Apt » History » Version 5
PASCHOS, Alexandros, 03/10/2016 11:36 PM
| 1 | 1 | PASCHOS, Alexandros | {{toc}} |
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| 2 | 1 | PASCHOS, Alexandros | |
| 3 | 2 | PASCHOS, Alexandros | h1. *2.2 Automatic Picture Transmission* |
| 4 | 3 | PASCHOS, Alexandros | |
| 5 | 3 | PASCHOS, Alexandros | 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). |
| 6 | 3 | PASCHOS, Alexandros | The two channels are composed of : |
| 7 | 5 | PASCHOS, Alexandros | * Channel A: A visible frequency range channel providing APT imagery during the day. |
| 8 | 5 | PASCHOS, Alexandros | * Channel B: An IR channel providing APT imagery at any time of the day and the night. |
| 9 | 4 | PASCHOS, Alexandros | |
| 10 | 4 | PASCHOS, Alexandros | h2. *2.2.1 Technical characteristics of APT* |
| 11 | 4 | PASCHOS, Alexandros | |
| 12 | 4 | PASCHOS, Alexandros | - frequencies : 137-138 MHz (audible frequencies)- Bandwith : 34 kHz |
| 13 | 4 | PASCHOS, Alexandros | - mode : NFM (NarrowBand Frequency Modulation) |
| 14 | 4 | PASCHOS, Alexandros | - Modulation : 2.4 kHz AM (Amplitude Modulation) subcarrier on FM carrier |
| 15 | 4 | PASCHOS, Alexandros | - a circularly polarized antenna is needed (RHCP) |
| 16 | 4 | PASCHOS, Alexandros | - data resolution : 4 km/pixel at a line rate of 120 lines/minute or 2 lines/second |
| 17 | 4 | PASCHOS, Alexandros | - transmitted power : 5W (37 dBm) |
| 18 | 4 | PASCHOS, Alexandros | |
| 19 | 4 | PASCHOS, Alexandros | NOAA 19 : 137.100 MHz |
| 20 | 4 | PASCHOS, Alexandros | NOAA 18 : 137.9125 MHz |
| 21 | 4 | PASCHOS, Alexandros | NOAA 15 : 137.620 MHz |
| 22 | 4 | PASCHOS, Alexandros | |
| 23 | 4 | PASCHOS, Alexandros | In following years, digital signals produced by Low Rate Picture Transmission (LRPT) systems will replace APT. |