In which direction are satellites turning and why
A satellite can stay in the same orbit for a long time because the gravitational pull of the earth ("gravity") balances the centrifugal force ("centrifugal force"). Since there is no braking air resistance in the orbit of the satellites outside the earth's atmosphere, the speed of the satellite remains constant for many years in a stable earth orbit.
The influence of gravity diminishes the further you get from the earth, while the centrifugal force increases with the speed of the satellite in orbit. Therefore, a satellite in near-earth orbit, generally at an altitude of around 800 km, has an extremely high force of attraction, which must be balanced by rapid movement along the orbit in order to generate the required centrifugal force. So there is a direct relationship between the distance to earth and the orbital speed of a satellite. At an altitude of 36,000 km the orbital time is 24 hours and thus corresponds exactly to the time of the earth's revolution. Such a satellite, which "stands" above the equator, does not move in relation to the earth, it is "geostationary".
The geostationary orbit
Geostationary orbits at an altitude of 36,000 km above the equator are known for their use of telecommunication satellites, e.g. for television. Signals from such satellites can be received practically all over the world. In order to have constant contact with the telecommunications satellite, it must always remain in the same position in relation to the earth's surface, so to speak "at one point in the sky".
A geostationary satellite offers the advantage for remote sensing that the earth is always "seen" from the same perspective. This means that you can take the same picture in short time intervals. This is particularly valuable for observing weather conditions.
A disadvantage of geostationary satellites is their large distance to the earth's surface, which considerably limits the technically feasible spatial resolution. Thanks to numerous weather satellites evenly positioned around the earth in geostationary orbits, we now have a global overview.
Many satellites transport passive sensor systems that work with sunlight. Hence, they move in a polar orbit around the earth. When measuring the reflected sunlight, the orbits must be adapted to the day-night rhythm. It is also important to be able to compare recordings made at the same time of day. For comparison, the lighting conditions must also be identical. The recordings must therefore be made at the same local time so that the height of the sun above the horizon is always the same. The inclination of the satellite orbit must also remain constant at the same angle to sunlight. These conditions can be met by sending a satellite into polar orbit.
While the satellite tracks its orbit and rotates around the earth, the earth also rotates on its own axis. After each complete orbit around the earth, the satellite scans a new strip of the earth's surface. After a certain number of orbits, the satellite has covered the entire surface of the earth. Some satellites work with a wide scanning field and can therefore completely cover the earth in just a few orbits; In contrast, high-resolution satellites only scan a narrow area at a time and need several days to explore the entire surface of our planet.
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