THE NAVEGATION SATELLITE TIMING AND RANGING NAVSTAR GLOBAL POSITIONING SYSTEM GPS

1
THE NAVEGATION SATELLITE TIMING AND RANGING
(NAVSTAR) GLOBAL POSITIONING SYSTEM (GPS) A.
Coles Tulane University
2
(No Transcript)
3
SATELLITES CONSTELLATION
Source Global Positioning Systemhttp//scour.myo
ngji.ac.kr/kwon/tsld001.htm Jay Hyoun Kwon
Department of Civil Environmental Engineering
Geodetic Science The Ohio State University
4
GPS POSITIONING
http//scour.myongji.ac.kr/kwon/tsld001.htm
5
(No Transcript)
6
(No Transcript)
7
(No Transcript)
8
(No Transcript)
9
(No Transcript)
10
(No Transcript)
11
(No Transcript)
12
(No Transcript)
13
WHAT IS A GPS
The Global Positioning System (GPS) was developed
by the United States Department of Defense as a
reliable means for accurate navigation. It is
based on an intricate network of 24 satellites
orbiting the earth at a very high altitude. These
satellites function 24 hours a day and
are designed to be resistant to jamming and
interference. GPS allows every square meter of
the earth's surface to have a unique address,
which offers limitless application possibilities,
when coupled with today's advanced
micro-computer systems.
Source GeoPlane Services (a division of
Universal ENSCO, Inc.)
http//www.geoplane.com/gps.html
14
Here's how GPS works
The basis of GPS is
triangulation
from satellites. Position is

calculated from distance
measurements (ranges)
to
satellites. Mathematically we need
four
satellite ranges to determine
exact position.
Three ranges are
enough if we reject
ridiculous
answers or use other tricks.
http//www.geoplane.com/gps.html
15
To triangulate, a GPS
receiver measures distance
using the travel time of
radio signals. Distance to a
satellite is determined by
measuring how long a radio signal
takes to reach us from that
satellite. To make the
measurement we assume that both
the satellite and our receiver are
generating the same
pseudo-random codes at exactly
the same time. By comparing how
late the satellite's pseudo-random
code appears compared to our
receiver's code, we determine how
long it took to reach us.
Multiply that travel time by
the speed of light and
you've got distance.
http//www.geoplane.com/gps.html
16

To measure travel time, GPS needs
very
accurate timing which it
achieves with some
tricks.
Accurate timing is the key to

measuring distance to satellites.
Satellites
are accurate because
they have atomic clocks
on board.
Receiver clocks don't have to be

too accurate because an extra
satellite range
measurement can
remove errors.
http//www.geoplane.com/gps.html
17
Along with distance, you
need to know exactly where
the satellites are in space.
High orbits and careful
monitoring are the secret.
To use the satellites as references
for range measurements we need
to know exactly where they are.
GPS satellites are so high up their
orbits are very predictable.
Minor variations in their
orbits are measured by the
Department of Defense. The
error information is sent to
the satellites, to be
transmitted along with the timing
signals.
http//www.geoplane.com/gps.html
18
Finally you must correct for any

delays the signal experiences as it
travels
through the atmosphere.
The earth's ionosphere
and
atmosphere cause delays in the
GPS
signal that translate into
position errors.
Some errors can
be factored out using
mathematics
and modeling. The configuration

of the satellites in the sky can
magnify other
errors. Differential
GPS can eliminate almost
all error.
http//www.geoplane.com/gps.html