"Heavy Ferret D"

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 D". The exact objective of these satellites is still unknown.

The satellites of this series used a larger hammerhead payload fairing than the preceding third version of the Heavy Ferret series.

This series was discontinued 1971. Possibly the Canyon satellites, which operated from an inclined geostationary orbit took over their duties.

Secondary Payloads

Some satellites carried secondary payloads mounted on the satellite:

• Heavy Ferret D 4 carried the RM 19 radiometer