MSG: The next generation...
On august 29 2002 the first of a new generation weathersatellites was launced: Meteosat Second Generation.
Eventually this generation will replace the old Meteosat satellites, designed 25 years ago.
The data transmitted by MSG satellites differs a lot compared to the current first generation Meteosats:
| First gen. (MET7 and lower) | Second gen. (MET 8 and higher)
|
| Data format | APT (analog), HRI | LRIT, HRIT
|
| Data related to frame | Fixed: 4 or 8 frames/line | No relation: packets
|
| Bits coded | No | Yes; Viterbi
|
| Scrambled | No | Yes
|
| Error correction | No | Yes; Reed Solomon
|
| Compression | No | Yes; JPEG
|
| Encryption | partly | LRIT: partly?
HRIT: ?
|
Because of the fixed data/frame relation, the straight-forward bit transmission and the absence of compression the HRI format is very simple to handle. It is even possible to plot the picture by means of a "picture drum", without the use of a computer.
This simplicity has also its draw-backs:
- Because of the fixed data/frame relation there is not much flexibility in transmitted picture sizes
- Because of the straight-forward bit transmission the sensitivity to noise is 'high'; the only way to increase the quality is by 'brute force': big antennas, powerfull transmitters and low-noise receivers.
- An errored bit is not corrected, not even recognized as such
- Because of the absence of compression the amount of data to transmit is high (i.e., the time to send one picture is long)
MSG transmits its data in a much more complex /sophisticated way, giving the following advantages:
- Viterbi-encoding (K=7, n=2) gives 5 dB better sig/noise ratio, at a cost of 3dB more bandwidth
- By adding extra bits in each block of data error correction is possible
- JPEG compression can lower the amount of bits to transmit up to a factor 10
(or more)
- The packet-like transmission makes it possible to efficiently transmit any size of picture, or other types of data, if needed.
Just like the old Meteosat sends its data in 2 flavours (APT, HRI)
MSG has also 2 flavours: LRIT and HRIT. These 2 digital formats are very similar.
Difference is that HRIT has 10 bits per pixel (LRIT: 8) and HRIT is lossless compressed (LRIT: lossy JPEG).
Also, the modulation is different: BPSK for LRIT, QPSK for HRIT. Which means that the receivers for both formats are different.
Because of a burned-out transmitter in MSG1 all data is now distributed using the Hotbird satellite. Advantage for amateurs is that the equipment needed to receive MSG is very easy to get:
- A small dish, same type as used for satellite TV (80 cm, I use a 60cm offset disc aimed to Eurobird at 9 east)
- Skystar 2 TV tuner, pluggable into a PC (there is also a version to be connected via USB). Note: Other than skystar2 tuners are not guaranteed to work together with the Tellique software!
- A 'fast' PC (1GHz is more than enough) with 'lots' of disk space (with a 80G disk you can play a lot)
- Software to translate the data from the TV tuner into files on disk (Tellique software) and a key to decrypt the data, bot available at Eumetsat for in total 100 euro (amateur use only)
- Software to translate the files into viewable pictures
For the translation software I did make 'xrit2pic'. It is free available, see further on my website.
