Global Positioning System

Global Positioning Global Positioning SystemSystem

By Farhan SaeedBy Farhan Saeed

GPSGPS

►Satellite based navigation system made Satellite based navigation system made up of a network of up of a network of 2424 satellites satellites

►Originally intended for military Originally intended for military applicationsapplications

► In the 1980’s, the US government made In the 1980’s, the US government made the system available for civilian use. the system available for civilian use.

►There are no subscription fees or setup There are no subscription fees or setup charges to use GPS charges to use GPS

Basic PrincipleBasic Principle

► GPS satellites circle the earth twice a day in a GPS satellites circle the earth twice a day in a very precise orbit and transmit signal very precise orbit and transmit signal information to earth. information to earth.

► GPS receiver compares the time a signal was GPS receiver compares the time a signal was transmitted by a satellite with the time it was transmitted by a satellite with the time it was received. The received. The time differencetime difference tells the GPS tells the GPS receiver how far away (receiver how far away (distancedistance) the satellite ) the satellite is. is.

► With distance measurements from a few more With distance measurements from a few more satellites, the receiver can determine the satellites, the receiver can determine the user’s position and display it as a latitude and user’s position and display it as a latitude and longitude.longitude.

Basic PrincipleBasic Principle

►A GPS receiver must be locked on to A GPS receiver must be locked on to the signal of at least the signal of at least threethree satellites satellites to calculate a to calculate a two-dimensionaltwo-dimensional position (latitude and longitude) and position (latitude and longitude) and track movement.track movement.

►With With fourfour or more satellites in view, or more satellites in view, the receiver can determine the user’s the receiver can determine the user’s three-dimensionalthree-dimensional position (latitude, position (latitude, longitude and altitude).longitude and altitude).

GPS AccuracyGPS Accuracy

►Today’s GPS receivers are extremely Today’s GPS receivers are extremely accurate and can give average accurate and can give average positional accuracy within positional accuracy within 15 metres15 metres or better.or better.

►With With Differential GPS (DGPS) Differential GPS (DGPS) receiver accuracies in the order of receiver accuracies in the order of 3 3 to 5 metresto 5 metres are possible. are possible.

GPS Satellite System GPS Satellite System

►The The 2424 satellites (21 active plus 3 satellites (21 active plus 3 operating spares) that make up the operating spares) that make up the GPS GPS space segmentspace segment are orbiting the are orbiting the earth about 12000 miles above us. earth about 12000 miles above us.

►They are constantly moving at approx. They are constantly moving at approx. 7000 m/h, making two complete orbits 7000 m/h, making two complete orbits in approximately 24 hours, i.e. orbital in approximately 24 hours, i.e. orbital period of approximately 12 hours.period of approximately 12 hours.

GPS Satellite SystemGPS Satellite System

►GPS satellites are powered by solar GPS satellites are powered by solar energy and are built to last energy and are built to last approximately 10 years. approximately 10 years.

►They have back up batteries on board They have back up batteries on board to keep them running in the event of to keep them running in the event of solar eclipses. solar eclipses.

►Small rocket boosters on each satellite Small rocket boosters on each satellite enable them to keep flying on the enable them to keep flying on the correct path.correct path.

GPS Satellite SystemGPS Satellite System

► The first GPS satellite was launched in 1978.The first GPS satellite was launched in 1978.► A full constellation of 24 satellites was A full constellation of 24 satellites was

achieved in 1994.achieved in 1994.► Each satellite is built to last approximately Each satellite is built to last approximately

10 years. Replacements are constantly 10 years. Replacements are constantly being built and launched into orbit.being built and launched into orbit.

► A GPS satellite weighs approximately 1500 A GPS satellite weighs approximately 1500 kg and is about 6 m across with solar panels kg and is about 6 m across with solar panels extended.extended.

► Transmitter power is only approximately 50 Transmitter power is only approximately 50 watts or less.watts or less.

Satellites SignalSatellites Signal

►GPS satellites transmit two low power radio GPS satellites transmit two low power radio signals, designated L1 and L2. signals, designated L1 and L2.

► Civilian GPS receivers “listen” on the L1 Civilian GPS receivers “listen” on the L1 frequency of 1575.42 MHz in the UHF band. frequency of 1575.42 MHz in the UHF band.

► The signals travel using direct (space) wave The signals travel using direct (space) wave propagation, often referred to as “line of propagation, often referred to as “line of sight” radio communication. sight” radio communication.

► Signals will pass through clouds, glass and Signals will pass through clouds, glass and plastic but will not go through most solid plastic but will not go through most solid objects.objects.

Satellites SignalSatellites Signal

► L1 contains a complex pattern of digital L1 contains a complex pattern of digital code signals, the “Protected” code signals, the “Protected” P P code and code and the “Coarse Acquisition” the “Coarse Acquisition” C/AC/A code. code.

► This GPS transmission contains This GPS transmission contains 33 different different types of coded information, which are types of coded information, which are essential for calculating the travel time from essential for calculating the travel time from the satellite to the GPS receiver on the the satellite to the GPS receiver on the earth. (earth. (Time of arrival)Time of arrival)

► The travel time multiplied by the speed of The travel time multiplied by the speed of light equals the satellites range (distance light equals the satellites range (distance from the satellite to the GPS receiver). from the satellite to the GPS receiver).

Coded InformationCoded Information

► A pseudorandom code – this is simply an ID A pseudorandom code – this is simply an ID code that identifies which satellite is code that identifies which satellite is transmitting information from which you are transmitting information from which you are receiving receiving

► Ephemeris data – this is information Ephemeris data – this is information continuously transmitted by each satellite, continuously transmitted by each satellite, containing important information about the containing important information about the status of the satellite (healthy or unhealthy), status of the satellite (healthy or unhealthy), current date and time. current date and time.

► Almanac data – this information tells your GPS Almanac data – this information tells your GPS receiver where each satellite should be at any receiver where each satellite should be at any time throughout the day. time throughout the day.

How does GPS work?How does GPS work?

► GPS receiver has to know two things about the GPS receiver has to know two things about the satellites, i.e. where they are (location) and satellites, i.e. where they are (location) and how far away they are (distance).how far away they are (distance).

► Your distance from a given satellite equals the Your distance from a given satellite equals the velocity of the transmitted signal multiplied by velocity of the transmitted signal multiplied by the time it takes the signal to reach you, i.e.the time it takes the signal to reach you, i.e.

►Distance = velocity of transmitted signal Distance = velocity of transmitted signal x travel timex travel time Velocity= 300,000,000 metres per secondVelocity= 300,000,000 metres per second Travel time = Time taken by signal to arrive Travel time = Time taken by signal to arrive

at the receiver.at the receiver.

Travel TimeTravel Time

► The transmitted digital code is called a The transmitted digital code is called a pseudo-random codepseudo-random code. When a satellite is . When a satellite is generating a pseudo-random code, the GPS generating a pseudo-random code, the GPS receiver is generating the same code and receiver is generating the same code and tries to match it up to the satellite’s code.tries to match it up to the satellite’s code.

► The GPS receiver then compares the two The GPS receiver then compares the two codes to determine how much it needs to codes to determine how much it needs to delay (or shift) its code in order to match delay (or shift) its code in order to match the satellite code. This delay time (shift) is the satellite code. This delay time (shift) is multiplied by the velocity of propagation of multiplied by the velocity of propagation of the radio wave to get the distance (range).the radio wave to get the distance (range).

GPS Receiver Clock GPS Receiver Clock

► Your GPS receiver clock does not keep the Your GPS receiver clock does not keep the time as precisely as the satellite clocks. So time as precisely as the satellite clocks. So each distance measurement needs to be each distance measurement needs to be corrected to account for the GPS receiver’s corrected to account for the GPS receiver’s internal clock error.internal clock error.

► The range measurement is referred to as a The range measurement is referred to as a pseudo-rangepseudo-range. To determine position using . To determine position using pseudo-range data, a minimum of four pseudo-range data, a minimum of four satellites must be tracked and the four satellites must be tracked and the four subsequent fixes must be recomputed until subsequent fixes must be recomputed until the clock error disappears.the clock error disappears.

GPS PositionGPS Position

► Like Radar Ranges!Like Radar Ranges!

Almanac DataAlmanac Data

►The unit stores data about where the The unit stores data about where the satellites are located at any given satellites are located at any given time.time.

► This data is called the almanac. This data is called the almanac. ►““cold” receiver cold” receiver ►““warm” receiverwarm” receiver

GPS receiver technologyGPS receiver technology

►Most modern GPS receivers are a Most modern GPS receivers are a parallel multi-channel design. parallel multi-channel design.

►These parallel receivers typically have These parallel receivers typically have between 5 and 12 receiver circuits, between 5 and 12 receiver circuits, each devoted to one particular each devoted to one particular satellite signal, so strong locks can be satellite signal, so strong locks can be maintained on all satellites at all maintained on all satellites at all times. times.

Sources of errors Sources of errors

►Ionosphere and troposphere Ionosphere and troposphere delaysdelays The satellite’s radio signal slows as it The satellite’s radio signal slows as it

passes through the atmosphere. Your passes through the atmosphere. Your GPS system uses a built-in model that GPS system uses a built-in model that calculates an average amount of delay to calculates an average amount of delay to partially correct for this type of error.partially correct for this type of error.


►Signal multi-pathSignal multi-path This occurs when the GPS radio signal is This occurs when the GPS radio signal is

reflected off objects such as large reflected off objects such as large topographical objects and surfaces before topographical objects and surfaces before it reaches your receiver. This effectively it reaches your receiver. This effectively increases the travel time of the GPS radio increases the travel time of the GPS radio signal, thereby causing errors.signal, thereby causing errors.


►Receiver clock errorsReceiver clock errors Your receiver’s built-in clock is not as Your receiver’s built-in clock is not as

accurate as the atomic clocks on board accurate as the atomic clocks on board the GPS satellites. Therefore, it may have the GPS satellites. Therefore, it may have very slight timing errors.very slight timing errors.


►Orbital errorsOrbital errors These are also known as ephemeris These are also known as ephemeris

errors, and are inaccuracies of the errors, and are inaccuracies of the satellite’s reported location. This could satellite’s reported location. This could be, for example, due to the satellite’s be, for example, due to the satellite’s orbit precessing in azimuth.orbit precessing in azimuth.


►Number of satellites visibleNumber of satellites visible The more satellites your GPS receiver can The more satellites your GPS receiver can

“see”, the greater the accuracy. “see”, the greater the accuracy. Topographical and geographical terrain, Topographical and geographical terrain, electronic interference and adverse electronic interference and adverse weather and precipitation can inhibit radio weather and precipitation can inhibit radio signal reception, causing position errors or signal reception, causing position errors or possibly no position indication at all. GPS possibly no position indication at all. GPS receivers typically do not work indoors, receivers typically do not work indoors, under water or underground.under water or underground.


►Satellite geometry/shadingSatellite geometry/shading Ideal satellite geometry exists when the Ideal satellite geometry exists when the

satellites are located at wide angles satellites are located at wide angles relative to each other, giving a position relative to each other, giving a position based on a wide angle of cut from several based on a wide angle of cut from several position lines. position lines.

This is often referred to as a situation This is often referred to as a situation where the position fix is based on a good where the position fix is based on a good Horizontal Dilution Of Position (HDOP). Horizontal Dilution Of Position (HDOP).


►Selective Availability (SA) – Selective Availability (SA) – Selective availability (SA) is an intentional Selective availability (SA) is an intentional

degradation of the signal once imposed degradation of the signal once imposed by the US Department of Defence. by the US Department of Defence.

The US government turned off SA in May The US government turned off SA in May 2000, which significantly improved the 2000, which significantly improved the accuracy of civilian GPS receivers. accuracy of civilian GPS receivers.


►Selective Availability (SA) – Selective Availability (SA) – However, this degradation could be re-However, this degradation could be re-

introduced at any time by the US introduced at any time by the US government and has led to the government and has led to the development of two initiatives, which help development of two initiatives, which help to overcome any future degradation of the to overcome any future degradation of the system for civilian users:system for civilian users:

the development of the development of DGPSDGPS the proposed development of an EU the proposed development of an EU

supported initiative called supported initiative called GalileoGalileo

GPS system accuracyGPS system accuracy

►100 metres100 metres - accuracy of the GPS - accuracy of the GPS system when subjected to accuracy system when subjected to accuracy degradation under the US government degradation under the US government Selective Availability (SA) programmeSelective Availability (SA) programme

►15 metres15 metres - typical GPS position - typical GPS position accuracy without SA. Available at accuracy without SA. Available at present to all civilian userspresent to all civilian users

►3-5 metres3-5 metres - typical differential GPS - typical differential GPS (DGPS) position accuracy(DGPS) position accuracy

ExerciseExercise

Chart datumChart datum

►Charts are essentially grids created Charts are essentially grids created from a starting reference point called a from a starting reference point called a datum.datum.

►Many charts still being used today were Many charts still being used today were originally created decades ago. originally created decades ago.

►Over time, technology has allowed us Over time, technology has allowed us to improve our surveying skills and to improve our surveying skills and create more accurate charts. However, create more accurate charts. However, there is still a need to adapt GPS there is still a need to adapt GPS receivers to use with older charts.receivers to use with older charts.

Chart DatumChart Datum

► A navigational chart is referenced to two A navigational chart is referenced to two datums – one horizontal, for latitude and datums – one horizontal, for latitude and longitude, and one vertical for depth and longitude, and one vertical for depth and height. height.

► Because the earth is not a regular shape the Because the earth is not a regular shape the accuracy of each datum will vary as you get accuracy of each datum will vary as you get further from the specific location for which it further from the specific location for which it was defined. was defined.

► OSGB36, European 1950, NAD27 etc.OSGB36, European 1950, NAD27 etc.► Satellite systems require a global datum and Satellite systems require a global datum and

GPS positions are based on the GPS positions are based on the World World Geodetic System 1984 (WGS-84)Geodetic System 1984 (WGS-84) which is which is a model of the entire earth.a model of the entire earth.

DGPSDGPS

►The fundamental principle of DGPS is The fundamental principle of DGPS is the comparison of the position of a the comparison of the position of a fixed point, referred to as the fixed point, referred to as the reference station, with positions reference station, with positions obtained from a GPS receiver at that obtained from a GPS receiver at that point. point.

GLONASSGLONASS

► The The GLOGLObal bal NANAvigation vigation SSatellite atellite SSystem ystem (GLONASS) is similar to GPS in that it is a (GLONASS) is similar to GPS in that it is a satellite based navigation system, providing satellite based navigation system, providing global 24 hour a day all weather access to global 24 hour a day all weather access to precise position, velocity and time information precise position, velocity and time information to a suitably equipped user. to a suitably equipped user.

► Any receiver capable of operating with both Any receiver capable of operating with both GLONASS and GPS would offer the best of GLONASS and GPS would offer the best of both worlds, with one system making up for both worlds, with one system making up for the limitations of the other at specific the limitations of the other at specific latitudes. latitudes.

GalileoGalileo

►Galileo is a proposed European Galileo is a proposed European satellite navigation system designed satellite navigation system designed purely for civilian use which is very purely for civilian use which is very much in the initial discussion stages.much in the initial discussion stages.

►Europe hopes to deploy by 2010. Europe hopes to deploy by 2010.

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Global Positioning System