SIM-201 Satellite Telephony Radio Frequency Satellites and Orbits GPS
Overview • Chapter 13 (continued) • Generations of cellular systems • Satellite telephones • Chapter 16 • Radio-Frequency • Satellite Systems and Orbits • GPS
Generations of Cellular Systems • Generations of cellular systems include: • AMPS 1st generation • GSM 2nd generation • W-CDMA 3rd generation • Cellular systems operate based on various protocols, and use RF (radio frequency) waves that propagate through the air for transmission of information. These systems typically use the 800-900 MHz or 1800-1900 MHz frequency band of the radio spectrum. • But what is the radio spectrum?
The Radio Spectrum • The Radio spectrum is composed of many frequency bands • Communication systems have the ability to transmit signals at various frequencies, and the FCC (Federal Communications Commission) regulates which frequencies to use Source: http://electronics.howstuffworks.com/radio-spectrum1.htm
Radio Spectrum Bands Low Frequency=large period High Frequency=small period ~400-2400 MHz frequency range • VHF • Television • FM radio • Aviation • UHF/SHF • Satellite • Television • Cell Phones • Microwave • EHF • Astronomy • MF • AM Radio • Maritime • HF • Maritime • Amateur Radio FM radio stations between: 88-108 MHz More Info: http://en.wikipedia.org/wiki/Radio_spectrum Electromagnetic Spectrum - http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html
1st generation • AMPS:Advanced Mobile Phone System • The only system available in the United States until about 1997 (commercial service started around 1979) • It is the first system used for cellular telephony, and it is analog. • Uses the 800 MHz frequency band of the spectrum • Is still being used today. However, the number of subscribers began to decrease in 1999 due to migration to digital. • Utilizes FDMA (Frequency division multiple access) to separate users • In FDMA, users are separated in frequency. i.e. mobile phones communicate at different frequencies than the others within each cell. The radio spectrum is shared among users.
2nd generation • GSM: Originally: “Group Special Mobile”, now: “Global System for Mobile Communications” • Is the most popular system worldwide • Originally developed in Europe • Was later introduced into the United States • Is a digital system for both voice and data transmission • Uses the 900 and 1800 MHz frequency bands • Utilizes TDMA (Time division multiple access) • In TDMA, each user can occupy the entire spectrum, but users are separated in time. i.e. mobile phones communicate in a different time slot than the others within each cell. Time is shared among the users.
3rd generation • W-CDMAWideband code division multiple access: • Provides much higher data rates • Supports a larger number of users • Enables features such as video, internet access, web browsing and complete worldwide operability • Widespread introduction began in 2005 • Utilizes CDMA (Code division multiple access) • In CDMA, users are separated by a unique code assigned to them. i.e. each mobile phone can utilize the entire spectrum, and can transmit all the time, but a unique code of each user is used to encode and decode the information during transmission
Satellite Telephones • Satellite telephones are utilized in circumstances where there is no access to a telephone or cellular network • For example ships at sea • Previously, these remote users would communicate via High frequency (HF) systems. A licensed radio operator would need to be present on board to operate these systems, and would need to know Morse code • Using radio waves in the HF range of the spectrum, communication could be realized between any two points on the earth
This was possible due to the unique property of HF waves to travel all the way around the world • Under appropriate conditions, HF waves can travel around the world by reflection between the earths surface and the ionosphere (which envelopes the earth above the atmosphere). The ionospheric reflection is analogous to the internal reflection that occurs in optical fibers - we will learn about fibers in chapter 15.
This method of communication however is unreliable, and currently satellites are utilized for communication in remote places • Using satellites, a ship at sea is easily reachable as any point on land • For some years INMARSAT corporation provided satellite radio communications for remote users (ground terminals large, and expensive, antenna setup needed as well) • Going to the next step, a Satellite based system called the Iridium system, conceived by Motorola corp. was proposed as an alternative to the conventional cellular systems • In the Iridium system, 66 low earth orbiting satellites are used for relaying information, analogous to a switching center in conventional cellular and telephone systems • The call does not pass through land lines, increasing the efficiency over other cellular systems
Opposed to the terrestrial cellular system, the cell sites in the Iridium system are overhead and are moving ! Source: http://www.geom.uiuc.edu/~worfolk/SaVi/
The Iridium system began operations in 1999, and presented the first generally available global system • However, the relatively high cost of service (around $3 per minute) and the relatively large size of the handsets, compared to terrestrial cell phones, drove the system to bankruptcy • Iridium failed to attract a sufficient amount of customers in competition with the rapidly spreading cellular systems across the globe • Shortly after this failure, a group of investors bought the system for $25 million, which had actually cost Motorola more than $5 billion to build • The system is expected to be profitable within a couple of years
Satellite Systems • Communication satellites provided the first long-distance, wide-bandwidth communication service • Testar 1 was the first commercial communications satellite to be launched into orbit • Satellites can be distinguished into 3 primary categories, depending on the type of orbit they reside in: • LEO • MEO • GEO
LEO:Low Earth Orbit satellites have a small area of coverage. They are positioned in an orbit approximately 3000km from the surface of the earth • They complete one orbit every 90 minutes • The large majority of satellites are in low earth orbit • The Iridium system utilizes LEO satellites (780km high) • The satellite in LEO orbit is visible to a point on the earth for a very short time • MEO:Medium Earth Orbit satellites have orbital altitudes between 3,000 and 30,000 km. • They are commonly used used in navigation systems such as GPS
GEO:Geosynchronous (Geostationary) Earth Orbit satellites are positioned over the equator. The orbital altitude is around 30,000-40,000 km • They complete one orbit every 24 hours. This causes the satellite to appear stationary with respect to a point on the earth, allowing one satellite to provide continual coverage to a given area on the earth's surface • One GEO satellite can cover approximately 1/3 of the world’s surface • They are commonly used in communication systems • Let’s look at a GEO animation: http://www.jpl.nasa.gov/basics/bsf5-1.htm
The Global Positioning System (GPS) • GPS is funded and controlled by the Department of Defense (DOD). • The system was originally designed for the U. S. military, however it has been allowed for commercial use in recent years. • GPS enables the user to determine its precise location anywhere on (or above) the Earth, using signals from at least 3 GPS satellites (preferably 4 or more) • Four GPS satellite signals are used to compute positions in three dimensions and the time offset in the receiver • For around $100, we can get a hand held gadget that will tell us exactly at what point we are located on the earth • How GPS works view video - http://www.youtube.com/watch?v=3zRlbboMvb0
The orbital period of the GPS satellites is around 12 hours • The satellites move with respect to the receivers on the earth, but at a slower speed compared to LEO’s • Typical civilian accuracy is around 100 ft, with occasional errors of up to 300 ft • For applications requiring higher accuracy such as aircraft landing etc., enhancements in the systems are made that can reduce the error to around 1 cm • Some applications of GPS include: • Aircraft navigation • Marine navigation • Driving • Surveying • Farming