| Dusk-Dawn polar orbit
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RADARSAT-1's orbit is a low-earth orbit of about 800 km above the Earth's surface. The plane of RADARSAT-1's orbit is aligned so that it is at right angles to the sun as shown in the diagram to the left. This orbital path is synchronised (to always face the sun) and to maintain this orientation throughout the year. Since RADARSAT-1 always travels along the terminator (the line between day and night) it is called a sun-synchronized dusk-dawn orbit. Also, because the orbit passes over the Earth's poles, it called a dusk-dawn polar orbit. |
| Satellite Moving Northward at Sunset
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The fact that RADARSAT-1 has a sun-synchronized dusk-dawn polar orbit leads to some effects which at first appear surprising. RADARSAT-1 has been placed in an orbit which carries from South to North, along the terminator, at 18:00 hours (local time). In other words, at 6:00 pm in the evening, as the sun sets below the Western horizon, RADARSAT-1 can be seen passing overhead in a South-to-North trajectory. |
| Satellite Moving Southward at Sunrise |
Now, the next time RADARSAT-1 passes overhead, along the terminator, will be at 6:00 am the next morning as the sun rises above the Eastern horizon. Here's what, at first, seems surprising; RADARSAT-1 is moving overhead in a trajectory which carries it from North to South! To understand what has happened, look at the first diagram (at the top of this page). Follow (in your "mind's eye") the orbit of RADARSAT-1 as it moves up the page (South-to-North) along the sunset terminator. Notice that RADARSAT-1 must descend (North-to-South) on the other side of the earth along the sunrise terminator. |
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The inertial frame of reference for all spacecraft is a frame of reference defined by the fixed stars. Recall that RADARSAT-1 is in a sun-synchronous orbit. That is, the plane of its polar orbit always faces the sun. The Earth however is orbiting the Sun. This causes the direction to the Sun, with respect to the fixed stars to continually change. The illustration to the left (not to scale!) shows how the plane of RADARSAT-1's orbit must precess (rotate) with respect to the frame of reference defined by the fixed stars. |
