Thursday, 7 July 2016

Tracking MUOS 5 in GTO [UPDATED]

click to enlarge

Over the past days, Paul Camilleri in Australia and me in the Netherlands have been tracking an object in GTO with a Mean Motion of 1.5 revolutions per day. It produces brief bright (mag +8) flashes each ~5 minutes. We are certain this is MUOS 5 (2016-041A) launched June 24 (see my earlier post here, about Paul's orbit insertion and Centaur fuel vent imagery).

The image below was shot by me from Leiden, the Netherlands, during the night of 4-5 July 2016. The object was at an elevation of only 16 degrees above the horizon:

click image to enlarge

Paul first imaged it from Australia on June July 3, when it passed a few degrees from the position where we expect MUOS 5 to be placed in GEO. I next imaged it from the Netherlands during the night of June July 4-5, low at my southeast horizon not far from Mentor 6. A few hours later, Paul observed it again from Australia. All these observations can be fitted to yield this GTO orbit:

MUOS 5                                               15242 x 35703 km
1 41622U 16041A   16186.93646397 0.00000000  00000-0  00000+0 0    08
2 41622   9.8319 324.4682 3211964 178.4686 182.8307  1.52727671    09

rms   0.003   from 14 observations Jul 3.46 - Jul 5.57 (arc 2.1 days)
Comparing this orbit to the initial GTO insertion orbit from June 24-25 provides a clear link. The RAAN values of both orbits agree to within a few degrees, and the apogee direction is also very similar, as can be see in the plot below:

click to enlarge

In the plot above, the red orbit is the June 24 initial GTO insertion orbit. Somewhere after June 25, the satellite manoeuvered (multiple times probably) to increase its perigee from 3900 km to 15240 km. The white orbit is the resulting "current" GTO orbit from the July 3-5 observations.

[ UPDATE 7 Jul 2016 17:25 UT: I have since done an analysis that suggests that a perigee-raising manoevre from the initial 3903 km to 15242 km could have happened on July 3, near 14:33 UT, in apogee. I suspect however that it was in reality a series of smaller manoeuvres [update July 8: series of manoeuvres confirmed here]]

The grey orbit is the eventual geosynchronous orbit in which MUOS 5 will be inserted a few days from now (probably with a position near longitude 172 W). It will probably make more manoeuvres for that purpose the coming days. [update: there is a possibility it actually did so only a few hours after our last observation on July 5] 

UPDATE July 8 17:00 UT: News has come in that something went wrong and MUOS 5 is snagged in GTO for now. More on the website and a brief follow-up post here].

The plot below shows how during this manoeuvering, the orbital inclination has been lowered, from 19.0 degrees initially, to 9.8 degrees currently. It will be further lowered to ~5.0 degrees upon GEO insertion:

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The object shows a clear brightness variation, from mag +8 to invisible, with a peak-to-peak period of ~5.0 minutes, indicating the satellite is spin-stabilized. [update:  Ted Molczan has noted that this 5-minute periodicity seems to be typical for the Lockheed A-2100 bus in GTO].The bright peaks are of short, specular and somewhat variable duration: lasting ~0.5 to 1 minute. During the lows, the object was not visible for my equipment.

The image sequence below, from my June July 4-5 imagery, shows a part of the described brightness behaviour:

click image to enlarge

As I have written earlier, MUOS 5 will likely be placed in a geosynchronous, 5-degree inclined orbit near longitude 172 W, probably within a few days from now or perhaps even on July 5th already [see the update already mentioned above: MUOS 5 has got stuck in GTO! See also the brief follow-up post here]. This is an initial check-out position. It will stay there for 4 to 6 months, and then be moved to longitude 72 E where it will be placed as an on-orbit spare. In 2015, we observed this with MUOS 4 (see previous posts here and here).

(this post was thriple updated, on 7 Jul 17:25 UT and 8 Jul 8:30 UT and 17:00 UT)

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