Geostationary Satellites - Frequently Asked Questions

  1. What is a Geostationary Satellite?
  2. What are the advantages/disadvantages of Geostationary Satellites for remote imaging?
  3. How many Meteorological Geostationary Satellites are there?
  4. What are Image Channels?
  5. Can I extract useful scientific (quantitative) information from these images
  6. Why do some images have blank areas or speckles?
  7. Why are some images missing?
  8. Why are the images not up-to-date?
  9. What are the “UK Reprojection” images?
  10. Where can I access the Meteosat SEVIRI HRV channel offsets?

  1. What is a Geostationary Satellite?

    Geostationary satellites are positioned at an exact height above the earth (about 36000 Km). At this height they rotate around the earth at the same speed as the earth rotates around its axis, so in effect remaining stationary above a point on the earth (normally directly overhead the equator).

    As they remain stationary they are ideal for use as communications satellites and also for remote imaging as they can repeatedly scan the same points on the earth beneath them.

    Polar Orbiting satellites by comparison have a much lower orbit, moving around the earth fairly rapidly, and scanning different areas of the earth at relatively infrequent periods.

  2. What are the advantages/disadvantages of Geostationary Satellites for remote imaging?

    As they are positioned at such a high altitude the spatial resolution (i.e. amount of detail shown) of their images (typically 2.5 Km per pixel) tends to be not as good as some polar orbiting satellites (typically 1 Km to 50m per pixel) which are much closer to the earth.

    However the advantage of their great height is that they can view the whole earth disk below them, rather than a small subsection, and they can scan the same area very frequently (typically every 30-60 minutes). This makes them ideal for meteorological applications.

    One big problem with Geostationary satellites is that since they are always positioned above the equator they can't see the north or south poles and are of limited use for latitudes greater than 60-70 degrees north or south. The further from the equator the lower the spatial resolution of each pixel and the greater the possibility of being hidden by the earth's curvature. So, for a typical Meteosat image a pixel near the equator may represent a 2.5Km square on the ground, but a pixel positioned for example in Northern Europe may represent 10Km on the ground and therefore provide less information (such as temperature, vegetation, wind speed, albedo, etc) per square metre. Move your house to a nice sunny spot on the equator and you'll get maximum value from your local geostationary satellite!

  3. How many Meteorological Geostationary Satellites are there?

    There are several such satellites positioned at regular intervals around the equator so that the whole earth is covered. Here at DSRS we receive data from 5 different Geostationary Meteorological satellites via EUMETSAT's EUMETCast satellite based distribution system.

    View the coverage of each of these satellites here - and read about them below:

    • Meteosat

      This European community satellite operated by EUMETSAT in Germany is positioned above Europe/Africa (approx 0 degrees Longitude).

      The original Meteosat satellite used an imaging instrument called “VISSR” which provided three image channels. Its service came to an end in 2006 having been replaced by a newer version called Meteosat Second Generation (MSG).

      MSG provides more image channels than the original Meteosat and provides images at higher spatial resolution, due to carrying a newer imaging instrument called “SEVIRI”. MSG transmits images every 15 minutes although EUMETSAT licensing restrictions mean we are only able to provide images on our website at longer intervals (previously 6-hourly, now 3-hourly).

      Further information on Meteosat can be found at EUMETSAT's web site.

    • GOES-WEST and GOES-EAST

      These are operated by the US NOAA agency and are positioned to the West and East of the USA/S.America (7135 degrees West and 75 degrees West. respectively). Further information on the GOES satellites can be found at NOAA's web site.

    • MTSAT

      This satellite is operated by the Japanese Meteorological Agency positioned over Japan/Australia. The first operational MTSAT satellite was positioned at 140 degrees East and its replacement MTSAT2 now operates at 145 degrees East. See also JAXA and MSC.

    • IODC - Indian Ocean Data Coverage

      The original first-generation Meteosat satellites have since been re-deployed over the Indian Ocean to provide this additional service. From 2005 to 2007 Meteosat-5 was positioned at 63 degrees East, and from 2007 onwards Meteosat-7 has been positioned at 57 degrees East to fulfil a similar role. As with MSG this data is now available at 3-hourly intervals (previously 6-hourly).

  4. What are Image Channels?

    The satellites typically scan the earth using different wave lengths (channels). Most geostationary meteorological satellites scan using:

    • VISIBLE wavelengths (0.5 - 0.9 um) (similar to that use by the human eye).
    • IR (thermal infra-red) (10.5 - 12.5 um). These wavelengths show differences in temperature.
    • WV (water-vapour) (5.7 - 7.1 um). These wavelengths show differences in water vapour absorption in the atmosphere.
  5. Can I extract useful scientific (quantitative) information from these images?

    Not in their present form. The images have been heavily compressed using a lossy algorithm, and they have also undergone histogram equalisation, both of which have changed the pixel values.

    If you require data for quantitative analysis (ie temperature, albedo, wind vectors, etc), we may be able to supply you with the original raw data. Please contact DSRS to enquire about this.

  6. Why do some images have blank areas or speckles?

    Firstly please note that images from the GOES satellites at three-hourly intervals will show the whole globe but at hourly intervals in between will only show North America. If you are looking for the latest image of South America please go back in time to the previous three-hourly image (i.e. one transmitted at 00:00, 03:00, 06:00, 09:00, 12:00, 15:00, 18:00 or 21:00 UTC.

    There are a couple of possible causes for this. It may be that you are looking at a visible channel image taken at a time when it was night time in the area you are looking at. Visible channel images show the Sun's light being reflected from the Earth's surface, and so only work during daylight hours. Note that all image times are in UTC rather than the local time of any particular area you may be looking at. If this is your problem then look at an IR (infrared) channel image instead or look at a different time of day.

    Another likely cause of this problem is that at certain times of year solar eclipse conditions interfere with the meteorological satellites operation, or the communication satellites used to relay their data from EUMETSAT to DSRS. This can cause noisy speckles, or whole image segments, or indeed entire images not to be received (see missing images below). This normally only occurs at around solar noon (12 UTC) for Meteosat. Unfortunately there is nothing that we can do to prevent this!

  7. Why are some images missing?

    There are several reasons why certain images may be missing:

    • License restrictions
    • No visible images at night
    • Transmission or reception problems
    • Eclipse conditions
    • Other reasons

    License restrictions mean that images from Meteosat satellites (which include the IODC service over India) may not be distributed more frequently than every three hours. These restrictions apply to all websites, not just Dundee! Images from GOES and other satellites can be distributed hourly or more frequently but Meteosat images transmitted between the three-hourly slots are only available after 24 hours.

    Note that as mentioned under “why do some images have blank areas or speckles?” above, visible channel images require light from the Sun to be illuminating the area that was being observed at the time when the image was taken. For some satellites (e.g. GOES), visible images are not included at all for times when it was night time in the area covered. Again note that all image times are in UTC, rather than the local time of any area you may be looking at.

    Missing images can also be caused by to transmission or reception issues that may have occurred at the time. Sometimes EUMETSAT have a problem receiving or disseminating the image, or else there is a problem with reception at our end.

    Occasionally images are considered too noisy due to interference possibly caused by eclipse conditions as discussed in “why do some images have blank areas or speckles?” above and are omitted entirely.

    Check the EUMETSAT Service News. If there is still an unexplained missing image then please feel free to contact us for assistance.

  8. Why are the images not up-to-date?

    First, please try to reload or refresh the web page in your web browser. If that does not work the please try again but this time hold down the CTRL (Control) key whilst doing so. If that does not work you can also try with the Shift key. This will force any proxy or cache servers to fetch the latest page. If you still see old images then there may be a problem with the satellite, a problem with EUMETCast transmission, or a problem with reception here at DSRS. Please feel free to use our comment form to contact us.

  9. What are the “UK Reprojection” images?

    “UK Reprojection” images are now provided for Meteosat Second Generation (MSG) 0 degrees East images.
    This example shows how these images differ from the original MSG images.

    What are they?
    These images show the area around the United Kingdom (including much of Western Europe) extracted from the MSG images and reprojected to look as though the UK was viewed from directly overhead rather than from above the equator, making visual interpretation of this area easier. We create these images by performing a reprojection process which involves geometrically distorting or “warping” the images from Geostationary Satellite View projection into a Cassini-Soldner projection centred on the UK.

    How do I access these images?
    These images are shown alongside the existing MSG 0 degrees East images on our website. Simply look at the latest MSG image or select another MSG image by date, and links to the UK reprojected images should appear alongside the links to the standard images. Note that UK reprojected images are not currently available for our entire MSG archive however - see below.

    From which months/years are these images available?
    We now produce these reprojected images automatically for all new MSG 0 degrees East images following reception, and they should be available on our website half an hour after the original images were received. We have also created these images for some of our existing MSG 0 degrees East image archive - therefore these images are generally available from mid-2009 onwards, and images for January 2008 and January 2009 are also available. We may be able to provide these images for more of our MSG archive in future.

    Which MSG image channels are available in this form?
    We produce these images for all MSG channels (and for the colour composite of channels 3,2,1). Since channel 12 is split into northern and southern sections, and the UK area is only within the northern region, the UK reprojected image is obviously only applicable for the northern section.
    The UK reprojection is produced for the visible channels (1, 2, 3, 12, and the Colour 321 composite) at times when the UK receives sufficient daylight. This is the case for 0900, 1200 and 1500 UTC images all year round, and in the summer months, for 0600UTC and 1800UTC as well. At other times the visible channel reprojections are not normally produced due to insufficient UK-wide daylight. The exact period of availability of visible channel reprojected images for the 0600UTC and 1800UTC times runs from 1st April until 10 September each year.

  10. Where can I access the Meteosat SEVIRI HRV channel offsets?

    The HRV channel offsets are accessible from the bottom of the channel listing pages. The offsets represent which x-axis pixel of the full resolution (large) image the centre longitude is located at.