Apt » History » Version 6
Version 5 (PASCHOS, Alexandros, 03/10/2016 11:36 PM) → Version 6/19 (PASCHOS, Alexandros, 03/10/2016 11:56 PM)
{{toc}}
h1. *2.2 Automatic Picture Transmission*
APT means Automatic Picture Transmission. It is a system, developed in the 1960s, made to transmit low-resolution analog image for weather satellites. A complete APT image takes around 12 minutes to be built up at a rate of 2 lines per second. The data is broadcasted by the satellite. The stream is obtained by the AVHRR/3 instrument. Two channels with a low resolution are emitting all the time using VHF signals at reduced rates (around 120 lines/minutes).
The two channels are composed of :
* Channel A: A visible frequency range channel providing APT imagery during the day.
* Channel B: An IR channel providing APT imagery at any time of the day and the night.
h2. *2.2.1 Technical characteristics of APT*
The - frequencies range is : 137-138 MHz (audible frequencies)and specificially 137.100 MHz for NOAA 19, 137.9125 MHz for NOAA 18, 137.620 MHz for NOAA 15. The overall RF bandwidth is equal to frequencies)- Bandwith : 34 kHz. The modulation kHz
- mode is : NFM (NarrowBand Frequency Modulation) and a
- Modulation : 2.4 kHz AM (Amplitude Modulation) subcarrier is modulated on FM carrier. A carrier
- a circularly polarized antenna is usually needed (RHCP) but other types have also been used. The
- data resolution is : 4 km/pixel at a line rate of 120 lines/minute or 2 lines/second anad the
- transmitted power : 5W (37 dBm). dBm)
h2. *2.2.2 APT Frame format* NOAA 19 : 137.100 MHz
NOAA 18 : 137.9125 MHz
NOAA 15 : 137.620 MHz
The broadacasted transmission is made of two channels for the image, synchronisation and telemetry information. A complete APT Video Line Time is 2080 pixels long (990 pixels for each image). One APT line is composed of one line for Channel A video followed by one line for Channel B video.
* Telemetry frame:
Each video channel A and B have their own telemetry frame
Each telemetry frames consists of 16 points (wedges): height of 128 video lines
Telemetry frame rate is 1 frame per 64 seconds
Each telemetry point is repeated on 8 successive APT lines
* Space and minute marker:
Time between two successive markers is one minute
Minute markers are repeated on 4 successive lines, with 2 lines black and 2 lines white
B video is always an IR channel (Ch. 4 usually), so spaces are white and minute markers black
When A Video is a visible channel, spaces are black and minute markers white; otherwise it appears like B video
* APT Synchronization:
Sync A precedes Channel A data. Is a 1040 Hz square wave - 7 cycles
Sync B precedes Channel B data. Is an 832 pps pulse train - 7 pulses
In following years, digital signals produced by Low Rate Picture Transmission (LRPT) systems will replace APT.
h1. *2.2 Automatic Picture Transmission*
APT means Automatic Picture Transmission. It is a system, developed in the 1960s, made to transmit low-resolution analog image for weather satellites. A complete APT image takes around 12 minutes to be built up at a rate of 2 lines per second. The data is broadcasted by the satellite. The stream is obtained by the AVHRR/3 instrument. Two channels with a low resolution are emitting all the time using VHF signals at reduced rates (around 120 lines/minutes).
The two channels are composed of :
* Channel A: A visible frequency range channel providing APT imagery during the day.
* Channel B: An IR channel providing APT imagery at any time of the day and the night.
h2. *2.2.1 Technical characteristics of APT*
The - frequencies range is : 137-138 MHz (audible frequencies)and specificially 137.100 MHz for NOAA 19, 137.9125 MHz for NOAA 18, 137.620 MHz for NOAA 15. The overall RF bandwidth is equal to frequencies)- Bandwith : 34 kHz. The modulation kHz
- mode is : NFM (NarrowBand Frequency Modulation) and a
- Modulation : 2.4 kHz AM (Amplitude Modulation) subcarrier is modulated on FM carrier. A carrier
- a circularly polarized antenna is usually needed (RHCP) but other types have also been used. The
- data resolution is : 4 km/pixel at a line rate of 120 lines/minute or 2 lines/second anad the
- transmitted power : 5W (37 dBm). dBm)
h2. *2.2.2 APT Frame format* NOAA 19 : 137.100 MHz
NOAA 18 : 137.9125 MHz
NOAA 15 : 137.620 MHz
The broadacasted transmission is made of two channels for the image, synchronisation and telemetry information. A complete APT Video Line Time is 2080 pixels long (990 pixels for each image). One APT line is composed of one line for Channel A video followed by one line for Channel B video.
* Telemetry frame:
Each video channel A and B have their own telemetry frame
Each telemetry frames consists of 16 points (wedges): height of 128 video lines
Telemetry frame rate is 1 frame per 64 seconds
Each telemetry point is repeated on 8 successive APT lines
* Space and minute marker:
Time between two successive markers is one minute
Minute markers are repeated on 4 successive lines, with 2 lines black and 2 lines white
B video is always an IR channel (Ch. 4 usually), so spaces are white and minute markers black
When A Video is a visible channel, spaces are black and minute markers white; otherwise it appears like B video
* APT Synchronization:
Sync A precedes Channel A data. Is a 1040 Hz square wave - 7 cycles
Sync B precedes Channel B data. Is an 832 pps pulse train - 7 pulses
In following years, digital signals produced by Low Rate Picture Transmission (LRPT) systems will replace APT.