Viewing page 18 of 117

This transcription has been completed. Contact us with corrections.

[[image]]

How does this Rotation Affect the Ground Track Drawing for Successive Orbits?

The Earth's rotation to the East causes the ground tracks to shift to the West after each successive orbit. The easiest way to measure the shift in the ground track is to measure how much the ascending and descending nodes move to the West. After one 90 minute orbit, the nodes shift 22.5° to the West. This means that because of the Earth's rotation, the shuttle only covers 337.5°(360°-22.5°=337.5°) of longitude along the Earth's equator during each 90 minute orbit. 

Eventually after several more orbits, a criss-cross pattern forms on the ground track map. Because of the Earth's rotation, a shuttle in a low earth orbit actually sees different parts of the world in every orbit. At the equator 1 degree corresponds to 69 miles so that for each orbit, the ground track shifts about 1500 miles along the equator. 

Ground Tracks for STS-81

The figure below shows the shuttle's predicted ground tracks for two orbits during STS-81. The solid line represents orbit 1 and the long dashed line represents orbit 2. Orbit 1 begins at its ascending node, -145°(145° West). Orbit 2 begins at the next ascending node (marked ascending node #2 in the figure), -167.5°(167.5° West). The descending node in orbit 2 moves 22.5° to the West of the descending node of orbit 1. 

[[image]]

7/17/96            Page 6.17

Transcription Notes:
[[Image – A world map set on a grid: on the left, each horizontal line marked in increments of 20 from -80 at the bottom to 80 at the top; each vertical line marked along the bottom in increments of ten, left to right, from -180 to 180; superimposed are two sine waves marked orbit 1 and orbit 2; as sending and descending nodes locations are marked]]