synthetic Aperture Radar (SAR) and the global LBR payload data acquisitions.
The ERS-2 payload data network is the same as the one used for ERS-1. It is managed by ESRIN and includes six ground stations, located at Salmijaervi (Kiruna, Sweden), Fucino (Italy), Maspalomas (Canary Islands, Spain), Tromso (Norway), and Gatineau and Prince Albert (Canada).
The ground stations' systems have been upgraded to allow them to handle ERS-2. In particular, the LBR processing chain installed at the stations has been re-hosted on a Silicon Graphics platform, replacing the old chain which was based on a minicomputer plus array processor. The new chain has a much higher CPU power and growth capability and generates the specified products for all sensors, including the new ERS-2 sensors GOME and ATSR-2, within the same time constraints as the old chain (100 minutes per orbit). The new chain also has the advantage that the LBR data are transcribed in near-real time at the stations on Exabyte cassettes for shipment to the relevant archiving facilities, while for ERS-1, the transcription was performed on a dedicated off-line facility installed at Fucino station. This will drastically improve the reliability and delivery time for raw data to PAFs, thus hastening the release of LBR off-line products to end users. Also the SAR processor, the Station Control and Monitoring System, and the Broadband Data Dissemination Network (BDDN) have been upgraded to ERS-2 mission functional requirements.
Except for Salmijaervi, which is operated by ESOC and is fully dedicated to ERS operations including telemetry, tracking and control (TT&C) activities, all of the other stations are multimission in nature. Under contract to ESRIN, they perform the ERS-1 and ERS-2 payload data acquisition, processing and dissemination, as well as hosting the ESA equipment for the requisite data exploitation. They also provide similar services for other international Earth-observation satellites, such as Landsat (USA), Spot (France), JERS-1 (Japan), and Tiros (USA).
The division of tasks and responsibilities between these stations takes into account the constraints related to the high- and low-rate payload data characteristics (Table 1). This network ensures global LBR data acquisition (mainly from the on-board recorder dumping) on a daily basis. A station's typical daily activities can be summarised as follows:
Table 1. Responsibilities of the ESA ground stations with respect to data acquisition and processing
The PAFs will continue to be the core of the product distribution system for ERS-2. Their role can be summarised as:
Each PAF receives the relevant ERS-2 payload telemetry data on a regular basis from the ground stations and ensures the long-term archiving, the routine production and the distribution of the data. Their activities are managed and monitored from ESRIN.
There are four PAFs, managed under contract to ESA.
F-PAF in Brest, France
It is operated by IFREMER, the French institute for research into the exploitation of the sea, and its tasks are:
The F-PAF has carried out a major modernisation of its facilities for ERS-2 by re-hosting the Altimeter Ocean Product (OPR) processing chain and associated management subsystem on a new hardware configuration.
UK-PAF in Farnborough, UK
It is operated by the National Remote Sensing Centre (NRSC) and its tasks are:
The UK-PAF has also made a great effort to update its facilities to accommodate ERS-2 by adopting new configurations to increase the throughput of their LBR chains and by procuring new archiving and processing chains for SAR data.
D-PAF in Oberpfaffenhofen, Germany
It is operated by DLR, and its tasks are:
In preparation for ERS-2, the D-PAF has developed the chains for GOME and PRARE products, procured new chains for the generation of SAR products and modernised its data management subsystem.
I-PAF in Matera, Italy
It is operated by the Italian Space Agency and is charged with:
The I-PAF has upgraded its system for ERS-2 and is developing GOME-derived products.
In addition to the ESA ground station network, a number of national ground stations, i.e. belonging to countries participating in the ERS Programme, and foreign ground stations, i.e. belonging to non-participating countries, have been set up around the world, or are planned, in order to acquire ERS-1 and ERS-2 SAR payload data.
The current situation is summarised in Table 2. Most of the stations have been used for ERS-1 and will be used again for ERS-2 under the terms and conditions of a standard Memorandum of Understanding (MOU) with ESA.
Table 2. Status of national and foreign ground stations acquiring or planned to acquire the ERS SAR data
The ground stations receive, from the EECF in Frascati, the input data needed to acquire, process and distribute the SAR data and they report back to the EECF on their station activities and status. The stations generate and distribute products developed nationally to ESA principal investigators, pilot projects and commercial users. In particular, low-resolution, near-real-time products are distributed as a service from the Tromso and Gatineau stations. Together with the ESA stations, the stations listed in Table 2 will provide the worldwide data coverage shown in Figure 2. Agreements are in place with the stations for the provision to ESA of copies of some of the raw data acquired so that ESA can, when required, serve its users directly. ESA PAFs have in this way acquired and archived valuable worldwide SAR data sets.
Figure 2. Total planned coverage by the ERS ground stations
Figure 3. The flow of global low-rate data. For fast-delivery products, the ESA ground stations acquire and process the ERS data and send it to the EECF. After conversion, the data is then sent to the UK or Italian meteorological office, which in turn distributes it to met. offices around the world. For off-line products, the ground stations send the data on cassettes to the PAFs for archiving and generation of the off-line products.
ERS-2 LBR and SAR products are distributed to users either on a routine basis or upon specific request. The full list of currently available products is shown in Table 3.
Table 3. ERS-1 and ERS-2 products currently available
The flow of the ERS-2 LBR data is summarised in Figure 3. The LBR data obtained from the Wind Scatterometer, the Radar Altimeter, and the Active Microwave Instrument (in Wave mode) are processed immediately after reception to so-called Fast Delivery level (UWI, URA, UWA) at the ESA stations. They are then collected at the ERS Central Facility and, after being converted in an upgraded BUFR formatter delivered by the UK Meteorological Office, are injected into the Global Telecommunication System (GTS) of the World Meteorological Organisation (WMO). They are also disseminated to selected facilities and users (including the PAFs, from which they can be obtained as off-line copies). Figure 4 shows the LBR fast-delivery products distributed during the first four years of ERS-1's lifetime.
For the ATSR data, real-time processing is performed at Tromso (10 orbits per day) for the generation of the Sea Surface Temperature Measurement, and the data is sent thereafter to ESRIN for conversion and distribution to the meteorological offices, or for temporary storage on-line for user access.
The LBR data sets (Radar Altimeter, ATSR and GOME) copied on Exabyte cassettes at the acquisition stations are sent to the PAFs for archiving and for the off-line generation of precision products. ATSR data are also sent to instrument providers, Rutherford Appleton Laboratory (UK) and Centre d'études des Environements Terrestres et Planetaires (F), for their internal investigations and support to ESA production activities.
The flow of ERS-2 SAR data is summarised in Figure 5. It is similar to the flow of ERS-1 SAR data. The SAR data are received at the ESA ground stations, processed to Fast-Delivery level, and disseminated via the Broadband Data Dissemination Network (BDDN, under ESRIN control), which allows the transmission of SAR Fast-Delivery images from Kiruna or Fucino nominally within 24 h of data sensing to nominated centres (one per country in Europe), using a Eutelsat satellite link for image transmission. The nominated centres then distribute the data to the end users. The raw data are sent to the PAFs for the off-line generation of ESA standard products (Raw, PRI, SLC, Geocoded).
Figure 6 shows the number of each ESA product type delivered to the users by the ESA processing facilities.
Figure 4. Distribution of ERS-1 low-bit-rate (LBR) fast-delivery products over ERS-1's four years of operation: UWI or Wind, URA or Radar Altimeter, and UWA or Wave user products
Figure 5. The flow of global high-rate data. To allow the data to reach the users quickly, the SAR images acquired at Kiruna and Fucino are transmitted directly to nominated centres via a Eutelsat satellite link. The centres then distribute the data to the users. In addition, the ground stations send the raw data to the PAFs for the off-line generation of standard products
Figure 6. Distribution of Synthetic Aperture Radar (SAR) products by type
The ERS User Services section provides support to the ERS-1 and ERS-2 user community through:
Figure 7 shows the ERS User Services organisation.
Figure 7. ERS-1 User Services. The various user categories are at the top, the ESRIN ERS Central Facility (EECF) is in the middle area, and the external facilities are at the bottom. The external facilities are controlled and operated via the telecommunications infrastructure
Upgraded User Services applications
The applications were upgraded in 1994 to support the parallel ERS-1 and ERS-2 missions. The software was revised thoroughly in order to make it fully 'multimission' and to improve performance and availability. The changes made are:
All upgrades entered into full operation well before the ERS-2 launch. All the concerned systems are now multimission systems and, thanks to the hardware and software enhancements, are able to cope with the activities and load caused by the contemporaneous management of the two satellites and of related ground segment activities without an increase in personnel.
Handling of user requests
The user requirements, particularly for SAR, are expressed as user requests, which define the required product and medium types together with the geographical area and window of time of interest. Most of the LBR products, on the other hand, are distributed on an orbit basis in monthly or yearly sets of data.
Figure 8. The processing flow for user requests for SAR products
Figure 8 shows the processing flow for user requests for SAR products. The requests may involve data that has already been acquired and archived; those requests are converted into production orders for the PAFs, where the required products are generated and dispatched to the end user. If the request concerns data yet to be acquired, the relevant acquisition is planned taking into account possible conflicts, alternatives, or anticipated needs and then confirmed in cooperation with the Mission Management and Control Centre (MMCC) at ESOC. Upon confirmation of data reception from the relevant ground station, the product order is placed and the products are delivered to the end user either via the BDDN (for fast-delivery products) or by the relevant processing facility.
As part of the mission planning, performed at ESRIN, the specific user requests are integrated with a 'baseline' mission plan, which covers the repeated coverage for multitemporal analysis. It permits also the optimal use of satellite resources, by limiting the number of SAR on/off switchings per orbit and exploiting the SAR on average for 9 min per orbit, with a maximum of 12 min per orbit. All areas covered by ground stations have already been acquired. The addition of new stations now permits coverage of most of the Earth's land surface. The contemporaneous availability of ERS-1 and ERS-2 provides a unique opportunity for 'tandem' operations. Planning SAR acquisitions for the two satellites over the same area, which the two satellites visit within a short time interval, permits new applications like interferometry.
The EECF also maintains a catalogue of the SAR data acquired worldwide and of the products archived at the PAFs. The catalogue is updated regularly, whenever the new acquisition reports are generated at the acquisition stations and the data is entered in the database. Users can query on-line the SAR catalogue or the Global Activity Plan (GAP), which contains up-to-date information on the planned operations of the different payload instruments.
Users can also browse through a simplified version of the SAR catalogue on their PC using the 'Display ERS-1 SAR Coverage' (DESC) software package. That tool supports the users in defining their requirements for products and services. With the start of ERS-2 operations, DESC was upgraded to DESCW, Display ERS Swath Coverage for Windows (Fig. 9). It is a multimission PC tool, covering at present the ERS-1, ERS-2 and Landsat missions, and running under MS-Windows. It provides a number of enhancements with respect to DESC, such as graphic definition of an area, search by such an area, additional mission specific filters, and on-line help. Its catalogue/ inventory files are updated weekly and put at the disposal of users on the ERS on-line server. Copies of the tool were distributed after the ERS-2 launch to a large number of users and are available upon request from the ERS Help Desk.
Figure 9. A sample of the new DESCW screen, with the three main windows active: the Full Map, which shows the location of the area being `zoomed'; the zoom window, which shows in greater detail the user's selected area and the swaths of the selected missions; and the Frame List, which lists all selected frames and related parameters
The ERS on-line server, accessible via Internet, Span or X.25, permits the downloading of the GAP, the weekly updates of the DESC and DESCW catalogue files, the ERS-1 SAR Low Resolution Images generated daily at Kiruna, and the Quick Look OPR products generated daily by the German PAF.
Printed material to support user activities and training, such as the ERS User Handbook, the ERS System Description, the ERS Products Specification document, and the CD Guide to ERS-1, is also available (see the order form where?? for the full list of documentation and tools available).
A new service, based on GDS/WWW (see 'ERS-2 Information Now Available on Internet' in this issue), began operation last year. It has improved over time and has been extensively used during the launch and early commissioning of ERS-2: it permits Mosaic or Netscape clients to access via Internet a set of daily-updated information, data and sample products. This includes information on ERS-2 deployment, initial operations and commissioning activities, as well as first images and results. This service also includes up-to-date data and information related to ERS-1.