"Heavy Ferret C"

These series of ELINT satellites, known for decades only as "Heavy Ferrets", are the third variation of Agena-based ELINT satellites, for which the proper name is still unknown. They are designated here as "Heavy Ferret C". The exact objective of these satellites is still unknown.

The satellites of this series used a longer payload fairing than the preceding Samos-F3 series.

The fourth satellite in this series featured an unusual bulge in the payload fairing, which hints at an additional secondary payload in this satellite.

This series was followed by a fourth version of the Heavy Ferret satellites.