Communication using satellites
Electromagnetic waves can be used for communication.
Mobile phones communicate to a mobile cell tower using radio waves, towers communicate with satellites using microwaves. Microwaves are used as they can pass through the atmosphere. This signal can be sent to a satellite and used to communicate around the world (more than one satellite is required for this).
Geostationary and geosynchronous orbits
What is a geosynchronous satellite?
- It has an orbit time of 24 hours.
- It returns to the same point in exactly 24 hours.
A geostationary orbit is a particular type of geosynchronous orbit.
How is a geostationary orbit different?
The difference between a geosynchronous orbit and a geostationary orbit is that an object in geostationary orbit never leaves its position. A geosynchronous orbit means that an object completes a full orbit in 24 hours, thus returning to its original location in the sky.
A base station can be in constant communication with a geostationary satellite, but only once every 24 hours with a geosynchronous satellite.
A microwave signal can be sent to a satellite in a geostationary orbit. These satellites are in orbit above the The line around the centre of the Earth, parallel to the Tropics of Cancer and Capricorn.. The height of their orbit (around 36,000 km) is just the right distance so that it takes them one day (24 hours) to make each orbit. This means that they stay above the same point on the Earth’s surface.
Geostationary satellites always appear in the same position when seen from the ground. This is why satellite television dishes can be set in a certain position and do not need to move.
Exam questions may require you to calculate the time taken for a microwave signal to travel from, for example, point A to point B on the Earth.
This diagram shows how it is possible to communicate between two points, A and B, on the Earth.. The signal must travel from point A to the satellite that is positioned 36,000 km above the Earth before returning to the Earth at point B.
Question
Calculate the time taken for a microwave signal to travel from point A to point B via the satellite.
The distance travelled by the microwave is 36,000 km × 2 (as it has to travel to the satellite and back) = 72,000 km.
You need to convert this into metres by multiplying by 1,000 (remember k = 1,000, this will be given on the exam paper if you’ve forgotten).
72,000 × 1,000 = 72,000,000 m (or 72,000 × 103 m or 72 × 106 m)
Next, you need to calculate the time taken for the signal to travel this distance.
\(\text{distance =}{\text{~speed}}\times{\text{time}}\)
Rearrange the equation to get
\(\text{time =}~\frac{\text{distance}}{\text{speed}}\)
The speed of the microwave is the same as the speed of all other electromagnetic waves, 3 × 108 m/s. You will be reminded of this in the exam.
Substituting the numbers into the equation gives
\(\text{time =}~\frac{\text{distance}}{\text{speed}}=\frac{{72\times10}^{6}}{{3\times10}^{8}}={0.24}{\text{~s}}\)
