Transcript No Slide Title

Definition
GPS stands for Global Positioning
System and provides accurate
locational information on the ground
using in the form of a
latitude/longitude
Problem
Often you need to know exactly where you
are or you will need to determine what the
length of a stretch of beach is or what is the
area of a wetland? This sounds simple but
is not nearly as easy as you think.
History
 The need for precise navigation was
critical for the precise delivery of
airborne missiles on their targets.
 As a consequence the US Department
of defence invested in a satellitebased system for precise navigation
and invested some $12 billion to this
end.
 This gave birth to the GPS system.
Benefits
 Normal navigation requires line of site
and theodolites – relatively complex
setup and skilled manpower required.
 GPS provides relatively low costs and
comparatively easy operation and
quick mapping of objects together
with three-dimensional capability.
 Provided you have a clear sky there is
24 hour coverage under virtually any
weather conditions.
What is NAVSTAR?
 A system of 24 satellites which orbit twice a
day (21 operation and three reserve).
 Each satellite is in an orbit 20 000 kms above
the surface of the earth and is in a diagonally
oblique orbit relative to the polar axis of the
earth.
 In effect each part of the earth should at any
time have access to at least 3 satellites.
What makes a GPS?
• The constellation of satellites which transmits
positional information using the speed of signal to
determine relative positions. Each Satellite has very
precise atomic clocks to make these measurements.
• Five control stations around the world:- Colorado
(master station), Ascension, Kwajalein, Diego Garcia
and Hawaii which calibrate the satellite’s clocks.
•Passive receiving handunits - which have low cost
quartz clocks and receive and intrepret signals. Often
have software to help collect, manage and interpret
locational information.
How does a GPS work?
Triangulation is used by
surveyors to map objects and
works on the following principles:
suppose you measure a distance
from one satellite and find it
to be 21,000 kms.
Given that the satellite has
only a certain range or view
of the earth (rather like we can
see only part of the moon surface
at any one time) this narrows
down our possible location to a
radius of 21,000 kms and centred
around the satellite.
Introducing the second Satellite
We now determine the
distance to a second
satellite and find that to
be 22,600 kms. This
will also have only a
selected footprint on the
earth and the effective
intersection of these tow
footprints will narrow
down our position on the
earth.
Three satellites are manadatory
Taking a measurement
from a third satellite which
might be 23,400 kms
away it narrows our
position down even
farther, to the two points
where the 23,400 km
sphere cuts through the
circle formed by the
intersection of the first two
spheres. Consequently we
can now determine that
we are somewhere on the
circle where these two
spheres intersect.
We really need Four
Satellites to determine
which of the two
positions!
How doe these satellites provide
positional information?
•Each satellite broadcasts its orbital position in
“pseudo code”
•The receiver on the ground calculates the time
the signal (pseudo code) took to get from the
satellite to ground and turns these time units
into distance based on the speed the light
travels at (“pseudorange”)
•Using information from 3 to 4 satellites allows
triangulation to the GPS receivers position.
What is the Accuracy of a GPS?
•A typical receiver that you purchased and used
prior to the selective availability being switched
off was accurate from 50 to 100 m.
• The same receiver with select availability
switched off is now accurate to 10-20 m.
• Using differential correction sub-meter
accuracy is possible.
What influences the Accuracy of a GPS?
•Paradoxically errors in the clocks produce an error of
no more than 1 m
•The position and number of satellites that are viewable
will increase accuracy – PDOP values and make big
differences
•The atmospheric conditions also significantly impact on
your accuracy of up to 4 meters
•Selective availability imposes an error of approximately
50 m
What is Selective Availability?
•the US Department of Defence who operate the
satellites deliberately generated a random error of about
50 metres.
•This error was referred to as selective availablity and
was applied to all civilian and non-USA defence force
activities but was switched off in May 2000.
•The primary purpose of selective availability was to
ensure than any USA hostile agents could not use their
technology to delivery missiles etc with any sort of real
precision. It would probably be switched on again if the
USA were to become involved in a military war.
How can you determine the accuracy of your
GPS?
•Most GPS receivers will provide a PDOP reading
•PDOP stands for Positional Dilution of Precision and is
assessed on a scale of 1 to infinity
•Your usual values for PDOP should be between 2 and 8
m as the range of best to worst that you can expect.
•The PDOP is a scale that measures the geometry of all
of the observed satellites. The more satellite that are
viewable and the higher their position above the horizon
the lower the PDOP value.
Are there differences between GPS receivers?
• Eight channels receivers were the normal, but are
increasingly being replaced by more accurate 12 channel
units.
•While the basic GPS provides positional accuracy in the
order of 50-100 metres this may not be accurate enough.
•More expensive receivers should allow various methods
of differential correction to overcome selective availability.
•The most expensive receivers also provide realtime
differential calculations and you will get to submeter
accuracy in the field and can transfer the data as GIS
coverages.
What is Differential Correction?
• Differential GPS works on the principle of having at least
two receivers, each obtaining information from at least four
respective satellites.
•These receivers are used such that one is stationary and
the second moves (rover) around collecting information.
•The stationary one being in a fixed position will know the
precise errors occurring at that particular time in the signal
and these can be applied to the rover unit.
How does a Differential GPS work?
•A typical you would setup a stationary GPS referred to as
the base station at a known location such as survey
beacon.
•The deviations from the known position are then
transmitted to the roving GPS and can “correct” the signal.
•This is referred to as real time differential GPS mapping
and no further corrections need to be applied.
•More recently companies have launched satellites of
there own which remain in a fixed position and can be
used as the base station and will transmit the signals to the
roving GPS to correct their positional accuracies in real
time.
GPS receivers are programmable.
•Modern mapping GPS are basically small hand held
computers with some form of information storage either as
a mini-hard disk or more commonly as Read Only Memory
ROM units.
•By pre-programming your GPS you can select one of
several features, e.g. a coastal line, a river edge, pathway
or road, or locational spots such as a survey beacon, a
marine buoy etc and these can be saved as separate
features to be stored within your GIS.
•A second advantage of the modern GPS is that you prestore positions and you can use to navigate to.
What can you do if you do not have a
differential GPS signal?
•To obtain a real-time differential GPS signals either
requires two receives or a secondary signal based on a
subscription service.
•It is, however, possible to obtain differential locational
information in South Africa with which to correct nondifferentially corrected GPS data.
•TELKOM loads down GPS information at various stations.
•Using this information on their website that matches
where you collected your data and the time frame when
you collected your data you can use the two sets of data to
post-process you data into differentially correct readings.
References
All About GPS
http://www.trimble.com/gps/howgps/gpsfram1.htm
A GPS Tutorial – Basics of high precision Global Positioning Systems
Javad Positioning Systems.
Copyright for pictures used
The three drawings were taken from the www.trimble.com website: No
portion of this service may be reproduced in any form, or by any means,
except as authorized below, without prior written permission from
Trimble Navigation Limited.