Minimize Ground Segment

Introduction

The ERS-1 programme has been designed to serve a large variety of users with a comprehensive range of products and services. In response to this and other very challenging mission objectives the ERS-1 ground segment has been established using a systems approach.

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The ground segment consists of an ensemble of facilities responsible for the acquisition, processing, distribution and archiving of the satellite data and of the derived products. Several factors have influenced the design of the ERS-1 ground segment:

  • the roles of the ESA establishments
  • the technical constraints of the satellite on data acquisition
  • specific user requirements for fast delivery services
  • national expertise in particular scientific fields
  • the desire to provide a primary user gateway to the system
  • the need for a high level of automation in routine operations
The responsibilities of the ESA establishment has influenced, to some extent, the distribution of the various tasks. The user interface and exploitation of the payload data has been implemented within the Earthnet at ESRIN in Frascati, Italy by the Earthnet Programme Office (EPO); while the satellite planning and control functions, including the control of the Kiruna station are the responsibility of the Mission at ESOC in Darmstadt, Germany.

The characteristics of the ERS-1 space segment in terms of its multi-sensor payload, orbit configurations and power requirements have imposed the implementation of a network of ground stations around the world to acquire the high bit rate SAR data (for which only direct readout is possible). In addition facilities have to be provided to permit the down-loading, once per orbit, of the on-board recorded low bit rate data.

Specific user requirements have called for the implementation of specific processing tools and fast delivery services at the ground stations operated by or for ESA to allow user centres to be furnished quickly with selected data products.

An important feature of the ground segment concept is that various user interfaces are considered key components. In practice the Central User Service (CUS) of the EECF will constitute the primary user gateway to the system.

Products will be disseminated either via fast delivery of selected products to nationally nominated user centres for operational applications or by off-line delivery to individual end users from the PAFs.

Data Acquisition and Fast Delivery Product Processing

ERS-1's orbit and the need for direct read-out of the SAR telemetry have necessitated a global network of ground stations, either within the ESA network or made available by national (ESA member) or foreign (non ESA members) entities. The ESA network, which has been established to ensure the acquisition of global LBR data and regional SAR data over Europe, consists of:

  •      Kiruna (Sweden)
  •      Fucino (Italy)
  •      Maspalomas (Canary Islands, Spain)
  •      Gatineau (Canada)
  •      Prince Albert (Canada)
The primary functions of the ESA ground stations are:
  • real-time data acquisition - when the satellite is visible from a ground station - the SAR data is transmitted at a rate of 105 Mbit/sec on a carrier at 8140 MHz and the LBR data at a rate of 1093 Kbit/sec (after spectrum spreading) on a carrier at 8040 MHz
  • acquisition of LBR data from the on-board tape recorder - the data is played back at a rate of 15 Mbit/sec and transmitted to the ground at 8040 MHz
  • data processing and generation of fast delivery products - a number of standard products within three hours of satellite acquisition for distribution to primary user centres.

It should be noted that LBR data cannot be recorded on-board, while previously recorded data is being replayed. Therefore, during this period the LBR data is directly transferred. The instrument data is sent to the PAFs for archiving and off-line product generation - the SAR data and FD products directly, the LBR data via Fucino, where they are transcribed onto optical disks.

 Data Dissemination Facilities

ERS data will be disseminated via various facilities at transmission and reception sites within the ground segment using satellite and telecommunication links. The low-rate fast-delivery products (LBRFDP) generated by the ESA ground stations will be centralised at the EECF (on the ISS) and re-distributed to nationally nominated user centres via standard land lines. An alternative of using a satellite link at low rate, e.g. 64 kbits/s, for quick dissemination, at least in Europe, is also being considered. One of the ERS-1 mission objectives is to provide to the user community FD products from ERS-1 within three hours from instrument observation. The nominal solution for the distribution of FD products distribution system, is using public network land lines. However, for the distribution of SAR products the capacity of the landlines is insufficient and satellite links are deemed necessary.

The Broadband Data Dissemination Network (BDDN) system is designed to transmit high rate FD products (HBRFDP) from Fucino, Kiruna and possibly Maspalomas ground stations to nominated receiving stations by means of a satellite telecommunication channel. Sources outside the footprint of the satellite will first transfer data to a suitable site within the footprint by appropriate point-to-point connections, e.g. land lines, packet switching networks etc. The Gatineau ground station will receive data through the dedicated line between the EECF and Canada. The EECF is connected with the other ESA Ground Stations and appropriate user centres via low speed land lines or equipment connections. Both Kiruna and Fucino ground stations are equipped with a SAR FD processor and with monitoring and control sub-systems for FD products distribution. In addition the stations will be connected through standard low speed lines with the EECF for schedule and control purposes.
 
 

Processing and archiving

As described above there are four PAFs: The functions of the PAFs have been harmonised by ESA and each has an agreed area of responsibility for archiving and product generation. They will be responsible for:
  • long-term archiving and retrieval of ERS-1 raw data, auxiliary information and relevant surface data
  • generation and distribution of off-line geophysical and precision products
  • support to long-term sensor performance assessment, calibration and geophysical validation, demonstration campaigns and pilot projects
  • interface with the EECF for updating of the catalogue and supporting user services.
The PAFs will share the responsibility for product generation, in order to make efficient use of national expertise and it is intended that their operations continue for 12 years after the launch of ERS-1.

A summary of the off-line products to be generated by each of the PAFs is shown in the figure and the services offered by each PAF are listed below it.

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  • D-PAF
    • primary archive of raw data acquired by the German Antarctic Research Station at O'Higgins
    • primary processing centre for SAR precision and geocoded image data, higher level altimetry products and precision orbit calculations.
       
  • F-PAF
    • primary archive for LBR data (SAR wave mode, Wind Scatterometer and Radar Altimeter) data over the oceans and associated FD products
    • secondary archive of the global ATSR data set
    • primary processing centre for LBR data over oceans
    • secondary processing centre for ATSR data
    • storage of relevant ESA provided campaign data.
       
  • I-PAF
    • regional archive of SAR and LBR data (raw, processed and FD) acquired over the Mediterranean by the Fucino station
    • regional processing of SAR and LBR products of the Mediterranean.
       
  • UK-PAF
    • primary archive for raw and processed SAR and ATSR data, LBR data over ice and land and SAR FD products
    • secondary archive for global LBR dataset
    • primary processing centre for SAR and LBR data over ice and land
    • secondary processing centre for wave data products
    • storage of campaign data.