Transcript Global Positioning System

Global Positioning
System
By Farhan Saeed
GPS
► 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 threedimensional 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 above us.
► They are constantly moving at approx. 7000
m/h, making two complete orbits in
approximately 24 hours, i.e. orbital period
of approximately 12 hours.
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).
Coded Information
►A
pseudorandom code – this is simply an ID code
that identifies which satellite is transmitting
information from which you are receiving
► 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 pseudorandom 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
► Like
Radar Ranges!
Almanac Data
► The
unit stores data about where the
satellites are located at any given time.
► This data is called the almanac.
► “cold” receiver
► “warm” receiver
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
 The satellite’s radio signal slows as it 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
 This occurs when the GPS radio signal is
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
 Your receiver’s built-in clock is not as accurate
as the atomic clocks on board the GPS
satellites. Therefore, it may have very slight
timing errors.
Sources of errors
► Orbital
errors
 These are also known as ephemeris 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
 The more satellites your GPS receiver can “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
 Ideal satellite geometry exists when the
satellites are located at wide angles relative to
each other, giving a position based on a wide
angle of cut from several position lines.
 This is often referred to as a situation where the
position fix is based on a good Horizontal
Dilution Of Position (HDOP).
Sources of errors
► Selective
Availability (SA) –
 Selective availability (SA) is an intentional
degradation of the signal once imposed by the
US Department of Defence.
 The US government turned off SA in May 2000,
which significantly improved the accuracy of
civilian GPS receivers.
Sources of errors
► Selective
Availability (SA) –
 However, this degradation could be reintroduced 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:
 the development of DGPS
 the proposed development of an EU supported
initiative called Galileo
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.
DGPS
► 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.
GLONASS
► 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
► 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.