An automatic picture transmission cloud cover receiving station

The automatic picture transmission (APT) sub-system carried on weather satellites transmits, at facsimile rate, cloud cover photographs of the Earth below. Frequency modulation of a carrier in the 137 MHz band transmitted at a level of 5 watts is used to give pictures representing an area approximately 2400 km (1500 miles) square with an average resolution of 3 km (2 miles). Major factors governing the design of a ground station which has been operational since early 1967 are presented. Signal/noise considerations show that a desirable combination is a tracking antenna having a gain of 10 dB followed by a receiving system having a noise figure not greater than 4 dB. Except for a doubling of line frequency a standard wire facsimile is used for final picture print-out. Many additions to, and refinements of, the basic station are possible. A few of these are outlined and assessed in terms of operational convenience.