thuyết trình về hệ thống GPS

► GPS receiver compares the time a signal was transmitted by a satellite with the time it was received.. The time difference tells the GPS receiver how far away distance the satellite

Global Positioning

System

By Farhan Saeed

► Satellite based navigation system made up of

a network of 24 satellites

► Originally intended for military applications

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

► There are no subscription fees or setup

charges to use GPS

Basic Principle

► GPS satellites circle the earth twice a day

in a very precise orbit and transmit signal information to earth

► GPS receiver compares the time a signal

was transmitted by a satellite with the

time it was received The time difference tells the GPS receiver how far away

(distance) the satellite is

► With distance measurements from a few

more satellites, the receiver can determine the user’s position and display it as a

latitude and longitude.

Basic Principle

► A GPS receiver must be locked on to the

signal of at least three satellites to calculate

a two-dimensional position (latitude and

longitude) and track movement.

► With four or more satellites in view, the

receiver can determine the user’s

three-dimensional position (latitude, longitude and altitude).

GPS Accuracy

► Today’s GPS receivers are extremely accurate and can give average positional accuracy

within 15 metres or better.

► With Differential GPS (DGPS) receiver

accuracies in the order of 3 to 5 metres are possible.

GPS Satellite System

► The 24 satellites (21 active plus 3 operating spares) that make up the GPS space segment are orbiting the earth about 12000 miles

GPS Satellite System

► GPS satellites are powered by solar energy

and are built to last approximately 10 years

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

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

GPS Satellite System

► The first GPS satellite was launched in 1978.

► A full constellation of 24 satellites was

achieved in 1994.

► Each satellite is built to last approximately

10 years Replacements are constantly

being built and launched into orbit.

► A GPS satellite weighs approximately 1500

kg and is about 6 m across with solar panels extended.

► Transmitter power is only approximately 50 watts or less.

Satellites Signal

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

► Civilian GPS receivers “listen” on the L1

frequency of 1575.42 MHz in the UHF band

► The signals travel using direct (space) wave propagation, often referred to as “line of

sight” radio communication

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

objects.

Satellites Signal

► L1 contains a complex pattern of digital

code signals, the “Protected” P code and the “Coarse Acquisition” C/A code

► This GPS transmission contains 3 different types of coded information, which are

essential for calculating the travel time

from the satellite to the GPS receiver on the earth (Time of arrival)

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

► Ephemeris data – this is information

continuously transmitted by each satellite, containing important information about the status of the satellite (healthy or

unhealthy), current date and time

► Almanac data – this information tells your GPS receiver where each satellite should be

at any time throughout the day

How does GPS work?

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

► Your distance from a given satellite equals the velocity of the transmitted signal

multiplied by the time it takes the signal to reach you, i.e.

► Distance = velocity of transmitted signal x travel time

 Velocity= 300,000,000 metres per second

 Travel time = Time taken by signal to arrive at the receiver.

Travel Time

► The transmitted digital code is called a

pseudo-random code When a satellite is generating a pseudo-random code, the GPS receiver is generating the same code and tries to match it up to the satellite’s code.

► The GPS receiver then compares the two

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

GPS Receiver Clock

► Your GPS receiver clock does not keep the time as precisely as the satellite clocks So each distance measurement needs to be

corrected to account for the GPS receiver’s internal clock error.

► The range measurement is referred to as a pseudo-range To determine position using pseudo-range data, a minimum of four

satellites must be tracked and the four

subsequent fixes must be recomputed until the clock error disappears.

GPS Position

Ranges!

GPS receiver technology

► Most modern GPS receivers are a parallel

multi-channel design

► These parallel receivers typically have

between 5 and 12 receiver circuits, each

devoted to one particular satellite signal, so strong locks can be maintained on all

satellites at all times

Sources of errors

► Ionosphere and troposphere delays

passes through the atmosphere Your GPS system uses a built-in model that

calculates an average amount of delay to partially correct for this type of error.

Sources of errors

► Signal multi-path

reflected off objects such as large

topographical objects and surfaces before

it reaches your receiver This effectively increases the travel time of the GPS radio signal, thereby causing errors.

Sources of errors

► Receiver clock errors

accurate as the atomic clocks on board

the GPS satellites Therefore, it may have very slight timing errors.

Sources of errors

► Orbital errors

errors, and are inaccuracies of the

satellite’s reported location This could

be, for example, due to the satellite’s orbit precessing in azimuth.

Sources of errors

► Number of satellites visible

“see”, the greater the accuracy

Topographical and geographical terrain,

electronic interference and adverse

weather and precipitation can inhibit radio signal reception, causing position errors or possibly no position indication at all GPS receivers typically do not work indoors,

under water or underground.

Sources of errors

► Satellite geometry/shading

satellites are located at wide angles

relative to each other, giving a position

based on a wide angle of cut from several position lines

where the position fix is based on a good Horizontal Dilution Of Position (HDOP)

Sources of errors

► Selective Availability (SA) –

degradation of the signal once imposed

by the US Department of Defence

2000, which significantly improved the

accuracy of civilian GPS receivers

Sources of errors

re-introduced at any time by the US

government and has led to the

development of two initiatives, which help

to overcome any future degradation of the system for civilian users:

GPS system accuracy

► 100 metres - accuracy of the GPS system

when subjected to accuracy degradation

under the US government Selective

Availability (SA) programme

► 15 metres - typical GPS position accuracy

without SA Available at present to all civilian users

► 3-5 metres - typical differential GPS (DGPS)

position accuracy

Exercise

Chart datum

► Charts are essentially grids created from a

starting reference point called a datum.

► Many charts still being used today were

originally created decades ago

► Over time, technology has allowed us to

improve our surveying skills and create more accurate charts However, there is still a

need to adapt GPS receivers to use with older charts.

Chart Datum

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

► Because the earth is not a regular shape the accuracy of each datum will vary as

you get further from the specific location for which it was defined

► OSGB36, European 1950, NAD27 etc.

► Satellite systems require a global datum and GPS positions are based on the World Geodetic System 1984 (WGS-84) which is a model of the entire earth.

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

► The GLObal NAvigation Satellite System

(GLONASS) is similar to GPS in that it is a

satellite based navigation system,

providing global 24 hour a day all weather access to precise position, velocity and time information to a suitably equipped user

► Any receiver capable of operating with both GLONASS and GPS would offer the best of

both worlds, with one system making up for the limitations of the other at specific

latitudes

► Galileo is a proposed European satellite

navigation system designed purely for civilian use which is very much in the initial

discussion stages.

► Europe hopes to deploy by 2010