National Reconnaissance Office mission NROL-61 utilizing United Launch Alliance ATLAS V rocket is planned on 21st April 2016 . Launch is planned from launch site SLC-41 in Cape Canaveral AFS.
There is nothing special in fact, that next NROL mission is classified. As far as payload, we can only have gossips and conjectures. According to some sources, this time place atop Atlas V will occupy one of satellites from Satellite Data System fleet. This highly classified program has started in 1976, when first SDS generation satellite was launched. Since then, probably at least next three generations were developed. First versions were equipped with 12 channels ultra-high frequency communication, were long for 7.6 m, heavy for 630 kg and cylindrical in shape. 980 W of power were provided by solar panels and batteries. Next generation was bigger, with weight at 2335 kg. SDS-2 characteristically features were three separate communication dishes (one was especially for K band communication). Two dish – type antennas were 4.5 m in diameter, while the third was 2 m in diameter. The solar panels generate 1238 watts of power. Satellites were (are?) operating on elliptical orbit, with perigee at 300 km and apogee at 39000 km. It is assumed that satellites were manufactured by Hughes. First generation was launched with Titan III rocket, Second was lifted by Space Shuttles and Titan-IV rocket. In missions of third generation Atlas and Delta rockets were utilized. SDS objective is uncertain. It can be a communication system providing high speed, real time data transmitting from U.S. satellites operating in polar areas. For example they can retransmit data from Key Hole optical reconnaissance satellites or terrestrial radar imaging reconnaissance satellites – Lacrosse and Onyx series. Last hypothesis would be partially confirmed. Lacrosse and Onyx were intended to retire after starting operational activity of radar reconnaissance satellites (FIA-R) from FIA (Future Imagery Architecture) program. It is possible that after delays in FIA development (after Boeing, manufacturer of FIA satellites, was not able to design FIA-R class satellites and FIA-R satellites were moved to Lockheed Martin) NRO is still operating with Lacrosse and Onyx.
United Launch Alliance is going to utilize in this mission Atlas-V rocket in 421 version. From number of version we can learn that this version will be equipped with 4 m payload fairing, will utilize two boosters and its propulsion will be one engine in second stage. In this version it will be capable to lift 6890 kg to GTO.