Transcript PowerPoint Presentation - Global Positioning System (GPS)

Global Positioning System (GPS)
1
Introduction
• The current global positioning system (GPS) is the culmination
of years of research and unknown millions of dollars.
• Navigational systems have been and continue to be developed
and funded by the U.S. government.
• The current system is managed by the U.S Air Force for the
Department of Defense (DOD).
• The current system became fully operational June 26, 1993
when the 24th satellite was lunched.
• While there are millions of civil users of GPS worldwide, the
system was designed for and is operated by the U. S. military.
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Introduction-History
1 9 73
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19801 9 82
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Decision to dev elop a satellite nav igation sy stem based on the sy stems TRANSIT,
TIMATION und 6 2 1B of t he U.S. Air For ce and the U.S. Navy.1 9 74 - 1 9 7 9 Sy stem tests
Fir st receiv er tests are perf ormed ev en bef ore t he f irst satellites are stationed in the
orbit. Tran smitte rs are installed on t he eart hs s surf ace called Pseudolites ( Pseudo
satellites)
A total of 11 Block I satellites are launched in t his period.
Launching of the f irst Block I satellite carry ing sensors to detect atom ic explosions. This
satellite is meant to c ont rol the abidance of the agreement of 1 9 63 betw een the USA
and the Sov iet Union to ref rain f rom any nuclear tests on the earth, submarine or in
space.
Decision to expand the GPS sy stem. Thereupon t he resources are considerably shortened
and the p rogram is rest ruct ured. At f irst only 1 8 satellites should be operated.
The number of satellites is again raised to 2 4 , as the f unct ionality is not satisf ying wi t h
only 18 satellites.
The f inancial sit uati on of the project is crit ical, as the usef ulness of the system is
questioned again and again by t he sponsors.
When a civ ilian airplane of the Korean Airline (Flight 0 0 7) was shot down af ter it had
gone lost ov er Sov iet terri t ory , it was decided to allow the civilian use of t he GPS sy stem.
The ac cident of the space shuttle "Challenger" means a draw back f or the GPS program,
as the space shuttles were s upposed to tran sport Block II GPS satellites to their orbit.
Finally the oper ato rs of t he pro gram rev ert to t he Delta rockets inte nded f or the
trans portat ion in t he f irst place.
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Introduction-History
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The f irst Block II satellite was installed and acti v ated.
Temporal deact iv ation of the selectiv e av ailabilit y (SA) during the Gulf war . In this period
civil receiv ers should be used as not enough milit ary receiv ers were av ailable. On July 01 ,
19 91 SA is act iv ated again.
The Init ial Operat ional Capabilit y (IOC) is announc ed. In the same y ear it is also def initely
decided to aut horize the wo rldwide civilian use f ree of charge.
The last Block II satellite completes the sat ellite constellation.
Full Operati onal Capabilit y (FOC) is announced.
Final deactiv at ion of the selectiv e av ailability and theref ore improv ement of th e accuracy
f or civ ilian users from about 10 0 m to 2 0 m.
Launching of the 5 0 st GPS satellite.
Launch of the f irst IIR-M GPS-satellite. This new t ype supports t he new military M-signal
and the seco nd civil signal L2C.
http://www.kowoma.de
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Introduction--cont.
• GPS provides specially coded satellite signals that can be
processed with a GPS receiver, enabling the receiver to
compute position, velocity and time.
• A minimum of four GPS satellite signals are required to compute
positions in three dimensions and the time offset in the receiver
clock.
• Accuracy and precision of data increases with more satellites.
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Three Parts
• Space segment
• Control segment
• User segment
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Space Segment
•
The Air force insures that at least 24
satellites are operational at all times.
• There are six orbital planes (with
nominally four space vehicles (SVs) in
each), equally spaced (60 degrees
apart), and inclined at about fifty-five
degrees with respect to the equatorial
plane.
– The satellite orbits are controlled so that
at least six should be available,
unobstructed location, at all times.
– Each satellite circles the earth twice a http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html
day.
• Each satellite broadcasts a unique signal that tells the receiver its
location and the exact time.
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Control Segment
The Master Control facility is located at Schriever Air Force Base
(formerly Falcon AFB) in Colorado.
Originally Schriever AFB
and four other stations
monitored and controlled
satellite positions.
During August and
September 2005, six more
monitor stations of the NGA
(National GeospatialIntelligence Agency) were
The monitoring stations compute
added to the grid.
precise orbital data (ephemeris) and
Now, every satellite can be SV clock corrections for each satellite
and update each satellite.
seen from at least two
monitor stations.
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Control Segment--cont.
• The Master Control station uploads ephemeris and clock
data to the SVs.
• The SVs then send subsets of the orbital ephemeris data
to GPS receivers over radio signals.
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User Segment
• The primary use of GPS is
navigation.
• Navigation receivers are made
for aircraft, ships, ground
vehicles, surveying, and for
hand carrying by individuals.
• The accuracy of a receiver
depends on the number of
channels, compatibility with
other navigational systems
(WAAS, GLONAS, etc.) and
design of the receiver (cost).
– Most civilian hand held units have an accuracy of 10 meters.
– Survey quality GPS units may be as good as one centimeter.
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User Segment--cont.
• The GPS User Segment consists of all GPS receivers.
– Surveying
– Recreation
– Navigation
• GPS receivers convert satellite signals into position, velocity,
and time estimates.
• Four satellites are required to compute the four dimensions of X,
Y, Z (position) and Time.
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User Segment--cont.
• Time and frequency dissemination, based on the precise clocks
on board the SVs and controlled by the monitor stations, is
another use for GPS.
• Astronomical observatories, telecommunications facilities, and
laboratory standards can be set to precise time signals or
controlled to accurate frequencies by special purpose GPS
receivers.
• The GPS signals are available to everyone, and there is no limit
to the number and types of applications that use them.
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Principles
• The GPS system operates on the principles of trilateration,
determining positions from distance measurements.
• This can be explained using the velocity equation.
Distance
Velocity=
T ime
• Rearranging the equation for distance:

Distance = Velocity x Time

• If the system knows the velocity of a signal and the time it takes
for the signal to travel from the sender to the receiver, the
distance between the sender and the receiver can be
 determined.
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Trilateration Example
• The signals from the GPS satellites travel at the speed of light-186,000 feet/second.
• How far apart are the sender and the receiver if the signal travel
time was 0.23 seconds?
Distance (ft) = Velocity (ft/sec) x Time (sec)
= 186, 000

ft
sec
x
0.23 sec = 42,780 ft
• We know that trilateration requires three distances.
• In the GPS the satellites are at known positions and the receiver
calculates its position by knowing the travel time for the signals
from at least four satellites.
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Satellite Signals
• Each satellite has its own unique signal.
• It continuously broadcasts its signal and also sends out a time
stamp every time it starts.
• The receiver has a copy of each satellite signal and determines
the distance by recording the time between when the satellite
says it starts its signal and when the signal reaches the receiver.
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GPS Trilateration
• Each satellite knows its position
and its distance from the center of
the earth.
• Each satellite constantly
broadcasts this information.
• With this information the receiver
tries to calculate its position.
• Just knowing the distance to one
satellite doesn’t provide enough
information.
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GPS Trilateration--cont.
• When the receiver knows its
distance from only one satellite, its
location could be anywhere on the
earths surface that is an equal
distance from the satellite.
• All the receiver can determine is
that it is some where on the
perimeter of a circle that is an
equal distance from the satellite.
• The receiver must have additional
information.
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GPS Trilateration--cont.
With signals from two satellites, the
receiver can narrow down its location
to just two points on the earths
surface.
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GPS Trilateration--cont.
• Knowing its distance from three
satellites, the receiver can
determine its location because
there is only two possible
combinations and one of them is
out in space.
• In this example, the receiver is
located at b.
• Most receivers actually require
four to insure the receiver has
full information on time, and
satellite positions.
• The more satellite positions that
are used, the greater the
potential accuracy of the position
location.
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Factors Influencing Position Accuracy
The number of satellites (channels) the receiver can track.
– The number of channels a receiver has is part of it’s design.
– The higher the number of channels---the greater the potential
accuracy.
– The higher the number of channels---the greater the cost.
The number of satellites that are available at the time.
– Because of the way the satellites orbit, the same number are not
available at all times.
– When planning precise GPS measurements it is important to check
for satellite availability for the location and time of measurement.
– If a larger number of channels are required (6-10), and at the time of
measurement the number available was less than that, the data will
be less accurate.
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Factors Influencing Position Accuracy--cont.
 The system errors that are occurring during the time the receiver
is operating.
– The GPS system has several errors that have the potential to
reduce the accuracy.
– To achieve high levels of precision, differential GPS must be used.
• Differential GPS uses one unit at a known location and a rover.
– The stationary unit compares its calculated GPS location with the
actual location and computes the error.
– The rover data is adjusted for the error.
• Real Time Kinematic (RTK)
• Post processing
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Location
Once the GPS receiver has located its position it is usually
displayed in one of two common formats:
– Latitude and longitude
– Universal transverse mercator (UTM).
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Latitude and Longitude
Latitudes and longitudes
are angles.
Both use the center of the
earth as the vertex, and
both utilize the equator,
but they use a different
zero reference.
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Latitude
Latitude gives the location of a place on
the Earth north or south of the Equator.
Latitude is an angular measurement in
degrees (marked with °) ranging from 0°
at the Equator to 90° at the poles (90° N
for the North Pole or 90° S for the South
Pole)
The earth’s circumference is approximately
24,859.82 miles around the poles.
Miles
24859.82 miles
=
= 69.05 miles/degree
Degree
360 degrees

Each degree of latitude, at the equator,
equals 69 miles.
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Latitude--cont.
• Each latitude north and south of
the equator forms a circle with
declining diameter.
• For each degree of longitude
north and south of the equator
the number of miles per degree
decreases.
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Latitude--Equator
 The Equator is an imaginary circle drawn around the planet at a
distance halfway between the poles.
 The equator divides the
planet into a Northern
Hemisphere and a
Southern Hemisphere.
 The latitude of the
equator is, by definition,
0°.
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Latitude--cont.
Four lines of latitude are named because of the role they play in
the geometrical relationship with the Earth and the Sun.
–
–
–
–
Arctic Circle — 66° 33′ 39″ N
Tropic of Cancer — 23° 26′ 22″ N
Tropic of Capricorn — 23° 26′ 22″ S
Antarctic Circle — 66° 33′ 39″ S
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Longitude
Longitude describes the location of a
place on earth east or west of a northsouth line called the Prime Meridian.
– Longitude is given as an angular
measurement ranging from 0° at the
Prime Meridian to +180° eastward and
−180° westward.
– In 1884, the International Meridian
Conference adopted the Greenwich
meridian as the universal prime meridian
or zero point of longitude.
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Longitude--cont.
The circumference of the earth at the
equator is approximately 24,901.55
miles.
Miles
24901.55 miles
=
= 69.17 Miles Degree
Degree
360 degrees
Each degree of longitude  69 miles

A longitude of 134o west would be 9,246 west of the prime meridian.

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Longitude--cont.
• There is an important difference
between latitude and longitude.
• The circumference of the earth
declines as the latitude
increase away from the
equator.
• This means the miles per
degree of longitude changes
with the latitude.
• This makes determining the
distance between two points
identified by longitude more
difficult.
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Mercator Projection
• A Mercator projection is a
‘pseudocylindrical’ conformal
projection (it preserves shape).
• Points on the earth are
transferred, on an angle from
the center of the earth, to the
surface of the cylinder.
• What you often see on postersize maps of the world is an
equatorial mercator projection
that has relatively little distortion
along the equator, but quite a bit
of distortion toward the poles.
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Mercator Projection
• What a transverse mercator
projection does, in effect, is
orient the ‘equator’ north-south
(through the poles), thus
providing a north-south
oriented swath of little
distortion.
• By changing slightly the
orientation of the cylinder onto
which the map is projected,
successive swaths of relatively
undistorted regions can be
created.
This system is very accurate for narrow
zones of longitude.
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UTM Zones
The world is divided into 60
zones of latitude, each 6o wide
at the equator, that extend from
84o N to 80o s.
These zones begin at 180o longitude and are numbered
consecutively eastward.
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UTM Zones--cont.
The conterminous United States
is covered by 10 UTM grid zones.
In the Northern Hemisphere each
zone's northing coordinate begins
at the equator as 0,000,000 and is
numbered north in meters.
The easting coordinates are measured from an artificial reference
line drawn perpendicular to the equator and centered in the zone at
the equator.
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UTM--cont.
• The UTM system uses a different grid for the polar regions.
• These areas are covered by a different conformal projection
called the Polar Stereographic.
• Since compass directions have little meaning at the poles, one
direction on the grid is arbitrarily designated "north-south" and
the other "east-west" regardless of the actual compass direction.
• The UTM coordinates are called "false northing" and "false
easting.”
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Using Location Information
Each system has its advantages and disadvantages.
UTM
Latitude and longitude
Advantages
•
•
With the proper
instruments, a person can
determine their position at
the site without using GPS.
Used by most maps
Advantages
Best method for determining
distances between two points.
Disadvantages
Not very useful for finding a
•
Disadvantages
Difficult to determine
distances between two or
more points.
location.
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Determining UTM Zone
• Treat west longitude as negative and east as positive.
• Add 180 degrees; this converts the longitude to a number
between zero and 360 degrees.
• Divide by 6 and round up to the next higher number.
• Example:
– The location of the intersection of Hall of Fame and Virginia on
OSU campus is 56 7 23.71 N and 97 05 16.079 W.
-97.088
+ 180 = 82.912
82.192
= 13.8 = 14
6

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Determining a UTM Grid Value
for a Map Point
• The UTM grid is shown on all
quadrangle maps prepared by
the U.S. Geological Survey
(USGS).
• On 7.5-minute quadrangle
maps (1:24,000 and 1:25,000
scale) and 15-minute
quadrangle maps (1:50,000,
1:62,500, and standardedition 1:63,360 scales), the
UTM grid lines are indicated at
intervals of 1,000 meters,
either by blue ticks in the
margins of the map or with full
grid lines.
• The 1,000-meter value of the
ticks is shown for every tick or
grid line.
http://erg.usgs.gov/isb/pubs/factsheets/fs07701.html
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Determining a UTM Grid Value
for a Map Point--cont.
• To use the UTM grid, you can place
a transparent grid overlay on the
map to subdivide the grid, or you
can draw lines on the map
connecting corresponding ticks on
opposite edges.
• The distances can be measured in
meters at the map scale between
any map point and the nearest grid
lines to the south and west.
• The northing of the point is the
value of the nearest grid line south
of it plus its distance north of that
line; its easting is the value of the
nearest grid line west of it plus its
distance east of that line.
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Determining Distance Using UTM
• In the illustration the UTM
coordinates for two points are
given.
• The distance can be determined
using Pythagorean Theorem
because UTM is a grid system.
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UTM Example--cont.
• Subtracting the easting proved the
length of the horizontal side:
208,000 meters.
• Subtracting the northing proves the
length of the vertical side: 535,000
meters.
• The distance between the two points
is:
Distance =
535, 0002  208, 0002
= 574011.32... or 574, 000 meters
Note: this is the plane distance. To find surface distance
a curve equation must be used.
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GPS Errors
•
Noise
•
Biases
•
Blunder
•
Clock
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Noise Error
• Noise errors are the combined effect of code noise (around 1
meter) and noise within the receiver noise (around 1 meter).
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Bias Error
• Selective Availability (SA)
– SA is the intentional degradation of the SPS signals by a time
varying bias. SA is controlled by the DOD to limit accuracy for nonU. S. military and government users.
– Selective availability is turned off.
• Ephemeris data errors: 1 meter
– Satellite orbits are constantly changing. Any error in satellite
position will result in an error for the receiver position.
• SV clock errors uncorrected by Control Segment can result in
one meter errors.
• Tropospheric delays: 1 meter.
– The troposphere is the lower part (ground level to from 8 to 13 km)
of the atmosphere that experiences the changes in temperature,
pressure, and humidity associated with weather changes.
– Complex models of tropospheric delay require estimates or
measurements of these parameters.
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Bias Error--cont.
• Unmodeled ionosphere delays: 10 meters.
– The ionosphere is the layer of the atmosphere from 50 to 500 km
that consists of ionized air. The transmitted model can only remove
about half of the possible 70 ns of delay leaving a ten meter unmodeled residual.
• Multipath: 0.5 meters.
– Multipath is caused by reflected signals from surfaces near the
receiver that can either interfere with or be mistaken for the signal
that follows the straight line path from the satellite.
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Blunder
• Blunders can result in errors of hundred of kilometers.
– Control segment mistakes due to computer or human error can
cause errors from one meter to hundreds of kilometers.
• User mistakes, including incorrect geodetic datum selection, can
cause errors from 1 to hundreds of meters.
• Receiver errors from software or hardware failures can cause
blunder errors of any size.
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Questions?
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