From the United Launch Alliance press release about the launch of AEHF-4:

United Launch Alliance (ULA) will use an Atlas V 551 rocket to launch the fourth Advanced Extremely High Frequency (AEHF) communications satellite for the U.S. Air Force from Space Launch Complex-41 at Cape Canaveral Air Force Station, Florida.

The AEHF system, developed by Lockheed Martin, provides vastly improved global, survivable, protected communications capabilities for strategic command and tactical warfighters. This jam-resistant system also serves international partners including Canada, the Netherlands and the United Kingdom.

AEHF-4 will be a protected communications relay to provide the highest levels of information protection to the nation's most critical users. The Lockheed Martin A2100 satellite gives senior leadership a survivable line of communications to military forces in all levels of conflict, including nuclear war. The system features encryption, low probability of intercept and detection, jammer resistance and the ability to penetrate the electromagnetic interference caused by nuclear weapons to route communications, real-time video, maps and targeting data to users on land, at sea or in the air.

The Atlas V 551 rocket will deliver AEHF-4 into an optimized, high-energy geosynchronous transfer orbit. ULA and the AEHF program produced this ascent profile to maximize mission flexibility over the satellite's lifetime.

Atlas V rockets successfully launched the first three AEHF satellites in 2010, 2012 and 2013 as the new constellation was formed in geosynchronous orbit 22,300 miles above Earth to augment and eventually replace the legacy MILSTAR communications satellite fleet. One AEHF satellite has greater capacity than the entire five-satellite MILSTAR constellation.