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Satellite Imagery FAQ - 3/5
Section - Ground Stations

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  How is satellite data recieved on the ground?
  
   _Intro to Ground Recieving Stations contributed by Peter Bolton
   <pbolton@clyde.pc.my>_
   
   1. GROUND RECEIVING STATIONS
   
   This document is an introduction to Ground Receiving Station (GRS)
   acquisition and processing of remote sensing satellites data such as
   SPOT, LANDSAT TM and ERS-1 SAR. Ground receiving stations regularly
   receive data from various satellites so as to provide data over a
   selected areas (a footprints approximately covers a radius of 2500 km
   at an antennae elevation angle of 5 degrees.) on medium such as
   computer tape, diskette or film, and/or at a specific scale on
   photographic paper. GRS are normally operated on a commercial basis of
   standard agreements between the satellite operators and the
   Governments of the countries in which they are situated. Subject to
   the operating agreements, local GRSs sell products adapted to end
   users needs, and provide remote sensing training, cartography, and
   thematic applications.
   
   2. GROUND RECEIVING STATION ARCHITECTURE
   
   A Ground Receiving Station consists of a Data Acquisition System
   (DAS), a Data Processing (DPS) and a Data Archive Center (DAC).
   
   2.1. DATA ACQUISITION SYSTEM
   
   DAS provides a complete capability to track and receive data from the
   remote sensing satellite using an X/S-band receiving and autotracking
   system on a 10 to 13meter antenna in cassegranian configuration. DAS
   normally store fully demodulated image data and auxiliary data on High
   Density Digital Tapes (HDDTs). However, in one small UNIX based
   system, data storage can be stored directly on disk and/or
   electronically transmitted to distant archives.
   
   2.2. DATA PROCESSING SYSTEM
   
   DPS keeps an inventory of each satellite pass, with quality assessment
   and catalog archival, and by reading the raw data from HDDTs,
   radiometrically and geometrically corrects the satellite image data.
   
   2.3.DATA ARCHIVE CENTRE
   
   The Data Archive closely related to DPS offers a catalog interrogation
   system and image processing capabilities through an Image Processing
   System (IPS).
   
   3. GROUND RECEIVING STATION PRODUCTS
   
   The GRS products can either be standard or value added products. Both
   are delivered on Computer Compatible Tapes (CCTs), CD ROM, cartridges,
   photographic films or photographic paper prints at scales of 1:250
   000, 1:100 000, 1:50 000 and 1:25000.
   
i.      Standard products
        - SPOT-1 and 2/HRV : data of CNES levels 0, 1A, 1B, 2A
        - Landsat TM : data of LTWG levels 0, 5,
        - ERS-1 SAR : Fast Delivery and Complex products.

ii.     Value added products
        - For SPOT
                .       P + XS : Panchromatic plus multi-spectral,
                .       SAT : a scene shifted along the track,
                .       RE : a product made of 2 consecutively acquired scenes,
                .       Bi-HRV : Digital  mosaic produced by assembling 2 sets
of
 2                              scenes acquired in the twin-HRV configuration.
                .       Stereoscopy : Digital terrain model (DTM) generation,
                .       Levels 2B, S and level 3B using DTMs.

        - For Landsat TM: levels 6, S and 7.
        - For ERS-1 SAR : geocoded data.

        - For any instrument:
                .       Image enhancement and thematic assistance,
                .       Geocoded products on an area of interest defined by the
                        customer (projection, scale, geocoding and mosaicking
                        according to the local map grid).

   4. GROUND RECEIVING STATION OPERATION
   
   Persons needing images for thematic applications in the field of
   cartography, geology, oceanography or intelligence, etc, will refer to
   the station catalog in order to find out if the data are available
   over the area concerned.
   
   There are two possibilities :
   
   The data exists.
          The customer fills in a purchase order and is then provided
          with the product on a medium such as CCT, film or paper print.
          If the data are available in the GRS catalog, a list of the
          related scenes and their hardcopies (named "quick looks") are
          provided.
          
   The data does not exist.
          a) For SPOT, the customer fills in a programming request form
          which is sent by GRS to the Mission Control Centre (MCC) in
          Toulouse, France. MCC returns a Programming Proposal to be
          submitted for approval. Upon approval, the confirmation is
          returned to MCC which in turn sends a programming order to the
          satellite for emitting the data during its pass over the GRS
          antenna.
          At the same time, MCC sends to GRS, the satellite ephemerides
          for antenna pointing and satellite tracking.
          In the case of SPOT, if the data does not exist within the
          Station catalog but are listed in the SPOT IMAGE worldwide
          catalog, GRS may request the level O product from SPOT IMAGE in
          TOULOUSE in order to process it locally.
          
          b) For other sensors, LANDSAT TM or ERS-1, the satellite
          ephemerides are known at GRS and the antenna is pointed
          accordingly in order to track all selected passes.
          
   Within the GRS, the raw satellite data are received by the Data
   Acquisition System (DAS), and recorded on High Density Digital Tapes
   (HDDTs). HDDTs are then sent to the Data Processing System (DPS),
   where an update of the Station catalog is made as well as a quick look
   processing.
   
   DPS is also in charge of automatic processing of selected raw data in
   order to produce images of standard level.
   
   Value added products with cartographic precision are produced within
   DPS using interpretation workstations which must be part of an
   operational Geographic Information System (GIS) combined to an Image
   Processing System (IPS).
   
   Once processed, the data, on CCT, are sent to the Data Archive Center
   (DAC) where they are delivered to the customers after a quality
   checking. At DAC, further processing may be applied to the data such
   as image stretching, statistical analysis, DTM, or a conversion from
   tape to film and paper prints in the photographic laboratory;
   "customized services" may also be offered.
   
     _________________________________________________________________
                                      
  Image Interpretation
  

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