At 21:39 UTC on September 13th, 2012, the NRO launched NROL-36 from Vandenberg AFB. The launch vehicle was an Atlas V containing a classified NRO payload in addition to a number of cubesats. Several analysts already suspected the classified payload of this launch to concern a new duo of US Navy NOSS satellites.
That suspicion appears to be born out by the first observations of the launched objects, which show two satellites in close formation, typical of a NOSS formation.
The Centaur last stage was the first object from this launch to be observed, by Björn Gimle in Sweden, who observed it 5 hours after launch, and then by BWGS chair Bram Dorreman in Belgium 1.5 hours later, who remarked it was variable in brightness. Alas I was clouded out in Leiden at that moment. Eleven hours after launch, Kevin Fetter in Canada observed it next. Over the following day Russell Eberst in Scotland and Scott Tilley in Canada added more observations.
The payloads were first observed by Kevin Fetter in Canada near 9:43 UTC (Sep 14), 12 hours after launch. Scott Tilley, also in Canada, next observed them on the 15th near 12:34 UT.
On the morning of the 16th, 2 days and 5 hours after the launch, it was clear in Leiden allowing me to join the chase. I (photographically) observed the Centaur r/b pass at 2:49 UTC (see image below) and then the payloads at 2:55 UTC (Sep 16).
The observations were done under difficult conditions: it was somewhat hazy, the passes were at very low altitude due east (only 14 degrees elevation for the Centaur and 21 degrees elevation for the payloads), and the phase angles for the objects were unfavourable, resulting in dim magnitudes (around +7 for the payloads). The resulting images (one of them below) are therefore not quite pretty: even with use of the 1.4/85mm SamYang lens the trails were so faint that they were marginally visible at best. As reported earlier by Scott Tilley, the leading object indeed seemed to be somewhat variable (on some images it wasn't visible at all).
USA 237 (Mentor 6?)
That same morning, following the NROL-36 related observations, I took a few images of the area near the bright geostationary Mentor 4 (USA 202, 2009-001A). These images not only recorded this 3-year-old Mentor, but also the geostationary satellite USA 237 (2012-034A), a classified NRO payload launched from Cape Canaveral as NROL-15 on 29 June 2012, employing an Atlas IV-Heavy as launch vehicle.
These were my first observations of this object: tracking of this object over the summer was done mostly by Greg Roberts in South Africa (who was the first to discover the object) and Peter Wakelin in Britain. Over the summer it had slowly been drifting westwards: as of mid-September 2012 it appears to have stabilized at 47.8 E. For me in Leiden it is currently located at 20 degrees elevation in azimuth 129 degrees (low SE), about 6 degrees separated from Mentor 4 in the sky.
The identity of this object is still provisional. There have been some thoughts that NROL-15 launched a stealth Misty satellite in LEO/MEO and then progressed to put a decoy (the object designated as USA 237) in geostationary orbit. USA 237 is very bright however (about mag. +8), ranking it among the brightest geostationary objects in the sky, the Mentor SIGINTs. As Ted Molczan showed, they seem very similar in terms of absolute brightness. The likeliness can be well seen in the photographic comparison below, which shows Mentor 4 and USA 237 (the crops come from one and the same image):
This gives reason to think that NROL-15/USA 237 is perhaps a sixth Mentor SIGINT (i.e., it is Mentor 6).
As by-catch of the NROL-36 payload and Centaur observations, I also obtained my first observations of the geostationary SDS satellite SDS 2-2 (USA 67, 1990-097B). It is located at an elevation of only 16.5 degrees, very low in the east for me. To my surprise, it was rather bright and easily visible in the 5-second exposures, of which the one below is one: