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SATELLITE METEOROLOGY – FAQ

# Apa yang dimaksud dengan satelit geostasioner?

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.

# Apa kelebihan/kekurangan dari satelit geostasioner untuk inderaja?

As they are positioned at such a high altitude the spatial resolution (ie 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!

# Ada berapa banyak Satelit Geostasioner untuk Meteorologi?

Currently there are 5 or 6 satellites positioned at regular intervals around the equator so that the whole earth is covered.

The main satellites are:

    * Meteosat

The European community satellite operated by EUMETSAT (formally part of the European Space Agency) in Germany is positioned above Europe/Africa (approx 0 degrees Longitude). This is the satellite that we receive data from in Dundee. Meteosat also rebroadcasts data from other geostationary satellites (although at less regular intervals than its own data). EUMETSAT are alone in deciding to encrypt the majority of their transmissions. Not very friendly behaviour we think. Although we can receive and decrypt such data our decryption license agreement restricts us to providing only their unencrypted data on the web. These are transmitted every 6 hours.

Meteosat transmits HRI data at 1694.5 MHz, at a data rate of 166.66 kbits/s. Further information on Meteosat can be found at EUMETSAT's web site

Meteosat's area of coverage map:
a

    * GOES-EAST

      Operated by the US NOAA agency, positioned over USA/S.America (75 deg. West. Further information on GOES-E can be found at NOAA's web site.

GOES-E's area of coverage map:
b

    * GOES-WEST

      positioned over the Pacific (135 deg. West). Further information on GOES-W can be found at NOAA's web site

GOES-W's area of coverage map:
c

    * MTSAT

      operated by the Japanese Space Agency NASDA positioned over Japan/Australia (140 deg. East). See also JAXA and MSC

MTSAT's area of coverage map:
d

    * IODC (Meteosat-5)

      Meteosat-5, now re-positioned at (63 deg. East) for Indian Ocean Data Coverage

IODC's area of coverage map:
e
* GOMS

      positioned at (76 deg. East)

# Apa itu kanal untuk membuat citra?

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.

# Citra tersebut ukurannya berapa?

Meteosat's digital HRI disseminations consist of a maximum resolution of 2500x2500 8-bit pixels (IR channels) and 5000x5000 (VIS) per channel. The separate European sector images (B format) are 2500x1250 (VIS) and 1250x625 (IR). The instantaneous field of view at the sub-satellite point is 2.5km (VIS) and 5km (IR,WV); resolution decreases with angular distance from this point.

Each transmission may consist of up to 3 image channels. The largest Meteosat transmissions are about 36Mbytes in size. Transmission rates are 166.66 Kbits/s from the satellite. It therefore takes about 25 minutes to receive such a transmission.

We have compressed the full size images to about 1MByte per channel, and also provide lower resolution sampled images (about 80-200Kbytes).

# Can I extract useful scientific (quantitive) information from these images?

Not in their present form. The images have been heavily compressed using an algorithm that degrades the pixel values.

If you require data for quantitive analysis (ie temperature, albedo, wind vectors, etc), we will shortly be able to supply the original raw data in a number of different forms. Please contact DSRS for more information.

# Mengapa beberapa citra ada yang terdapat bintik-bintik?

During February/March, solar eclipse conditions interfers with the satellites broadcasts creating these noisy speckles. This normally only occurs at around solar noon (12 GMT) for Meteosat. There is nothing that we can do to prevent this! For more details see the Meteosat anomalies document at Eumetsat.

# Mengapa ada beberapa citra yang tidak ada?

There are times when there are glitches in our system, but more likely explanations are that Eumetsat had a problem receiving or disseminating the image, or that the image was considered too noisy due to interference, possibly caused by eclipse conditions, see above.

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