• Table of contents
• 2.2 Automatic Picture Transmission
  • 2.2.1 Technical characteristics of APT

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.

2.2.1 Technical characteristics of APT¶

- frequencies : 137-138 MHz (audible frequencies)- Bandwith : 34 kHz
- mode : NFM (NarrowBand Frequency Modulation)
- Modulation : 2.4 kHz AM (Amplitude Modulation) subcarrier on FM carrier
- a circularly polarized antenna is needed (RHCP)
- data resolution : 4 km/pixel at a line rate of 120 lines/minute or 2 lines/second
- transmitted power : 5W (37 dBm)

NOAA 19 : 137.100 MHz
NOAA 18 : 137.9125 MHz
NOAA 15 : 137.620 MHz

In following years, digital signals produced by Low Rate Picture Transmission (LRPT) systems will replace APT.