Chapter 3

 Help you understand how satellites are used in the following areas: ◦ Global Positioning (GPS) ◦ Weather ◦ Data transfer systems ◦ Television  You should be able to explain the advantages and disadvantages of using satellites for these applications.

 GPS was first used by the US military to locate ground troops.  It was then released to civilians in  Russia has its own service (GLONASS) and India plans to launch its IRNSS service in 2012 with the European service (Galileo) launching in 2013.

 One of the most common use of GPS technology is in satnav systems found in cars.  However, it is also used in map making, navigation for ships and aeroplanes and also by the military.  Smart phones are also making use of it and apps such as Gowalla are allowing people to ‘tag’ places of interest and mark them onto Google Maps.

 The GPS service makes use of 24 satellites which orbit Earth twice a day.  At any one point there should be at least 4 satellites in line of sight with a GPS device.  The GPS device will attempt to communicate with at least 3 of the devices.  It then uses a method known as trilateration to determine its current location. The green dot represents your current location. This is determined by the location where all three circles cross over each other. The more satellites connected the better the accuracy of the location result.

ProsCons Accuracy can be very good (30cm accuracy is possible) Blind spots can interfere with signals (e.g. tunnels, buildings, etc) If you are lost you can use a GPS device ‘un-lose’ yourself or you can use a GPS tracking device to find a lost person. The atmosphere can effect accuracy (e.g. thunder storms, rain, etc)

ProCons There is no need for the driver to use a map. They can concentrate on driving. Maps might not be up to date resulting in poor results from the SatNav system

 These satellites are used to monitor weather formations.  They take images of cloud formations which are then analysed.  They are also able to gather data on pollution, ice cover, water temperature and sea levels.

 Weather satellites are either in geostationary orbit or polar orbit.  Geostationary means that the satellite is in a fixed location above the equator.  Polar orbiting means the satellites orbit from north to south and then south to north.

ProsCons Early warning systems for hurricanes can be put in place to allow evacuations to take place. The cost of building and launching a satellite is very high (in the millions). Energy use can be monitored to track how much light is being produced in countries. Weather patterns are not always predictable so the data can only be used as a guide. Fires can be monitored to track movement. There are concerns that these satellites are being used for other reasons such as tracking the movement of people. Climate change can be tracked over a period of time using the data weather satellites collect.

 All satellites transfer data.  The type of data that is transmitted varies but essentially they all pass data using the same types of methods.  For more information please refer to the presentation covering Wireless Communications.

 Because satellites have a high bandwidth more data can be transmitted compared to a regular aerial.  This means a viewer can receive many more channels.  Set-top boxes allow you to view information about the programme you are watching and also provide you with a list of what programmes are currently on and will be broadcast in the future.  Some boxes also allow you to record and now receive Hi-definition television.  Sky Digital are planning on releasing a new 3D service in 2010.

 Sky Digital makes use of the Astra satellite system.  You can see the satellites and their coverage below.  The larger the dish you have the more channels you can receive. The following link will allow you to see where all the satellites are in terms of their location and their orbit. /jtrack/3d/JTrack3D.html Please note: Java needs to be installed and enabled on your web browser for this to work.

 Television shows were traditionally broadcast via radio waves which came from television masts.  These masts only provided a limited number of frequencies which meant that not many channels could be broadcast.  They also required line of sight meaning they had to be positioned high up on hills or built very tall.  The UK Government are phasing out this type of transmission and replacing with a completely digitalised service which will provide people with more channels at a better quality.

 The satellites used for broadcasting television signals are in geosynchronous orbit meaning they remain in one place relative to the Earth.  Their orbit is about 35,000 km above the Earth.  This high orbit means they have a better line of sight compared to traditional aerials. This is a nice little web page that explains how it works in more detail: This is a nice little web page that explains how it works in more detail:

 The satellites receive the transmissions from the ground and then broadcast them out to the receiving dish.  It uses a system called Direct Broadcast Satellite (DBS).  The signals are encrypted so that channels can be subscribed to. This allows companies to charge customers extra for pay-per-view programmes. This is a nice little web page that explains how it works in more detail: This is a nice little web page that explains how it works in more detail:

 Satellite dishes are designed to receive the encrypted broadcast signal but it is the job of the set-top box to decode the data.  The box receives the encryption key which allows it to decode subscribed channels.

 Broadcast signals are also compressed so that more channels can fit onto one frequency.  The set-top box will also un-compress the signal so that channels can be watched.  A misconception is that data can be sent via the satellite dish. This is untrue – data is sent via the telephone cable (look at the back of your sky box!)

ProsCons Hundreds of channels are available due to the high bandwidth that satellites provide. Equipment is initially expensive to purchase or a subscription contract has to be taken out first. Television signals can be received in remote locations because line of sight is better with satellites compared to terrestrial television masts. Satellite signals can not be transmitted to different rooms unless each room has it’s own set-top box. Viewers can choose what channels they want to subscribe to. Severe weather conditions, such as electrical storms, can effect the quality of the signal. Companies can charge customers for Pay- Per-View events because signals can be encrypted. Satellite dishes must be fixed to the house. This can sometimes be unsightly, especially when the dish must be placed at the front of the house.

 Describe three features of navigation software.  Why will the location on a map be lost when travelling through a tunnel?  How can driving be made safer using GPS devices such as SatNavs?  State four types of data that can be collected by weather satellites.  Why are photographs collected by polar orbiting satellites much higher in quality compared to photographs collected by satellites in geostationary orbit?

 Why are more channels available through satellite television that terrestrial television?  List the equipment required to receive satellite television in the home.  How are signals broadcast using DBS?