Orbital Focus - International Spaceflight Facts and Figures
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STSS Demonstrator Mission

Tyneside, UK
2022 Aug 15
Monday, Day 227

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STSS Demonstrator:


Description of missile detection and interception system

STSS Demonstrator home page
Radio Monitoring of Operations

The Space Tracking and Surveillance System (STSS) Demonstrator mission is to demonstrate new techniques for detecting, from orbit, missile launches. Until now, this task has generally been undertaken from high orbit - either geosynchronous or highly eccentric inclined orbit.

Because the Delta deposited SV2 into a slightly lower orbit than its companion it started life by overtaking it within a short time and moving increasingly further ahead by virtue of its lower orbital period.

Separation between SV1 and SV2

STSS Satellites in orbitA document published on the Web by Aerospace Corp early in 2008 indicated that the operational separation of two satellites would be 35 degrees of orbital arc (best thought of as the angle between vertical lines from each satellite to the centre of the Earth).

A Northrop Grummann web document mentions an extended period of testing. The result of tracking is to indicate that a considerable time may elapse from launch to settling down in the operational configuration. This is similar to what was observed when tracking USA 198, an SDS satellite, where many weeks passed between it reaching orbit and settling down into a similar routine to its companion satellites. In the event, four months passed between launch 2009 Sep 25 and the station-keeping manoeuvre that occurred 2010 Feb 28.

The plot below shows measurements of the radio-determined separation over the four months, using timed observations almost exclusively from the Zarya location in the UK. It shows the SV2 satellite's lead in terms of of orbital angle. During the first 3-4 days of the mission, there was some tracking from Sweden and South Africa and the USA. Greg Roberts (SA) provided particularly useful Doppler observations in the first few days that helped in monitoring both the separation and the true transmission frequencies.

STSS Satellite separation

The plot tracks back to zero at a time very close to release from the launch vehicle. What it tells us is that, apart from the initial separation manoeuvres by the Delta rocket that put SV2 into the lower orbit, no other significant orbital adjustments took place up to Oct 7. This was a sensible strategy that cut out use of onboard propellant because the Delta did the initial work.

2009 Nov 11, the separation peaked at 63 degrees of orbital angle. Had they separated much further then line of sight between the two would have been lost as the Earth got in the way. At that point, an orbit adjustment by SV2 caused the gap to start closing until 2010 Jan 28 when SV2 again fired its thrusters and commenced station keeping operations with SV1.

Notably, all major orbital adjustments to this point were carried out by SV2.

The separation calculates as 36.25 degrees, a little more than the 35 degrees given in an Aerospace Corp document. However, there are alternative ways of expressing the measured separation and there may be some rounding of numbers involved so, to all intents and purposes they are the same figure.

The following diagram comes from the Aerospace Corporation document and illustrates the orbital geometry with 35 degrees separation. Note - the document opens in a separate tab/window.

"The demonstrator satellites provide above-the-horizon stereo (two-viewer) coverage in the green area, mono (single-viewer) coverage in the yellow areas, and no coverage in the red area at any given instant. Stereo viewing provides better track accuracy than mono viewing. The map relates only to the view of the ground.

Stereo coverage increases with target altitude as does the coverage radius in general. Altitude also determaines whether the target is viewed against an Earth background (below the horizon) or a space background (above the horizon). While the demonstrator satellites are not technically operational, increased numbers above two would provide a rapid increase in coverage capability, especially for stereo viewing.

Log of Radio Tracking Events

A log of significant events noted while STSS was being checked out prior to reaching the operational separation can be found in the left hand menu of this page.

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