CHAPTER 8
Global Positioning
System
LEARNING OUTCOME
What is GPS and its application?
What does GPS provide?
History
Basic concept of GPS
How GPS works?
WHAT IS GPS?
GPS, which stands for Global Positioning
System, is the only system today able to show
you your exact position on the Earth anytime,
in any weather, anywhere.
The three parts of GPS are:
Satellites
Receivers
Software
WHAT IS GPS?
is a U.S. space-based Global Navigation Satellite
System.
It provides reliable positioning, navigation, and
timing services to worldwide users on a continuous
basis in all weather, day and night, anywhere on or
near the Earth.
Global Navigation Satellite Systems (GNSS) is the
standard generic term for satellite navigation
systems that provide autonomous geo-spatial
positioning with global coverage.
GNSS allows small electronic receivers to determine
their location (longitude, latitude, and altitude)
to within a few meters using time signals
transmitted along a line-of-sight by radio from
satellites.
Receivers on the ground - fixed position can also be
determine precise time.
WHAT IS GPS?
A worldwide radio-navigation system formed
from a constellation of 24 satellites and
their ground stations
GPS uses these "man-made stars" as reference
points to calculate positions accurate to a
matter of meters.
In fact, with advanced forms of GPS you can
make measurements to better than a
centimeter.
In US-maintained byUS Department of
Defense.Malaysia?
WHAT IS GPS?
3D position anywhere on the earth at any time
of the day or night by receiving and
processing signals from orbiting GPS
satellites
GPS provides specially coded satellite
signals that can be processed in a GPS
receiver, enabling the receiver to compute
position, velocity and time
4 GPS satellite signals are used to compute
positions in three dimensions and the time
offset in the receiver clock
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THE HISTORY
The first satellite navigation system, Transit, used by
the United States Navy, was first successfully tested in
1960.
It used a constellation of five satellites and could
provide a navigational fix approximately once per hour.
In 1967, the U.S. Navy developed the Timation satellite
which proved the ability to place accurate clocks in
space, a technology that GPS relies upon.
In the 1970s, Omega Navigation System, based on phase
comparison of signal transmission from pairs of stations,
- first worldwide radio navigation system.
Friedwardt Winterberg proposed a test of General
Relativity using accurate atomic clocks placed in orbit
in artificial satellites. To achieve accuracy
requirements, GPS uses principles of general relativity
to correct the satellites' atomic clocks.
BASIC CONCEPT OF GPS
A GPS receiver calculates its position by
precisely timing the signals sent by the GPS
satellites high above the Earth. Each
satellite continually transmits messages which
include:
- the time when the message was sent
- precise orbital information (the
ephemeris)
- the general system health and rough
orbits of all
GPS satellites (the almanac)
BASIC CONCEPT OF GPS
The receiver measures the transit time of each
message and computes the distance to each
satellite.
Geometric trilateration is used to combine these
distances with the satellites' locations to obtain
the position of the receiver.
This position is then displayed, perhaps with a
moving map display or latitude and longitude;
elevation information may be included. Many GPS
units also show derived information such as
direction and speed, calculated from position
changes.
BASIC CONCEPT OF GPS
Three satellites might seem enough to solve for
position, since space has three dimensions.
However, even a very small clock error multiplied
by the very large speed of lightthe speed at
which satellite signals propagateresults in a
large positional error.
Therefore receivers use four or more satellites to
solve for the receiver's location and time. The
very accurately computed time is effectively
hidden by most GPS applications, which use only
the location. A few specialized GPS applications
do however use the time; these include time
transfer, traffic signal timing, and
synchronization of cell phone base stations.
HOW GPS WORKS?
Global Positioning System satellites
transmit signals to equipment on the
ground.
GPS receivers passively receive
satellite signals; they do not
transmit. GPS receivers require an
unobstructed view of the sky, so they
are used only outdoors and they often
do not perform well within forested
areas or near tall buildings. Not our
lecture room..
GPS operations depend on a very
accurate time reference, which is
provided by atomic clocks at the U.S.
Naval Observatory. Each GPS satellite
has atomic clocks on board.
HOW GPS WORKS?
Each GPS satellite transmits data that
indicates its location and the current time.
All GPS satellites synchronize operations so
that these repeating signals are transmitted
at the same instant. The signals, moving at
the speed of light, arrive at a GPS receiver
at slightly different times because some
satellites are farther away than others.
The distance to the GPS satellites can be
determined by estimating the amount of time
it takes for their signals to reach the
receiver. When the receiver estimates the
distance to at least four GPS satellites, it
can calculate its position in three
dimensions.
There are at least 24 operational GPS
satellites at all times. The satellites,
operated by the U.S. Air Force, orbit with a
period of 12 hours. Ground stations are used
to precisely track each satellite's orbit.
S
ATELLITES
There are quite a number of satellites
out there in space. They are used for a
wide range of purposes: satellite TV,
cellular phones, military purposes and
etc. Satellites can also be used by GPS
receivers.
Facts:
There are some 2,500 satellites of all
types and purpose orbiting the earth.
There are over 8,000 foreign objects
orbiting the earth consisting of items
like nose cones and panels from old
satellites, an astronaut's glove, spanner
and more!
WHAT IS GPS USED FOR?
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IN-CAR NAVIGATION
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BUILDING/ENGINEERING SET OUT
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CRUSTAL MOTION
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MACHINE GUIDANCE
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AIR NAVIGATION
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MAPPING
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DEFORMATION MONITORING
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MISSILE GUIDANCE
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GPS ACCURACY
The accuracy of a position determined with GPS depends
on the type of receiver.
Most hand-held GPS units have about 10-20 meter
accuracy. Other types of receivers use a method called
Differential GPS (DGPS) to obtain much higher accuracy.
DGPS requires an additional receiver fixed at a known
location nearby. Observations made by the stationary
receiver are used to correct positions recorded by the
roving units, producing an accuracy greater than 1
meter.
When the system was created, timing errors were
inserted into GPS transmissions to limit the accuracy
of non-military GPS receivers to about 100 meters. This
part of GPS operations, called Selective Availability,
was eliminated in May 2000.
GPS ACCURACY
Method
Stand Alone (SPS)
DGPS
Carrier Phase DGPS
Position Accuracy
10-20m @ 95%
0.5 - 3m
5mm + 0.01ppm
THREE GPS SEGMENTS/ELEMENTS
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SPACE SEGMENT
Satellites orbiting the Earth
The nominal GPS Operational Constellation consists
of 24 satellites that orbit the earth in 12 hours,
all of which are in orbits of 20,200 km above the
Earth
To achieve the best coverage, the satellites are
located in 6 orbital planes (with nominally 4
satellites in each plane), equally spaced (60
degrees apart), and inclined at about 55 with
respect to the equatorial plane
This constellation provides the user with between
5 and 8 satellites visible from any point on the
earth
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SPACE SEGMENT
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SPACE SEGMENT
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CONTROL SEGMENT
A number of tracking stations at fixed
locations around the world form the control
segment of GPS
The Master Control Station (MCS) is located
in the Consolidated Space Operations Center
at Schriever Air Force Base in the USA
Monitor and control the constellation
Establish and maintain GPS time
Determine the positions of the satellites at
any given time
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CONTROL SEGMENT
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CONTROL SEGMENT
The Master Control station uploads ephemeris
and clock data to the satellites. The
satellites then send subsets of the orbital
ephemeris data to GPS receivers over radio
signals.
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USER SEGMENT
Consists of anyone, civilian or military, who use
a GPS receiver to determine their position
GPS receivers convert satellite signals into
position, velocity, and time
Military applications include :
Navigation (land, sea, air)
Missile guidance
Search and rescue
Civilian applications include :
Construction work (control survey,
mapping, setting out)
Position, time
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REFLECTION AND READING
What is GPS and its application?
How many segments does a GPS has and discuss
what are the segments.
Further Reading
Chapter 7 (7.1-7.2) J. Uren book
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POSITION FROM SATELLITES
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POSITION FROM SATELLITES
Position in XYZ is converted within the
receiver to geodetic latitude, longitude and
height above the ellipsoid
Latitude and longitude are usually provided
in the geodetic datum on which GPS is based
(WGS-84)
Just as with GPS coordinates, local
coordinates or coordinates used in a
particular country, maps are based on a local
ellipsoid, designed to match the geoid in the
area
Existing coordinates are usually in a local
coordinate system and therefore the GPS
coordinates have to be transformed into this
local system
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POSITION FROM SATELLITES
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POSITION FROM SATELLITES
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GPS SIGNALS
Each GPS satellite transmits
data that indicates its location
and the current time. All GPS
satellites synchronize
operations so that these
repeating signals are
transmitted at the same
instant.
Physically the signal is just a
complicated digital code, or
in other words, a
complicated sequence of
on and off pulses.
GPS SIGNALS
GPS satellites transmit two radio signals.
These are designated as L1 and L2. A Civilian
GPS uses the L1 signal frequency (1575.42
MHz) in the UHF band.
The signals travel by line of sight, meaning
they will pass through clouds, glass, plastic
etc but will not travel through solid objects
such as buildings and mountains.
The GPS signal contains three different bits
of information a pseudo random code,
almanac data and ephemeris data.
SIGNAL STRUCTURE
The satellites broadcast two carrier waves
constantly
These carrier waves are in the L-Band (used
for radio), and travel to earth at the speed
of light
GPS receivers use the different codes to
distinguish between satellites
The codes can also be used as a basis for
making pseudorange measurements and therefore
calculate a position
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SIGNAL STRUCTURE
Fundamental
Frequency
10.23 MHz
x 154
x 120
?1
?10
L1
1575.42 MHz
?0.19 m
C/A - Code
1.023 MHz
?300 m
L2
1227.60 MHz
?0.24 m
P-Code
10.23 MHz
?30 m
P-Code
10.23 MHz
?30 m
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50 BPS
Satellite Message
SIGNAL ERROR
1.
2.
3.
Factors that can degrade the GPS signal and thus affect
accuracy include the following:
Ionosphere and troposphere delays The satellite signal
slows as it passes through the atmosphere. The GPS system
uses a built-in model that calculates an average amount
of delay to partially correct for this type of error.
Signal multi path This occurs when the GPS signal is
reflected off objects such as tall buildings or large
rock surfaces before it reaches the receiver. This
increases the travel time of the signal, thereby causing
errors.
Receiver clock errors A receiver's built-in clock is
not as accurate as the atomic clocks onboard the GPS
satellites. Therefore, it may have very slight timing
errors.
4.
5.
6.
7.
SIGNAL ERROR
Orbital errors Also known as ephemeris errors,
these are inaccuracies of the satellite's
reported location.
Number of satellites visible The more
satellites a GPS receiver can "see," the better
the accuracy.
Buildings, terrain, electronic interference, or
sometimes even dense foliage can block signal
reception, causing position errors or possibly no
position reading at all. GPS units typically will
not work indoors, underwater or underground.
Satellite geometry/shading This refers to the
relative position of the satellites at any given
time
SIGNAL ERROR
8.
9.
10.
Ideal satellite geometry exits when the
satellites are located at wide angles relative
to each other.
Poor geometry results when the satellites are
located in a line or in a tight grouping.
Intentional degradation of the satellite signal
Selective Availability (SA) is an intentional
degradation of the signal once imposed by the
U.S. DoD. SA was intended to prevent military
adversaries from using the highly accurate GPS
signals. The government turned off SA in May
2000, which significantly improved the accuracy
of civilian GPS receivers.
SURVEYING WITH GPS
are 3 techniques
available for taking
measurements with GPS
Static
Rapid static
Real-time kinematic (RTK)
There
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STATIC
For
determining the length of
baselines that are longer than 20 km
The reference receiver is located at
a known control point and a rover is
set up at a point whose coordinates
are to be determined
Collection of data done
simultaneously during a survey which
must be to at least 4 common
satellites and which can be up to
several hours duration
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STATIC
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RAPID STATIC
Baselines
should not be longer than
20 km
The shorter the baselines are, the
better
This technique is similar to static
positioning but has shorter
occupation times of 10 30 minute
This technique is slightly less
accurate than the static method
because it is more sensitive to
environmental conditions
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RAPID STATIC
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REAL-TIME KINEMATIC (RTK)
Used
when a lot of points are to be
surveyed in a relatively small area
Used where the accuracy requirement
is not as high as for static surveys
Require a reference receiver to be
located at a known position, but in
this case, instead of remaining
stationary, the rover is moved
around the site recording position
at discrete points
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REAL-TIME KINEMATIC (RTK)
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APPLICATION
IN
GPS technique
Static
ENGINEERING SURVEYING
Applications
Locating reference points for
primary control networks, for
deformation survey, for sea level
and tidal monitoring
Rapid static
Densifying control networks,
provides coordinates of reference
points and base stations in
kinematic methods
Real-time
Data collection in topographical
kinematic (RTK) surveys and for measurement 53
survey
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