3.1 Methodology¶

The FM signal is demodulated to a 2400Hz audio signal (in wave format). The receiver is responsible for the demodulation of the FM carrier and the output of an AM sub-carrier with the information signal within containing the image data. Every half a second, a row of the image is transmitted.

Figure 3.1 APT Frame Format

3.1.1 AM Demodulation¶

It is important to determine when each row starts for decoding the image. As said before every half of a second a row of the image is transmitted. But in each transmitted row (see Figure above) there are two different images. The first image is the one using the AVHRR visible frequencies (optical) and the second image is coming from the IR. In order to determine the beginning of each row of data for each (visible or IR) image the transmitted message signal contains a distinct syncronization pulse (see Figure 2.2 APT Synchronization).
The signal’s cross correlation with the synchronization square waveform gives us the frequency (number of maximums) of the repeated signal, and therefore the start of each row.

An illustration of the implementation followed is shown in the next Figure.

Figure 3.2 Implementation Diagram 1

3.1.2 Image Decoding and Display¶

3.1.3 Referenes¶