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Miscellaneous Tracking


Tyneside, UK
2017 Nov 25
Saturday, Day 329

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USA 155 and the Solar Eclipse of 2011 January 4

Through the Seesat-l satellite observing discussion group, Gerhard Holtkamp pointed out that the partial Solar Eclipse would result in satellites occupying slots in the equatorial geosynchronous belt also experiencing a blocking of the Sun by the Moon.

His point was that observers would see a visual dimming of those satellites. Radio tracking experience with USA 155, one of the United States' SDS satellites stationed at 10° west longitude, suggested that it might be possible to detect the eclipse's effect on its S-band transmitter.


Radio Observation

USA 155's radio transmitter is observed to drift in frequency around the time of the equinoxes as the satellite enters the Earth's shadow because the output from the solar panels ceases and the chemical batteries run down as a result. There is a description of the phenomenon here.

The plot below charts the progress of the 2011 January eclipse as it affected USA 155.

Red points and lines are actual frequency measurements. Unfortunately, the presence of the Doppler shifted signal from USA 179 was not anticipated and it swamped the data collection system. On the other hand, a computer screen-based visual representation was also being recorded, allowing some of the gap to be filled. The screen-based tracking is at a lower resolution so shadow entry can only be determined to ±5 minutes.

USA 155 Solar Eclipse

The scattered points either side of the line are background noise, visible because the USA 155 signal is quite weak when viewed through the omni-directional aerial that was used. Just before entry into shadow, which was penumbral at all times, the frequency was actually drifting downwards very slowly, indicating that a small charge was building up in the onboard storage batteries.

Gerhard Holtkamp's prediction for the USA 155 eclipse event was for shadow entry at 13:59 UTC, and exit at 14:59 UTC. Observation fits the prediction. At 14:03 ±5 minutes, USA 155's frequency started to rise. It levelled out at 14:59 UTC when output from the solar batteries was back to normal. It then took several hours to return to its original value as a small amount of charge trickled into the storage batteries.
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