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What is GPS?
The Global Positioning System (GPS) is a location system based on
a constellation of about 24 satellites orbiting the earth at altitudes
of approximately 11,000 miles. GPS was developed by the United States
Department of Defense (DOD), for its tremendous application as a
military locating utility. The DOD's investment in GPS is immense.
Billions and billions of dollars have been invested in creating
this technology for military uses. However, over the past several
years, GPS has proven to be a useful tool in non-military mapping
applications as well.
GPS
satellites are orbited high enough to avoid the problems associated
with land based systems, yet can provide accurate positioning 24
hours a day, anywhere in the world. Uncorrected positions determined
from GPS satellite signals produce accuracies in the range of 50
to 100 meters. When using a technique called differential correction,
users can get positions accurate to within 5 meters or less.
Today,
many industries are leveraging off the DOD's massive undertaking.
As GPS units are becoming smaller and less expensive, there are
an expanding number of applications for GPS. In transportation applications,
GPS assists pilots and drivers in pinpointing their locations and
avoiding collisions. Farmers can use GPS to guide equipment and
control accurate distribution of fertilizers and other chemicals.
Recreationally, GPS is used for providing accurate locations and
as a navigation tool for hikers, hunters and boaters.
Many would
argue that GPS has found its greatest utility in the field of Geographic
Information Systems (GIS). With some consideration for error, GPS
can provide any point on earth with a unique address (its precise
location). A GIS is basically a descriptive database of the earth
(or a specific part of the earth). GPS tells you that you are at
point X,Y,Z while GIS tells you that X,Y,Z is an oak tree, or a
spot in a stream with a pH level of 5.4. GPS tells us the "where".
GIS tells us the "what". GPS/GIS is reshaping the way
we locate, organize, analyze and map our resources.
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How GPS Determines a Location
In a nutshell, GPS is based on satellite ranging - calculating the
distances between the receiver and the position of 3 or more satellites
(4 or more if elevation is desired) and then applying some good
old mathematics. Assuming the positions of the satellites are known,
the location of the receiver can be calculated by determining the
distance from each of the satellites to the receiver. GPS takes
these 3 or more known references and measured distances and "triangulates"
an additional position.
As an example, assume that I have asked you to find me at a stationary
position based upon a few clues which I am willing to give you.
First, I tell you that I am exactly 10 miles away from your house.
You would know I am somewhere on the perimeter of a sphere that
has an origin as your house and a radius of 10 miles. With this
information alone, you would have a difficult time to find me since
there are an infinite number of locations on the perimeter of that
sphere.
Second, I tell you that I am also exactly 12 miles away from the
ABC Grocery Store. Now you can define a second sphere with its origin
at the store and a radius of 12 miles. You know that I am located
somewhere in the space where the perimeters of these two spheres
intersect - but there are still many possibilities to define my
location.
Adding additional spheres will further reduce the number of possible
locations. In fact, a third origin and distance (I tell you am 8
miles away from the City Clock) narrows my position down to just
2 points. By adding one more sphere, you can pinpoint my exact location.
Actually, the 4th sphere may not be necessary. One of the possibilities
may not make sense, and therefore can be eliminated.
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For
example, if you know I am above sea level, you can reject a point
that has negative elevation. Mathematics and computers allow us
to determine the correct point with only 3 satellites.
Based on this example, you can see that you need to know the following
information in order to compute your position: A)
What is the precise location of three or more known points (GPS
satellites)?
B) What is the distance between the known points and the position
of the GPS receiver?
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How the Current Locations of GPS Satellites are Determined
GPS satellites are orbiting the Earth at an altitude of 11,000 miles.
The DOD can predict the paths of the satellites vs. time with great
accuracy. Furthermore, the satellites can be periodically adjusted
by huge land-based radar systems. Therefore, the orbits, and thus
the locations of the satellites, are known in advance. Today's GPS
receivers store this orbit information for all of the GPS satellites
in what is known as an almanac. Think of the almanac as a "bus
schedule" advising you of where each satellite will be at a
particular time. Each GPS satellite continually broadcasts the almanac.
Your GPS receiver will automatically collect this information and
store it for future reference.
The Department of Defense constantly monitors the orbit of the satellites
looking for deviations from predicted values. Any deviations (caused
by natural atmospheric phenomenon such as gravity), are known as
ephemeris errors. When ephemeris errors
are determined to exist for a satellite, the errors are sent back
up to that satellite, which in turn broadcasts the errors as part
of the standard message, supplying this information to the GPS receivers.
By using the information from the almanac in conjuction with the
ephemeris error data, the position of a GPS satellite can be very
precisely determined for a given time.
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Computing the Distance Between Your Position and the GPS Satellites
GPS determines distance between a GPS satellite and a GPS receiver
by measuring the amount of time it takes a radio signal (the GPS
signal) to travel from the satellite to the receiver. Radio waves
travel at the speed of light, which is about 186,000 miles per second.
So, if the amount of time it takes for the signal to travel from
the satellite to the receiver is known, the distance from the satellite
to the receiver (distance = speed x time) can be determined. If
the exact time when the signal was transmitted and the exact time
when it was received are known, the signal's travel time can be
determined.
In order to do this, the satellites and the receivers use very accurate
clocks which are synchronized so that they generate the same code
at exactly the same time. The code received from the satellite can
be compared with the code generated by the receiver. By comparing
the codes, the time difference between when the satellite generated
the code and when the receiver generated the code can be determined.
This interval is the travel time of the code. Multiplying this travel
time, in seconds, by 186,000 miles per second gives the distance
from the receiver position to the satellite in miles.
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Four (4) Satellites to give a 3D position
In the previous example, you saw that it took only 3 measurements
to "triangulate" a 3D position. However, GPS needs a 4th
satellite to provide a 3D position. Why??
Three
measurements can be used to locate a point, assuming the GPS receiver
and satellite clocks are precisely and continually synchronized,
thereby allowing the distance calculations to be accurately determined.
Unfortunately, it is impossible to synchronize these two clocks,
since the clocks in GPS receivers are not as accurate as the very
precise and expensive atomic clocks in the satellites. The GPS signals
travel from the satellite to the receiver very fast, so if the two
clocks are off by only a small fraction, the determined position
data may be considerably distorted.
The atomic clocks aboard the satellites maintain their time to a
very high degree of accuracy. However, there will always be a slight
variation in clock rates from satellite to satellite. Close monitoring
of the clock of each satellite from the ground permits the control
station to insert a message in the signal of each satellite which
precisely describes the drift rate of that satellite's clock. The
insertion of the drift rate effectively synchronizes all of the
GPS satellite clocks.
The same procedure cannot be applied to the clock in a GPS receiver.
Therefore, a fourth variable (in addition to x, y and z), time,
must be determined in order to calculate a precise location. Mathematically,
to solve for four unknowns (x,y,z, and t), there must be four equations.
In determining GPS positions, the four equations are represented
by signals from four different satellites.
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Using
Differential GPS to Increase Accuracy
As powerful as
GPS is, +/-50 - 100 meters of uncertainty is not acceptable in many
applications. How can we obtain higher accuracies?
A technique
called differential correction is necessary
to get accuracies within 1 -5 meters, or even better, with advanced
equipment. Differential correction requires a second GPS receiver,
a base station, collecting data at a stationary position on a precisely
known point (typically it is a surveyed benchmark). Because the
physical location of the base station is known, a correction factor
can be computed by comparing the known location with the GPS location
determined by using the satellites.
The differential correction process takes this correction factor
and applies it to the GPS data collected by a GPS receiver in the
field. Differential correction eliminates most of the errors listed
in the GPS Error Budget discussed earlier. After differential correction,
the GPS Error Budget changes as follows:
GPS Error Budget
| Source |
Uncorrected |
With
Differential |
| Ionosphere |
0-30
meters |
Mostly
Removed |
| Troposphere |
0-30
meters |
All Removed |
| Signal
Noise |
0-10
meters |
All Removed |
| Ephemeris
Data |
1-5 meters |
All Removed |
| Clock
Drift |
0-1.5
meters |
All Removed |
| Multipath |
0-1 meters |
Not Removed |
| SA |
0-70
meters |
All Removed |
By eliminating many of the above errors, differential correction
allows GPS positions to be computed at a much higher level of accuracy.
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Measuring GPS Accuracy
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As
discussed above, there are several external sources which introduce
errors into a GPS position. While the errors discussed above always
affect accuracy, another major factor in determining positional
accuracy is the alignment, or geometry, of the group of satellites
(constellation) from which signals are being received. The geometry
of the constellation is evaluated for several factors, all of which
fall into the category of Dilution Of Precision, or DOP.
DOP is an indicator of the quality of the geometry of the
satellite constellation. Your computed position can vary depending
on which satellites you use for the measurement. Different satellite
geometries can magnify or lessen the errors in the error budget
described above. A greater angle between the satellites lowers the
DOP, and provides a better measurement. A higher DOP indicates poor
satellite geometry, and an inferior measurement cofiguration.
Some GPS receivers can analyze the positions of the satellites available,
based upon the almanac, and choose those satellites with the best
geometry in order to make the DOP as low as possible. Another important
GPS receiver feature is to be able to ignore or eliminate GPS readings
with DOP values that exceed user-defined limits. Other GPS receivers
may have the ability to use all of the satellites in view, thus
minimizing the DOP as much as possible. |
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The
GPS Error Budget
The GPS system has been designed to be as nearly accurate as possible.
However, there are still errors. Added together, these errors
can cause a deviation of +/- 50 -100
meters from the actual GPS receiver position. There are several
sources for these errors, the most significant of which are discussed
below:
Atmospheric Conditions
The ionosphere and troposphere both refract the GPS signals. This
causes the speed of the GPS signal in the ionosphere and troposphere
to be different from the speed of the GPS signal in space. Therefore,
the distance calculated from "Signal Speed x Time" will
be different for the portion of the GPS signal path that passes
through the ionosphere and troposphere and for the portion that
passes through space.
Ephemeris Errors/Clock Drift/Measurement Noise
As mentioned earlier, GPS signals contain information about ephemeris
(orbital position) errors, and about the rate of clock drift for
the broadcasting satellite. The data concerning ephemeris errors
may not exactly model the true satellite motion or the exact rate
of clock drift. Distortion of the signal by measurement noise
can further increase positional error. The disparity in ephemeris
data can introduce 1-5 meters of positional error, clock drift
disparity can introduce 0-1.5 meters of positional error and measurement
noise can introduce 0-10 meters of positional error.
Selective Availability
Ephemeris errors should not be confused with Selective Availability
(SA), which is the intentional alteration of the time and epherimis
signal by the Department of Defense. SA can introduce 0-70 meters
of positional error. Fortunately, positional errors caused by
SA can be removed by differential correction.
Multipath
A GPS signal bouncing off a reflective surface prior to reaching
the GPS receiver antenna is referred to as multipath. Because
it is difficult to completely correct multipath error, even in
high precision GPS units, multipath error is a serious concern
to the GPS user.
The chart below lists the most common sources of error in GPS
positions. This chart is commonly known as the GPS Error Budget:
GPS
Error Budget
| Source |
Uncorrected
Error Level |
| Ionosphere |
0-30
meters |
| Troposphere |
0-30
meters |
| Measurement
Noise |
0-10
meters |
| Ephemeris
Data |
1-5 meters |
| Clock
Drift |
0-1.5
meters |
| Multipath |
0-1 meter |
| Selective
Availability |
0-70
meters |
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Levels of GPS Accuracy
There
are three types of GPS receivers which are available in today's
marketplace. Each of the three types offers different levels of
accuracy, and has different requirements to obtain those accuracies.
To this point, the discussion in this book has focused on Coarse
Acquisition (C/A code) GPS receivers. The two remaining types of
GPS receiver are Carrier Phase receivers and Dual Frequency receivers.
C/A
Code receivers
C/A Code receivers typically provide 1-5 meter GPS position accuracy
with differential correction. C/A Code GPS receivers provide a sufficient
degree of accuracy to make them useful in most GIS applications.
C/A
Code receivers can provide 1-5 meter GPS position accuracy with
an occupation time of 1 second. Longer occupation times (up to 3
minutes) will provide GPS position accuracies consistently within
1-3 meters. Recent advances in GPS receiver design will now allow
a C/A Code receiver to provide sub-meter accuracy, down to 30 cm.
Carrier Phase
receivers
Carrier Phase receivers typically provide 10-30 cm GPS position
accuracy with differential correction. Carrier Phase receivers provide
the higher level of accuracy demanded by certain GIS applications.
Carrier
Phase receivers measure the distance from the receiver to the satellites
by counting the number of waves that carry the C/A Code signal.
This method of determining position is much more accurate; however,
it does require a substatially higher occupation time to attain
10-30 cm accuracy. Initializing a Carrier Phase GPS job on a known
point requires an occupation time of about 5 minutes. Initializing
a Carrier Phase GPS job on an unknown point requires an occupation
time of about 30-40 minutes.
Additional
requirements, such as maintaining the same satellite constellation
throughout the job, performance under canopy and the need to be
very close to a base station, limit the applicability of Carrier
Phase GPS receivers to many GIS applications.
Dual-Frequency
receivers
Dual-Frequency receivers are capable of providing sub-centimeter
GPS position accuracy with differential correction. Dual-Frequency
receivers provide "survey grade" accuracies not often
required for GIS applications.
Dual-Frequency
receivers receive signals from the satellites on two frequencies
simultaneously. Receiving GPS signals on two frequencies simultaneously
allows the receiver to determine very precise positions.
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GPS and Canopy
GPS receivers require a line of sight to the satellites in order
to obtain a signal representative of the true distance from the
satellite to the receiver. Therefore, any object in the path of
the signal has the potential to interfere with the reception of
that signal. Objects which can block a GPS signal include tree
canopy, buildings and terrain features.
Further, reflective surfaces can cause the GPS signals to bounce
before arriving at a receiver, thus causing an error in the distance
calculation. This problem, known as multipath, can be caused by
a variety of materials including water, glass and metal. The water
contained in the leaves of vegatation can produce multipath error.
In some instances, operating under heavy, wet forest canopy can
degrade the ability of a GPS receiver to track satellites.
There are several data collection techniques which can mitigate
the effects of signal blockage by tree canopy or other objects.
For example, many GPS receivers can be instructed to track only
the highest satellites in the sky, as opposed to those satellites
which provide the best DOP. Increasing the elevation of the GPS
antenna can also dramatically increase the ability of the receiver
to track satellites.
Unfortunately, there will be locations where GPS signals simply
are not available due to obstruction. In these cases, there are
additional techniques which can help to solve the problem. Some
GPS receivers have the ability to collect an offset point, which
involves recording a GPS position at a location where GPS signals
are available while also recording the distance, bearing and slope
from the GPS antenna to the position of interest where the GPS
signals are not available. This technique is useful for avoiding
a dense timber stand or building.
Further, a traditional traverse program can be used to manually
enter a series of bearings and ranges to generate positions until
satellite signals can again be received. This position data can
then be used to augment position data collected with the GPS receiver.
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GPS for GIS
Up to this point, the discussion has focused on
describing how GPS determines a location on the surface of the Earth.
Now the discussion can shift to the process of describing what is
at the location. The "what" is the object or objects which
will be mapped. These objects are referred to as "Features",
and are used to build a GIS. It is the power of GPS to precisely
locate these Features which adds so much to the utility of the GIS
system. On the other hand, without Feature data, a coordinate location
is of little value.
Feature
Types
There are three types of Feature which can be mapped: Points, Lines
and Areas. A Point Feature is a single GPS coordinate position which
is identified with a specific Object. A Line Feature is a collection
of GPS positions which are identified with the same Object and linked
together to form a line. An Area Feature is very similar to a Line
Feature, except that the ends of the line are tied to each other
to form a closed area.
Describing
Features
As stated above, a Feature is the object which will be mapped by
the GPS system. The ability to describe a Feature in terms of a
multi-layered database is essential for successful integration with
any GIS system. For example, it is possible to map the location
of each house on a city block and simply label each coordinate position
as a house. However, the addition of information such as color,
size, cost, occupants, etc. will provide the ability to sort and
classify the houses by these catagories.
These
catagories of descriptions for a Feature are know as Attributes.
Attributes can be thought of as questions which are asked about
the Feature. Using the example above, the Attributes of the Feature
"house" would be "color", "size",
"cost" and "occupants".
Logically,
each question asked by the Attributes must have an answer. The answers
to the questions posed by the Attributes are called Values. In the
example above, an appropriate Value (answer) for the Attribute (question)
"color" may be "blue".
The
following table illustrates the relationship between Features, Attributes
and Values:
| Feature |
Attribute |
Value
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| House
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Color |
Blue
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Size |
3 BDR
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Cost |
$118K
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Occupants |
5
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By collecting the same type of data for each house which is mapped,
a database is created. Tying this database to position information
is the core philosophy underlying any GIS system. Feature
Lists
The field data entry process can be streamlined by the use of
a Feature List. The Feature List is a database which contains
a listing of the Features which will be mapped, as well as the
associated Attributes for each Feature. In addition, the Feature
List contains a selection of appropriate Values for each Attribute.
The Feature List can be created on the CMT hand-held GPS data
collector, or on a PC. Below is an example of a Feature List as
it appears in PC-GPS:
When
a Feature List is used in the field, the first step is to select
the Feature to be mapped. Once a Feature is selected, the Attributes
for that Feature are automatically listed. A Value for each Attribute
can then be selected from the displayed list of predetermined
Values.
The
use of a Feature List streamlines the data entry process and also
ensures consistent data entry among different users in the same
organization.
Exporting
to a GIS System
The final step in incorporating GPS data with a GIS system is
to export the GPS and Feature data into the GIS system. During
this process, a GIS "layer" is created for each Feature
in the GPS job. For example, the process of exporting a GPS job
which contains data for House, Road and Lot Features would create
a House layer, a Road layer and a Lot layer in the GIS system.
These layers can then be incorporated with existing GIS data.
Once
the GPS job has been exported, the full power of the GIS system
can be used to classify and evaluate the data.
Reference:
http://www.cmtinc.com/gpsbook/index.htm
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