Principles of GPS
Principles of GPS
What is GPS?
How does it work?
Principles of GPS
•
GPS: Global Positioning System is a worldwide radio-navigation system formed from a constellation of
24 satellites and their ground stations.
•
A simplistic explanation: GPS uses these “man-made stars” as reference points to calculate positions
accurate to a matter of meters.
•
Radio-based navigation system funded and developed by DoD
– First Satelite Launch in 1978
– Initial operation in 1993
– Fully operational in 1995
•
System is called NAVSTAR
– NAVigation with Satellite Timing And Ranging
– Referred to as GPS
•
Series of 24 satellites, 6 orbital planes, 4 satellite vehicles (SV) on each plane
•
Works anywhere in the world, 24 hours a day, in all weather conditions and provides:
– Location or positional fix
– Velocity
– Direction of travel
– Accurate time
Principles of GPS
The Global Positioning System
is comprised of three segments.
Control Segment
Space Segment
User Segment
Three Segments of the GPS
Space Segment
User Segment
Control Segment
Ground
Antennas
Master Station
Monitor Stations
Control Segment
• The Control segment is made up of a
Master Control Station (MCS), four
monitor stations, and three ground
antennas (plus a reserve antenna at
Cape Canaveral used primarily for prelaunch satellite testing) used to uplink
data to the satellites.
• The Master Control Station, or MCS
(also known as the Consolidated
Satellite Operations Center) is located
at the US Air Force Space Command
Center at Schriever Air Force Base
(formerly Falcon AFB) in Colorado
Springs, Colorado. It's responsible for
satellite control and overall system
operations.
Ground
Antennas
Master Station
Monitor Stations
Control Segment
• Monitor stations (MS) are
unmanned remote sensors that
passively collect raw satellite
signal data and re-transmit it in
real time to the MCS for
evaluation. Monitor stations
basically function as very precise
radio receivers, tracking each
satellite as it comes into view.
• Ground antennas are remotely
controlled by the MCS. Ground
antennas transmit data
commands received from the
Master Control Station to GPS
satellites within their sky view
Ground
Antennas
Master Station
Monitor Stations
Control Segment
US Space Command
Cape Canaveral
Hawaii
Kwajalein Atoll
Diego Garcia
Ascension
Is.
Control Segment
Master Control Station
Monitor Station
Ground Antenna
Control Segment
Control Segment
The MCS uplinks data to GPS satellites
which includes:
• Clock-correction factors for each
satellite; necessary to ensure that all
satellites are operating at the same
precise time (known as “GPS Time”).
• Atmospheric data (to help correct
most of the distortion caused by the
GPS satellite signals passing through
the ionosphere layer of the
atmosphere).
Control Segment
The MCS uplinks data to GPS satellites which
includes:
• Almanac, which is a log of all GPS satellite
positions and health, and allows a GPS
receiver to identify which satellites are in its
hemisphere, and at what times.
• An almanac is like a schedule telling a GPS
receiver when and where satellites will be
overhead. Transmitted continuously by all
satellites, the almanac allows GPS receivers
to choose the best satellite signals to use to
determine position.
Control Segment
The MCS uplinks data to GPS satellites which includes:
• Ephemeris data is unique to each satellite, and provides
highly accurate satellite position (orbit) information for that
GPS satellite alone. It does not include information about
the GPS constellation as a whole. Ephemeris information is
also transmitted as a part of each satellite’s time signal.
• By using the information from the GPS satellite
constellation almanac in conjunction with the ephemeris
data from each satellite, the position of a GPS satellite can
be very precisely determined for a given time.
Space Segment
Space Segment
• The space segment is an earth-orbiting constellation of
24 active and five spare GPS satellites circling the earth
in six orbital planes. Each satellite is oriented at an
angle of 55 degrees to the equator. The nominal
circular orbit is 20,200-kilometer (10,900 nautical
miles) altitude. Each satellite completes one earth orbit
every twelve hours (two orbits every 24 hours). That's
an orbital speed of about 1.8 miles per second, so that
each satellite travels from visible horizon to horizon in
about 2 hours.
• Each satellite has a design life of approximately 10
years, weighs about 2,000 pounds, and is about 17 feet
across with its solar panels extended.
Space Segment
• Each satellite transmits as part of its signal to ground stations and all users
the following information:
-Coded ranging signals (for triangulation)
Sequences of 0s and 1s (zeroes and ones), which allow the receiver to determine the travel time
of radio signal from satellite to receiver. They are called Pseudo-Random Noise (PRN) sequences or PRN
codes.
-Ephemeris position information
-Atmospheric data
-Clock correction information defining the precise time of satellite signal
transmission (in GPS Time), and a correction parameter to convert GPS Time to
UTC.
-An almanac containing information on the GPS constellation, which includes
location and health of the satellites.
User Segment
•
•
•
•
•
•
•
•
•
•
Military.
Search and rescue.
Disaster relief.
Surveying.
Marine, aeronautical and terrestrial navigation.
Remote controlled vehicle and robot guidance.
Satellite positioning and tracking.
Shipping.
Geographic Information Systems (GIS).
Recreation.
Four Primary Functions of GPS
•
Position and coordinates.
•
The distance and direction between any two points.
•
Travel progress reports.
•
Accurate time measurement.
Global Navigation Satellite Systems
• NAVSTAR
– USA
• GLONASS
– Russians
• Galileo
– Europeans
• Beidou
– Chinese
How does GPS Work?
Triangulation
• A GPS receiver's job is to locate four or more of these
satellites, figure out the distance to each, and use this
information to deduce its own location.
• This operation is based on a simple mathematical
principle called triangulation or trilateration.
• Triangulation in three-dimensional space can be a little
tricky, so we'll start with an explanation of simple twodimensional trilateration.
An Example of 2D Triangulation
• Imagine you are somewhere in the United States and you
are TOTALLY lost -- for whatever reason, you have
absolutely no clue where you are.
• You find a friendly local and ask, "Where am I?" He says,
"You are 625 miles from Boise, Idaho."
• This is a nice, hard fact, but it is not particularly useful by
itself. You could be anywhere on a circle around Boise
that has a radius of 625 miles
Where in the U.S. am I?
• To pinpoint your location better, you ask somebody else
where you are.
• She says, "You are 690 miles from Minneapolis,
Minnesota.“ If you combine this information with the
Boise information, you have two circles that intersect.
Where in the U.S. am I? (Cont’d)
• If a third person tells you that you are 615 miles from
Tucson, Arizona, you can eliminate one of the
possibilities, because the third circle will only intersect
with one of these points. You now know exactly where
you are…
Where in the U.S. am I? (Cont’d)
• You are in Denver, CO!
• This same concept works in three-dimensional space, as
well, but you're dealing with spheres instead of circles.
Another 2D Example
• Consider the case of a mariner at sea (receiver)
determining his/her position using a foghorn
(transmitter).
• Assume the ship keeps an accurate clock and mariner has
approximate knowledge of ship’s location.
Fog
Foghorn Example
• Foghorn whistle is sounded precisely on the minute mark
and ship clock is synchronized to foghorn clock.
• Mariner notes elapsed time from minute mark until
foghorn whistle is heard.
• This propagation time multiplied by speed of sound is
distance from foghorn to mariner’s ear.
Distance = Speed x Time
Foghorn Example (Cont’d)
• Based on measurement from one such foghorn, we know
mariner’s distance (D) to foghorn.
• With measurement from one foghorn, mariner can be
located anywhere on the circle denoted below:
D
Foghorn 1
Foghorn Example (Cont’d)
• If mariner simultaneously measured time range from 2nd
foghorn in same way.
• Assuming, transmissions synchronized to a common time
base and mariner knows the transmission times. Then:
A
D
Foghorn 1
Possible Location of Mariner
D2
Foghorn 2
B
Foghorn Example (Cont’d)
• Since mariner has approximate knowledge of ship’s
location, he/she can resolve the ambiguity between
location A and B.
• If not, then measurement from a third foghorn is needed.
D2
D
Foghorn 1
Foghorn 2
B
D3
Foghorn 3
How Foghorn Relates to GPS
• The foghorn examples operates in 2D space. GPS
performs similarly but in 3D.
• The foghorn examples shows how time-of-arrival of
signal (whistle) can be used to locate a ship in a fog. In
this time-of-arrival of signal, we assumed we knew when
the signal was transmitted.
• We measured the arrival time of the signal to determine
distance. Multiple distance measurements from other
signals were used to locate the ship exactly.
Foghorn Example: Consider Effect of
Errors
• Foghorn/mariner
discussion assumed
ship’s clock was precisely
synchronized to
foghorn’s time base.
• This may not be the case
 errors in TOA
measurements.
• If we make a fourth
measurement, we can
remove this uncertainty.
D2+e2
D+e1
Foghorn 1
Foghorn 2
D3+e3
Foghorn 3
Estimated
Location Area
of Ship
3D Triangulation
• Fundamentally, three-dimensional trilateration is not
much different from two-dimensional trilateration, but
it's a little trickier to visualize.
• Imagine the radii from the examples in the last section
going off in all directions. So instead of a series of circles,
you get a series of spheres.
GPS Triangulation
• If you know you are 10 miles from satellite A in the sky,
you could be anywhere on the surface of a huge,
imaginary sphere with a 10-mile radius.
10 miles
Earth
GPS Triangulation (Cont’d)
• If you also know you are 15 miles from satellite B, you can
overlap the first sphere with another, larger sphere. The
spheres intersect in a perfect circle.
15 miles
10 miles
GPS Triangulation (Cont’d)
• The circle intersection implies that the GPS
receiver lies somewhere in a partial ring on
the earth.
Perfect circle formed from
locating two satellites
Possible
Locations of
GPS Receiver
GPS Triangulation (Cont’d)
• If you know the distance to a third satellite, you get a
third sphere, which intersects with this circle at two
points.
Triangulation
Satellite 1
Satellite 3
Satellite 2
Satellite 4
GPS Triangulation (Cont’d)
• The Earth itself can act as a fourth sphere -- only one of
the two possible points will actually be on the surface of
the planet, so you can eliminate the one in space.
• Receivers generally look to four or more satellites,
however, to improve accuracy and provide precise
altitude information.
GPS Receivers
• In order to make this simple calculation, then, the GPS
receiver has to know two things:
– The location of at least three satellites above you
– The distance between you and each of those
satellites
• The GPS receiver figures both of these things out by
analyzing high-frequency, low-power radio signals from
the GPS satellites.
GPS Receivers (Cont’d)
• Better units have multiple receivers, so they can pick up
signals from several satellites simultaneously.
• Radio waves travel at the speed of light (about 186,000
miles per second, 300,000 km per second in a vacuum).
• The receiver can figure out how far the signal has
traveled by timing how long it took the signal to arrive.
(Similar to foghorn example.)
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