The fifth and final next-generation narrowband communications satellite being built for the U.S. Navy by Lockheed Martin has entered its first system test phase.

The Lockheed Martin team recently mated the Mobile User Objective System (MUOS) system module, which carries mission system equipment, and the core—which houses propulsion—with a key antenna for the satellite. The multi-beam assembly hosts 16 ultra-high frequency (UHF) antennas. MUOS is designed to carry the legacy Boeing UHF payload, as well as a new wideband code division multiple access (WCDMA) payload that will provide 3G-like services to soldiers around the globe.

MUOS-1 was launched Feb. 24, 2012; MUOS-2 followed on July 19. The legacy UHF payloads are in use, but the Navy is still conducting tests of the WCDMA capability, says Steven Davis, a spokesman for the service’s Space and Naval Warfare Systems Command. The Navy must finish testing the capability on both satellites prior to declaring it fit for operational use.

Lockheed Martin handed over MUOS-2 and ground stations to the Navy Dec. 2, after its contractor test period.

The company is on contract to deliver five MUOS satellites; four comprise an operational constellation and the fifth is an orbit-ready spare. The contracts are worth at least $3.3 billion.