So-called Gapfiller transponders on three Marisat satellites launched in 1976 continue to be leased from the COMSAT Corporation, although these are now relegated to a backup role.
In June 1970, residual IDCSP and TACSATCOM assets were made available to the United States (US) Navy by Joint Chiefs of Staff (JCS) direction, to provide an interim Operational Capability (IOC). This was followed by the approval of the Fleet Satellite Communications (FLTSATCOM) Program by the Secretary of Defense in September 1971.
The TACSATCOM spacecraft (TACSAT-1), located in the Pacific Area failed in December 1972. At that time the Navy was supplementing its capabilities by using the UHF channels on the Lincoln Labs Experimental Satellite LES-6. With the loss of TACSAT-1 and an anticipated end-of-life
September 1973 for LES-6, the Navy was faced with a five year period in the Pacific and a potential four year period in the Atlantic during which no UHF SATCOM relay would be available. This hiatus was based on the assumption that the first FLTSATCOM satellite would be available by December 1978. To minimize the gap in the space segment continuity the Navy decided, in late December 1972, to initiate efforts to lease UHF SATCOM services. A contract for a two ocean coverage (Atlantic and Pacific) was signed with COMSAT General in March 1973 and subsequently modified in September 1976 to include coverage of the Indian Ocean. This UHF capability was to be provided on the Maritime Satellite (MARISAT).
The leased UHF services (GAPFILLER) commenced for the Atlantic in March 1976 (at which
time LES-6 was deactivated); for the Pacific in June 1976; and for the Indian Ocean in
September 1976. All GAPFILLER satellites were physically and electrically identical. Each
provided one 500 kHz wideband and two 25 kHz narrowband channels. Uplink operating
frequencies were in the 292-312 MHz band and downlink frequencies were in the 249-270 MHz
band. This interim capability was phased out by the FLTSATCOM satellites, the first was
launched into orbit in February 1978.