Satellite Communications Location Technology Terrestrial Communications
GSM
The Global System for Mobile communications (GSM) open standards were initiated through the formation of a GSM study group in 1982 under the Conference of European Posts and Telegraphs (CEPT), and became the responsibility of the European Telecommunications Standards Institute (ETSI) in 1989. Commercial service began in 1991 and the use of the system has expanded to the point where more than 700 GSM networks have been established in Europe, North America, South America, Iceland, Asia, Africa and Australia. These networks interoperate based on a common Memorandum of Understanding. This type of network can be referred to as a 2nd generation network (2G), where the first generation was an analogue service.
GSM is a circuit switched digital cellular service capable of supporting voice and data at rates of 9.6kbps. Multiple user access to the system is provided through a combination of Time Division and Frequency Division Multiplexing (TDMA and FDMA).
GSM also supports the Short Message Service (SMS) which allows users to send and receive alphanumeric messages up to 160 bytes in length. This service has become very popular and an important source of revenue for mobile operators. Mobile terminated cell broadcast messages can be sent up to a length of 82 bytes.
The cellular nature of this system allows cell/location specific information to be transmitted to handsets.HSCSD
High Speed Circuit Switched Data (HSCSD) is similar to GSM data transfer but faster. It has not been widely implemented by mobile network operators, but may be in the future.GPRS
The General Packet Radio Service (GPRS), standardised through ETSI, operates over the GSM network with minimal upgrade to base stations and using GPRS capable handsets. It is sometimes referred to as a 2.5G network denoting it's stepping stone status between GSM (2G) and 3rd generation networks (3G). It can be used for carrying packet data protocol (such as IP and X.25) information from/to a GPRS terminal to/from other GPRS terminals and/or external packet data networks. GPRS is a packet switched data network and is therefore generally more effective than circuit switched GSM, for transmission of small quantities of intermittent data (such as position reports). Users are pseudo 'always-on' connected and are charged based on the amount of data sent/received rather than call length as is the case with GSM.
The data rate available to the user is generally much higher than with GSM, dependent on a number of factors, including the handset capability, available network resource and service agreement with the mobile operator. In theory the data rate can go up to 150kbps or higher.EDGE
Enhanced Data Rates for GSM Evolution (EDGE) is another 2.5G standard which would run on GSM networks and claims to give data rates up to 473kbps. EDGE has not been widely implemented, with mobile network operators looking at missing this step and going directly to 3G.UMTS
The Universal Mobile Telecommunication System (UMTS) will be the third and most capable generation (3G) of mobile cellular network. UMTS combines circuit switching for voice only services and Internet Protocol (IP) connectivity for IP based services. The move to an IP based network offers many advantages such as service integration with fixed networks.
It is planned that UMTS will provide users with data rates of up to 144 kbps in macrocellular (35 km cell radius) environments, up to 384 kbps in micro-cellular (1 km cell radius) environments and up to 2 Mbps in indoor or pico-cellular (100 m cell radius) environments. These data rates will enable UMTS to provide services such as paging, video conferencing, video telephony, news distribution, file transfer and in particular Internet type services.
This type of network requires major infrastructure investment compared with 2.5G networks and is currently in the trials stage of deployment. A number of factors have delayed the arrival of UMTS, including lack of compatible handsets, general technical issues, frequency spectrum license issues and market conditions.
Within Europe, ETSI is the main focus for the specification and standardisation of UMTS and feeds into the global standardisation process being carried out by the Third Generation Partnership Programme (3GPP) and co-ordinates with the International Telecommunications Union (ITU). Standards for a terrestrial component (T-UMTS) and satellite component (S-UMTS) are evolving. The satellite element is at an earlier stage in its evolution, but there are many organisations from the satellite and cellular communications industries working on S-UMTS development with specifications starting to emerge from the ETSI SES S-UMTS WG (The working group within ETSI that is concentrating on the S-UMTS development). S-UMTS should offer a fully integrated and standardised satellite element.RDS
RDS development was started over 20 years ago by the European Broadcasting Union (EBU), with the first systems appearing on the market in 1987. RDS equipped radios are widespread and incorporated in most new cars. RDS information is transmitted over VHF/FM sub-carriers.
RDS allows the automatic tuning of the radio to Alternative Frequencies (AF) with display of the selected station name. Utilising the Travel Programme (TP) and Travel Announcement (TA) flags, Traffic and Travel Information (TTI) can be passed to the driver. RDS-EON (Enhanced Other Networks) enables the user to operate their radio in an automatic switch mode for program type or travel information. RDS-TMC (Radio Data System-Traffic Message Channel) was the result of European Commission support for the development of technology to achieve pan-European TTI data services with language independence, and allows in-car units to translate standardised incident and location codes into spoken warnings or displayed messages. RDS-TMC operates at a data rate of 37bps.
RDS systems are currently the most widespread telematics systems in terms of vehicle equipment but are generally restricted to basic services due to their low bandwidth
DAB
DAB has been developed by a consortium known as EUREKA with funding from the EC. The EUREKA work resulted in the publication of a European Standard, which has now gained worldwide acceptance. DAB is a reliable rugged multi-service digital broadcasting system for reception by mobile, portable and fixed receivers from a simple non-directional antenna.
In the UK, DAB is broadcast using band III (around 221MHz) formerly used for black and white television signals. Other countries including Germany and Canada have allocated L-band (1452-1492MHz).
Most of the available bandwidth for DAB is taken up with audio transmissions, with a small proportion allocated for data.
Bluetooth
Bluetooth is a de-facto standard as well as a specification for small-form factor, short-range radio links between mobile PCs, mobile phones and other portable devices. It operates in the globally available Industrial-Scientific-Medical (ISM) frequency band at 2.4GHz, using a spread spectrum frequency-hopping signal. The range for bluetooth is less than 30 feet and the theoretical data rate is 1 to 2Mbps.Wireless LAN
There are a number of wireless Local Area Network (LAN) standards, with the IEEE 802.11a and IEEE 802.11b gaining increasing popularity. These utilise a spread spectrum Orthogonal Frequency Division Multiplexing (OFDM) scheme and have a range of about 60 feet and 300 feet respectively in their standard configuration. The frequencies of operation are around 5GHz and 2.4GHz for 802.11a and 802.11b respectively, with 802.11a having theoretical data rates up to 54Mbps and 802.11b up to 11Mbps. Actual data throughput for these wireless LANs is much less due to overheads associated with the protocols, and varies with the distance to the access point.A brief summary of commercial mobile two-way satellite communications is given below
Iridium
The Iridium satellite infrastructure consists of 66 Low Earth Orbit (LEO) satellites in 6 near polar planes with inter-satellite links giving global coverage. The satellites carry out on-board switching and routing of the calls from the handsets. The ground segment includes gateways for interconnection to the Public Switched Telephone Network (PSTN).
Services supported by the Iridium service include global voice, data, messaging and fax. Voice and paging services over Iridium have been commercially available since November 1998 and a data service since November 1999.
Packet mode data transfer is available when using the Iridium gateway for access to the Internet and circuit switched data services when using an independent Internet Service Provider (ISP). The maximum rate for the circuit switched data services is 2.4kbps, and 10kbps for certain types of compressed packet data when accessing the Internet through the Iridium gateway.
The radio frequency used by the handsets/terminals is L-band.Inmarsat
Inmarsat formed as a maritime-focused intergovernmental organisation in 1979 and was the world's first global mobile satellite communications operator. Inmarsat is now a privatised company and has evolved to become a major provider of global satellite communications for commercial, distress and safety applications, at sea in the air and on land. Inmarsat's primary constellation of four Imarsat-3 satellites in geostationary orbit gives global coverage, with the exception of the poles. Inmarsat's fourth generation of satellites are currently under construction, and will provide broadband communications capability. The ground segment includes gateways for interconnection to the Public Switched Telephone Network (PSTN).
A range of services are available through a number of independent service providers. These services include messaging, paging, video and voice.
The radio frequency used by the handsets/terminals is L-band.Orbcomm
The Orbcomm satellite network consists of 30 LEO spacecraft, which link to a worldwide network of Gateway Earth Stations. Orbcomm provides global two way packet data and messaging services with 6-250 byte typical message size using Internet and X.400 protocols.
The system uses the VHF band for communications to/from the user and provides 2.4kbps uplink and 4.8kbps downlink data rates. The downlink rate may be increased to 9.6kbps in the future.Globalstar
The Globalstar satellite constellation consists of 48 LEO satellites in 8 orbital planes. The system provides service coverage between around 70 degrees North to 70 degrees south, excluding some mid-ocean regions, regions of Africa and Indonesia. The ground segment includes gateways for interconnection to the Public Switched Telephone Network (PSTN). Commercial service started in 1999.
User frequency bands are L-band for uplink and S-band for downlink.
Services include voice and data, dependent on service provider. Data services can be circuit-switched or packet based dependent on location and service provider, with a maximum data rate of 9.6kbps.Thuraya
The Thuraya space segment currently consists of one operational geostationary satellite positioned at 44 degrees East, launched in October 2000. The first commercial services began rolling out in April 2001. The ground segment includes gateways for interconnection to the Public Switched Telephone Network (PSTN).
Service coverage is currently over most of Europe, Central Asia, Middle East, Indian subcontinent and North and Central Africa. Services offered are voice, fax, SMS and data with rates up to 9.6kbps. Handsets/terminals have integrated GPS receivers and are generally dual mode GSM and Thuraya.
The radio frequency used by the handsets/terminals is L-band.Automatically identifying your location is generally achieved through a process of triangulation (1) . This involves comparing the travel time of radio signals from known locations and therefore calculating the distances from those locations. The most well known method of achieving this is through the use of Global Positioning Satellites (GPS). The most commonly used infrastructure is the US Navstar system although there is also a Russian system called GLONASS and plans are underway for a European system called Galileo. For a good explanation of how a GPS system works see the Trimble website: www.trimble.com/gps.
An alternative location technology that uses terrestrial cellular phone network infrastructure to locate mobile handsets is called Enhanced Observed Time Difference (E-OTD). This method requires a modification to be made to the mobile phone handset. Obviously this method only works in areas that have terrestrial cellular network coverage. The accuracy of this method varies widely depending on the spacing of the cells and the environment (urban, rural etc.). A good explanation of E-OTD can be found here: www.cursor-system.com/sitefiles/cursor/tech_eotd.htm.
GPS started life chiefly as a military system but its use has become widespread for civilian applications. The US system has two levels of positional accuracy, with the highest accuracy signal being reserved for military use. The signal for civilian use originally had a deliberate error superimposed on the signal that reduced the achievable accuracy. This feature called Selective Availability (SA) was eventually removed in May 2000, increasing the level of accuracy for civilian users by up to 10 times. Standard accuracy from GPS will generally tell the user their location to within 20m.
Real-time location accuracy can be increased through the use of special GPS receivers which can receive correction data as well as the signals from GPS satellites. Correction data is derived through the use of reference stations whose position is known to a high degree of accuracy. These stations monitor the GPS satellite signals and with reference to their very accurately known location calculate and broadcast corrections to remote GPS receivers. This is known as Differential GPS (DGPS). DGPS is only available in areas in which infrastructure has been put in place.
In the US there is a system known as the Wide Area Augmentation System (WAAS) which is a form of DGPS giving enhanced accuracy and reliability. This system was developed primarily for aeronautical navigation but is available for any user with the appropriate equipment. The aim of the WAAS is to provide location accuracy to at least 7m horizontal and vertical. The correction signals are broadcast through the use of geostationary satellites (Inmarsat) and can be received with standard GPS antennas. Although WAAS is not yet certified, there is an operational test signal available for use and a number of WAAS capable commercial receivers. WAAS is scheduled to be fully operational late in 2003.
In Europe there are plans for a similar system to WAAS called European Geostationary Navigation Overlay Service (EGNOS). This will broadcast correction and reliability data for both the Navstar and GLONASS systems via geostationary satellites (Inmarsat) on a standard GPS frequency to be received by standard GPS antennas. The accuracy of location derived using EGNOS should be within 5m. EGNOS is scheduled to become fully operational in 2004.
(1) Purists would not regard the GPS system as using 'triangulation', but 'trilateration' or 'resection'
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