Automatic Picture Transmission

 Automatic Picture Transmission (APT) is used by Satellites to enable a fully automated unattended reception of the transmitted pictures.
At the start of the transmission a start tone is transmitted for some seconds, which is recognised by the receiving unit.
At the end of the transmission a stop tone is sent that switches the receiving unit back to standby mode. 
The Meteosat and other Geostationary satellites (GOES/GMS/GOMS) have also Digital High Resolution transmissions, which needs special equipment.

6. Polar Orbiting Satellites (APT/HRPT)

It is also possible to receive data from various Polar Orbiting satellites and this may well be the cheapest option, but it does have limitations.
The hardware for this consists of a VHF antenna (the most popular being a crossed dipole), a receiver (137 MHz band) and a suitable software package for displaying on a PC.
Satellites in Polar Orbit are much closer to the Earth surface (around 800-1000 km) and so the image scanned is of a much higher resolution.
The satellite scans what is directly beneath it so in higher latitudes the curvature of the Earth does not pose a problem of the data being invalid, as is the case with Geostationary data.
The NOAA Polar Orbiters are placed in a sun synchronous orbit which means that they will pass overhead at approximately the same time of day.
Unlike the imagery from Meteosat which lends itself to creating animation’s, Polar Orbiter data cannot be animated easily.
The satellite when it passes may be within view of your antenna for about 12 minutes but it will not be following the exact same path overhead and so each captured image will differ.
In order to receive data from Polar Orbiters your ground station needs to be visible from the satellite so if  you wanted data from other parts of the world you would either have to take your equipment with you to that part of the world or swap data with another station.
 The Polar Orbiter satellites have also Digital High Resolution transmissions (HRPT), which needs special equipment.