TacSat-4 is a Navy lead Joint project managed by NRL's SED. TacSat-4's mission is to augment current SATCOM capabilities and to advance Operationally Responsive Space systems. TacSat-4 provides 10 Ultra High Frequency (UHF) communications channels that can be used for any combination of communications, data exfiltration, or Friendly Force Tracking (FFT).

TacSat-4's HEO (Highly Elliptical Orbit) orbit augments geosynchronous SATCOM by providing near global—but not continuous—coverage, including the high latitudes. TacSat-4 improves on current SATCOM by providing communications-on-the-move (COTM) for existing radios without antenna pointing. TacSat-4 provides flexible up- and down-channel assignments, increasing its ability to operate in some interfered environments. The tasking system, coupled with the orbit, allows dynamic reallocation of communication channels to different theaters worldwide and within 24 hours, enabling rapid SATCOM augmentation to support unexpected operations such as when natural events occur.

Sponsors & Management: The Office of Naval Research is funding the payload, management, and first year of operations. OSD-OFT/DDR&E funded the standardized spacecraft bus. The ORS Office & Air Force are providing the launch on a Minotaur-IV. The Naval Research Lab (NRL) is the program manager.

Spacecraft & Orbit: TacSat-4 is in the Small-Sat class at 450kg with 1000W array and steerable 12-foot payload antenna. The "Low HEO" orbit has a 4 hour period (6 orbits per day), altitude range from 700km to 12,050km, and is inclined at 63.4 degrees with an argument of perigee at 210 degrees. For a single satellite, a given location on the ground in the Northern Hemisphere is generally in view for about 2 hours per pass and sees three consecutive passes each day.

User Terminals: TacSat-4 enables COTM with existing radios, such as the PRC-117 and 152, and does NOT require user antenna pointing at the typical rates of 16 kbps or less. Advanced capabilities such as converting voice comms to SIPRNET, bridging channels, FFT, or specific buoy data exfiltration collections require a custom terminal for each 2000nm radius area. Given the state of wideband radio development, users are expected to use TacSat-4's 5-MHz channel only for purposes of experimentation. Commercial ODTML transmitters are available to provide a turn-key, IP-based data exfiltration option.

Availability: Launch is scheduled for Fall 2009 on a Minotaur IV from Kodiak, Alaska.