Understanding Geostationary Satellites

Geostationary satellites are a specific type of satellite that orbits the Earth. Their key characteristic makes them incredibly useful for applications like telecommunications, broadcasting, and weather monitoring.

What Makes a Satellite Geostationary?

A satellite is considered geostationary if it appears to remain in a fixed position in the sky relative to an observer on Earth. To achieve this, the satellite must meet several conditions:

• It must orbit in the same direction as the Earth rotates (west to east).
• It must be in a circular orbit directly above the Earth's equator.
• Most importantly, its orbital period must be exactly equal to the Earth's rotational period (one sidereal day, approximately 23 hours, 56 minutes, and 4 seconds).

If a satellite fulfills these conditions, its angular velocity matches the Earth's angular velocity. This means that as the Earth rotates below, the satellite moves at the same angular speed, effectively staying above the same point on the Earth's surface.

Analyzing the Options

Let's look at the given options in the context of geostationary satellites:

• Option 1: same distance from earth's centre
  Geostationary orbits are indeed circular, meaning the distance from the Earth's center is constant. However, other types of circular orbits (like Low Earth Orbit or Medium Earth Orbit) also maintain a constant distance but are not geostationary because their orbital period is much shorter than Earth's rotation period. So, while true, this is not the defining characteristic that makes it "geostationary".
• Option 2: same angle with geodetic stations
  Geodetic stations are ground reference points used for surveying and navigation. A geostationary satellite maintains a nearly constant angle of elevation and azimuth from a specific ground station within its view, because it appears fixed in the sky. However, stating it has the "same angle" with *all* geodetic stations doesn't make sense, as the angle varies depending on the station's location relative to the satellite. This option is not a standard or accurate description.
• Option 3: same mass as global weight
  This option uses non-standard terminology ("global weight") and relates to the satellite's mass, which is an intrinsic property and has no bearing on its orbit type being geostationary. This option is incorrect.
• Option 4: same speed as earth's rotation
  Specifically, a geostationary satellite has the same angular speed as the Earth's rotation. This allows it to complete one orbit in the same time the Earth takes to rotate once, making it appear stationary. While "speed" here refers to angular speed for the comparison to Earth's rotation, in the context of the options, this is the correct interpretation describing the core characteristic that makes the satellite appear fixed relative to the rotating Earth.

Conclusion

The defining characteristic of a geostationary satellite that enables it to appear stationary above a fixed point on Earth is that its orbital period matches the Earth's rotational period. This is best described by stating it has the same speed (angular speed) as Earth's rotation.

Therefore, the correct option is that geostationary satellites have the same speed as earth's rotation.

Revision Table: Geostationary Satellite Properties

Additional Information: Geostationary vs. Geosynchronous

While often used interchangeably, there's a subtle difference between geostationary and geosynchronous orbits:

• Geosynchronous Orbit: Any orbit with a period equal to the Earth's sidereal rotation period (24 hours sidereal time). A satellite in a geosynchronous orbit will be over the same longitude at the same time each day. The orbit doesn't have to be circular or equatorial. If it's elliptical or inclined, the satellite will appear to move in a figure-eight pattern in the sky.
• Geostationary Orbit: A specific type of geosynchronous orbit that is both circular and equatorial. This combination ensures the satellite remains directly above a fixed point on the equator, hence appearing stationary.

All geostationary orbits are geosynchronous, but not all geosynchronous orbits are geostationary.