Receiving (Live) Weather Data
Weather data, which can be received live (and free), can be split into several groups :
- Weather Charts via Short wave (APT Marine Fax)
- Satellite Rebroadcasts via Short-wave
- SYNOP Data via Short-wave (RTTY)
- NAVTEX (RTTY) on 518 kHz
- Geostationary Satellites (APT-WEFAX/PDUS)
- Polar Orbiting Satellites (WEFAX/HRPT)
1.Weather Charts via Short-wave (Marine Fax)
The transmission mode used for weather charts on short wave are transmit line by line.
To correctly receive the chart you must have the same drum speed (RPM) (for correct slanting), and the same index of co-operation (IOC) (for vertical resolution) as the transmitting station.
Usual RPM speed is 120 with a IOC 576. RPM 60 and 90 are often in use by CIS stations, also some Charts are transmit with a IOC 288.
Press Photo Fax transmissions use RPM 60 and IOC 352
Weather Charts on Short-wave are transmitted as Frequency Modulation (FM).
That means that the transmitter is keyed between two frequencies, which 1500 Hz corresponds for Black and 2300 Hz for White
For half tone (grey scale) Satellite pictures, the transmitter frequency shifted between the frequency for black (1500 Hz) and the frequency for white (2300 Hz).
The difference/2 between the frequency for white and the frequency for black is called Deviation. The standard deviation on Short-wave is 400 Hz (2x400=800 1500 to 2300 Hz).
For Long wave transmissions the standard Deviation is 150 Hz.
To receive Weather Charts on Short-wave you need a Single Side Band (SSB) Receiver to receive the FM modulated audio Sub-carrier
Select the Upper Side Band (USB) and tune in the (carrier) frequency 1.9 kHz lower as listed
The audio frequency output (Record/Speaker) is then fed to a interface or the Sound Card input, that converts the different audio frequencies into digital format that can be processed by the computer.
Automatic Picture Transmission (APT) is used to enable a fully automated unattended reception of the transmitted pictures.
At the start of the transmission a start tone is transmitted for some seconds, which is recognised by the receiving unit.
At the end of the transmission a stop tone is sent that switches the receiving unit back to standby mode. See also the HF-FAX listings or the HF-FAX schedules.
Weather Charts have only Black and White contours, for good resolution 2 Bit decoding is the best way for clear charts
2. Satellite rebroadcasts via Short-wave
A view stations around the world (USCG, Tokyo, Honolulu, Taipei and Delhi Meteo) transmit also Satellite Rebroadcasts from the GOES and GMS Geostationary Satellites.
To receive these grey scale pictures you should decode them with 8 Bit greyscale.
For Satellite rebroadcasts the standard RPM is 120 and IOC 576.
3. SYNOP Data via Short-wave (RTTY)
Several Meteo stations transmit 24 hour daily Teletype (RTTY) Synopsis Data on short wave from weather stations around the globe.
These (5 number) data streams are called as example SYNOP AAXX or SHIP BBXX and are WMO standard.
To decode the SYNOP data you need a SSB receiver and also an RTTY decoder which decode the numbers into clear text or to convert graphical maps. See also the SYNOP page