click image to enlarge
Last Saturday evening I however targetted USA 184 (2006-027A), a classified US military satellite in HEO which hovered almost in the zenith for my locality during the observation. It is the tiny trail indicated by the arrow in the image above, taken with my Canon EOS 60D and a 2.8/180mm Zeiss Sonnar MC. Stars in the image belong to Ursa maior.
A Highly Elliptical Orbit (HEO) is an orbit which is highly eccentric ("elliptical") with a low perigee at only a few hundred kilometers altitude (usually in the southern hemisphere) and a high apogee, often in the 20 000 to 39 000 km altitude range. The orbit is typically inclined by about 63 degrees. USA 184 is in a 63.58 degrees inclined, 1590 x 38 760 km orbit.
Satellites in such an orbit spend a long time near the apogee of the orbit. As a result, they hover high above the northern hemisphere for many hours a day. Just like a geosynchronous orbit, this allows long duration coverage of a (large) area. The difference with a geosynchronous orbit is that a HEO orbit is well suited to cover high polar latitudes, while a geosynchronous orbit has a poor coverage of such high latitudes. HEO orbits are therefore typically used for applications that demand long-duration coverage of high Northern latitudes. It concerns communications satellites (notably by the Russians), SIGINT satellites and Infrared Early Warning satellites.
USA 184 falls in the latter two categories. It is a TRUMPET-FO (the FO stands for "follow-on", i.e. it is an improved version of the older TRUMPET) SIGINT satellite. In addition, it has a piggyback SBIRS (Space Based Infrared System) package, which is dedicated to the detection of ICBM launches by their Infrared signatures. It is one of two HEO sensors in the SBIRS system (the other one is on USA 200, 2008-010A), in addition to the two dedicated SBIRS satellites in geostationary orbit (SBIRS-GEO 1 and SBIRS-GEO 2, 2011-019A and 2013-011A).
At the time of the observation, USA 184 was at an altitude of 38 355 km over the Northern Atlantic at 62.74 N, 4.84 W. It was almost in its apogee, and hovered at 76 degrees elevation in the sky. This is the approximate view from the satellite at that time:
note: the orbital diagrams were made with JSatTrak software and amateur orbital elements calculated by Mike McCants.