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Answers Your Questions

From: Marc Sarrasin, of Winnipeg, Manitoba
To: Matt Abbott, flight director

Question: Hi. I enjoy reading about activities onboard the ISS. I'm a little familiar about the ISS' orientation such as LVLH, XPOP and "barbecue". What exactly does it mean? Does it affect how bright ISS is when it passes overhead?

Answer: Hi Marc!

LVLH stands for Local Vertical, Local Horizontal. When we fly this attitude, the “nose” (+X axis) of ISS is pointed in the velocity vector, and the “bottom” (+Z axis) of ISS is pointed at the Earth, which is similar to the attitude an airplane flies in level flight. An astronaut looking out the Lab window, which is on the bottom of the Lab, will always see the Earth. In order to keep the +Z pointed at the Earth, ISS rotates at 4 degrees/minute (360 degrees per 90 minute orbit). LVLH is the attitude most often flown by ISS.

What is sometimes referred to as “barbecue” is an LVLH attitude but with a 90-degree yaw. Viewed from above, it would appear as if ISS is flying sideways. It received the “barbecue” moniker because the attitude is sometimes used as an alternative to XPOP to provide more even heating on the ISS structure. The +Z axis still faces the Earth.

XPOP stands for X-axis Perpendicular to Orbit Plane. In this attitude, the “nose” of ISS is again 90 degrees from the direction of travel, but ISS no longer rotates at 4 degrees/minute to maintain the +Z axis pointing at the Earth. Instead, the attitude appears to be nearly fixed with respect to the stars, and the +Z axis only points towards the Earth once per orbit, and that’s at orbit noon. Depending on where in the orbit ISS is, an astronaut would see the Earth or the night sky out the window. We fly XPOP at certain Sun orientations to improve illumination on the solar arrays.

The attitude, as well as the orientation of the solar arrays and the relative position of the Sun, does affect the brightness as it passes overhead since it alters how much sunlight is reflected back towards an observer.

Mike Lammers
Attitude Determination and Control Officer (ADCO)

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Curator: Kim Dismukes | Responsible NASA Official: John Ira Petty | Updated: 09/03/2003
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