Automatic picture transmission
Automatic Picture Transmission ( APT ; English for automatic picture transmission ) is an outdated method for the transmission of analogue pictures from weather satellites , which was and is used in American, Russian and Chinese satellites.
history
In 1963, TIROS 8 was the first satellite to use the APT method to transmit images of the earth. Nimbus 1 followed in 1964 . In the following years a number of American TIROS / NOAA satellite satellites, Russian Meteor , Resurs and Okean satellites, and Chinese Fengyun satellites followed.
As of 2017, the APT procedure will only be used in the NOAA satellites
- NOAA 15 (137,620 MHz),
- NOAA 18 (137.9125 MHz) and
- NOAA 19 (137,100 MHz)
used.
There is a new and improved version of the APT process called Low Rate Picture Transmission .
technology
The permanent, as broadcast sent, right-hand circularly polarized signal is a carrier frequency of 2.4 kHz frequency modulated . The emitted power is typically between five and six watts.
The image data transmission takes place in APT frames. An APT frame defines the horizontal, line-by-line transmission of two video channels, typically referred to as channel A and channel B or video A and video B. Each line contains a synchronization, an image line as well as telemetry and calibration data for each of the two video channels.
| designation | Data words | description |
|---|---|---|
| Sync A | 39 | 1040 Hz synchronization |
| Space A and Marker | 47 | Space radiation or minute markers |
| Channel A Video | 909 | Video image line channel A |
| Telemetry Frame A | 45 | Telemetry and calibration data frame A. |
| Sync B | 39 | 832 Hz synchronization |
| Space B and Marker | 47 | Space radiation or minute markers |
| Channel B video | 909 | Video image line channel B |
| Telemetry Frame B | 45 | Telemetry and calibration data frame B |
The telemetry data is sent over 84 seconds. Each block of telemetry data is repeated for eight consecutive lines. The first eight blocks of telemetry data contain a wedge function, which increases from the value 1/8 to 1/8 up to 100%. Block nine contains the value zero. The following blocks contain sensor calibration data and information about the sensor channel used.
A total of 1040 words per channel results for one line, so both channels are 2080 words long and are transmitted in 0.5 seconds. This corresponds to a transfer rate of 4160 baud . The number of lines in a frame is 128, so the total transmission time is 64 seconds.
The synchronization of frames is done by minute markers. Every 60 seconds, which corresponds to 120 lines, two black and two white pieces of information are sent in consecutive lines.
The APT method is used today for the transmission of image data from an AVHRR / 3 . For this purpose, the sensor data are linearized by the Manipulated Information Rate Processor (MIRP) for the upcoming transmission. Every third line of the original image is transmitted with a resolution of four kilometers per pixel. The digital-to-analog conversion of the ten-bit resolution takes place via the eight most significant bits of the pixel.
reception
The reception and decoding of APT signals from the NOAA satellites is possible several times a day, even for private individuals and on a low budget. All that is required for this is compatible receiving equipment and decoding software, which is available free of charge, among other things. The procedure and results are described on the Internet in a series of articles by various authors.
Individual evidence
- ↑ Stojče Dimov Ilčev: Global Satellite Meteorological Observation (GSMO) Theory . Springer, 11 October 2017, ISBN 978-3-319-67119-2 .
- ↑ Arthur P. Cracknell: advanced very high resolution radiometer AVHRR . CRC Press, April 23, 1997, ISBN 978-0-7484-0209-0 , pp. 55-.
- ↑ RTL-SDR Tutorial: Receiving NOAA Weather Satellite Images. In: rtl-sdr.com. May 13, 2013, accessed October 26, 2017 .