[gps-talkusers] everything you ever wanted to know about GPS accuracy
- From: Michael May <mikemay@xxxxxxxxxxxxxxxx>
- To: "GPS-talkusers-freelists.org" <GPS-talkusers@xxxxxxxxxxxxx>
- Date: Fri, 23 Dec 2005 15:11:18 -0800
The following document will be posted soon on the Sendero download page.
GPS and Map Accuracy
Compiled by Sendero Group
December 2005
Contents:
1. Highlights
2. BrailleNote GPS manual, section 3
3. GPS-Talk User's list, 2 emails on WAAS
4. Excerpt from an adaptive technology book chapter by May and LaPierre
5. Article "GPS where is it taking you?" by Joseph Mendoza
1. Highlights
The highlights section of this document is a quick reminder of key
points to remember about GPS and map accuracy followed by documents
from which this information has been drawn. To minimize frustration
and maximize GPS reception, read and understand the information below.
When first acquiring satellites, find a location that is open to the
horizon on as many sides as possible, with the receiver away from
your body and facing up. Remain stationary if possible.
Once you have acquired satellites, place the GPS receiver on the
sliding shoulder pad with the face of the receiver toward the sky on
top of your shoulder.
Satellites will be acquired most quickly when you have recently
acquired in the same location.
Satellites will take longer to acquire when you have traveled several
hundred miles from your last acquisition location.
You may receive the No Fix message when the battery voltage of the
receiver is low but not flat.
If you cannot get a fix after 10 minutes, return to the main menu,
turn the receiver off and then on again, make sure your battery has a
good charge, position the receiver in the open and load the GPS program again.
Weather has little to no affect on GPS reception.
You need at least 3 satellites to determine a position. When you have
more satellites than 3, the best four signals are used to determine a
position. In other words, 9 satellites are not 3 times as good as 3 satellites.
Direction of travel cannot be reliably determined less than 2 miles
per hour. Signals are poled once a second so brisk walking or
traveling in a vehicle help in providing a good GPS direction of travel.
Some GPS receivers retain the last position fix and some do not. It
is a good practice to turn the receiver off when going indoors so the
last position remembered is a good outdoor location.
GPS is available around the world but at any given moment, the
configuration of the moving satellites can change the accuracy at
your location. You can stand in one place and watch your position
quality continuously change.
Tall buildings do two things to satellite signals. First, they block
and second they scatter the signals, called multi path. If you have
tall buildings on one or two sides, you may still receive a GPS fix.
In mid town Manhattan you may lose a fix in mid block but pick it up
again at the intersection. Walking as far away from the building as
possible near the street is helpful. Signals also bounce around among
tall buildings meaning that you may see 5 satellites but the
reflected bits of signal are not as accurate as direct signals. The
same can apply to signals picked up indoors or by super sensitive receivers.
Be aware that heavily tinted windows or defrost elements within
windows may interfere with GPS reception in vehicles.
GPS receivers pick up fine through hats, back packs and the like.
Street map accuracy varies from place to place. Street addresses are
converted into GPS coordinates based upon a process called Geocoding,
meaning that a computer is projecting where an address should
theoretically be and not where it may actually be located. The same
applies to points of interest. A business's address may often not be
at its front door especially for businesses located in malls.
2. BrailleNote GPS manual, Sections 3 and 6
This section covers some of the concepts, issues and background
information about the Global Positioning System.
3.1 Use Good Judgment.
This product is an excellent navigation aid, but it does not replace
the need for careful orientation and good judgment. Never rely
solely on GPS or any one device for navigating.
3.2 About GPS Satellites.
The U.S. Government operates the Global Positioning System (GPS), and
is solely responsible for the accuracy and maintenance of
GPS. Certain conditions can make the system less accurate.
There are 24 U.S. military satellites in the Global Positioning
System, 12 in each hemisphere, and they circuit the earth twice a
day. Although your receiver may be able to pick up as many as 12
satellites, three or more signals are necessary to determine the GPS
position. Some GPS receivers offer additional accuracy through the
use of a fixed (geostationary) satellite called WAAS, Wide Area
Augmentation System.
It is possible to get a reading of Good with 4 or more satellites or
Poor with 5 or more satellites. A Good reading is better than a Poor
reading no matter the number of satellites. The quality of the
satellite signal determines the quality rating announced to you. You
will only hear Very Good if the extra WAAS satellite is engaged. The
user has no control over the WAAS satellite detection. This feature
will automatically report when it is available from the satellite.
A similar system in Europe called EGNOS will be operational in the
near future and will also be indicated with the Very Good rating. In
the past, the military distorted the GPS signals used by civilians
for national security reasons, a process which is called Selective
Availability. This was turned off in May 2000 but it may be
reinstated if they wish. This hasn't happened as of the writing of
this manual 5 years after they improved the signals.
3.3 Signal Blind Spots.
Since the GPS receiver detects information from satellites orbiting
the earth, the antenna needs to have a relatively unobstructed view
of the sky. Large obstructions such as buildings, cliffs, and
overhangs may interfere with signal reception, reducing accuracy or
eliminating tracking altogether. This is called the "urban canyon
effect." For example, if you were in a city with 50 story buildings
on all sides, the satellite signals would be blocked
completely. However, if there is a tall building on one side only,
you may still be able to receive enough signals from other satellites
to determine a position.
Changing your location by even a few feet can make a difference. For
instance, walking on the outer edge of a sidewalk versus up against
the building may help. Positioning yourself on a corner at an
intersection may also help. There is a better view of satellites
while in a car in the street versus up against a building.
The location of the receiver on your body may also make a difference
to reception. If you are among tall buildings or near an overhang,
you may begin tracking faster if you hold the receiver up and away
from your body. Face away from the building. Once the receiver
begins tracking, it should continue doing so attached to the
BrailleNote shoulder strap. If you need to have it inside a backpack
or enclosed because of rain or snow, the GPS signals should be picked
up through clothing or vinyl materials.
It so happens that among tall buildings in big cities, where GPS is
less reliable, there are more people to ask for directions. Out in
the open where there are fewer people to ask for directions, the GPS
availability and accuracy is best. Isn't this convenient?
3.4 Using GPS indoors or in a Vehicle.
GPS signals cannot usually be picked up in-doors; however you may be
able to pick up signals inside a house with a wooden roof or inside a
bus with a fiberglass roof. You may also be successful in picking up
signals with the receiver in the window of a bus or train. There
should be no problem with the receiver on the dash or window of a
car. (Note: highly tinted and mirrored windows can block satellites.)
Although you can often pick up a signal from a plane, you must get
permission to use the GPS receiver on most commercial airlines. You
may pick up signals from the window of an aircraft but you have a
limited view of the sky because of the small window.
3.5 Picking Up Satellite Signals.
The GPS receiver needs to track at least 3 satellites to determine a
position. Some receivers may provide an approximate position with
less than 3 satellites. Once the receiver acquires a position, the
information is sent to your BrailleNote through the serial cable or
Bluetooth connection. It can take anywhere from 10 seconds to 10
minutes before a position is tracked depending upon how long it has
been since you last turned the receiver on and how clear a view of
the sky you have.
During this acquisition period, you will get the message: no fix,
acquiring satellites. If your GPS receiver is not connected or there
is a problem between the receiver and the BrailleNote, you will see
the message: "Turn on your GPS receiver or press V for Virtual Mode".
In this case, make sure your receiver is turned on and that the data
cable is firmly attached to your BrailleNote. If you still cannot
acquire satellites, see the Getting Started document for your
receiver, which is in the Documents folder of the storage card.
Once you have acquired a GPS position fix and have started moving,
the receiver calculates the change in your position approximately
every second using the satellite signals. The average of your heading
is calculated to minimize the variations in heading due to GPS
fluctuations. In 10 to 30 feet (3 to 10 meters) your direction of
travel is determined and can be announced on the BrailleNote. Until
you begin moving in a consistent direction, the direction you are
facing cannot be determined. So keep in mind which direction you
were heading before you stopped walking. When you stop moving, your
heading should be locked on your most recent direction of
travel. If you spin around in circles, you must begin walking for
30 feet (10 meters) or so before your new direction of travel can be
announced accurately. When you make a turn, walk for several seconds
before asking for a heading. Direction of travel under 2 miles per
hour may be unreliable.
3.6 GPS and map data accuracy.
To maximize the information and navigation benefits of your
BrailleNote GPS, it is important to understand the strengths and
weaknesses of the Global Positioning System. You can improve your
use of the GPS location information by knowing how the system works.
The accuracy scale in the BrailleNote GPS software is geared so you
know how much to trust the information. If BrailleNote GPS says
"Poor, 3 satellites" before giving you your location, consider that
you might really be tens of yards or meters away. Move a little and
try again to double check. Try to get in a more open area so you can
get better accuracy. Bear in mind that you must be tracking
satellites for 30 feet (10 meters), or more before your direction of
travel can be determined. The ratings are: No Fix, Poor, Fair, Good
and Very Good.
6.1 How to Wear the GPS.
For pedestrian travel, the GPS receiver tracks satellites best when
located on your shoulder. You will find a belt clip on a pouch to
secure your GPS receiver onto the BrailleNote strap.
6.2 General Receiver information.
See the GPS receiver manual for details and functions not addressed
in this user guide.
To get started:
Make sure the GPS cable connection to the BrailleNote is secure.
1) Turn the receiver on. See the Getting Started manual to learn
about the receiver controls.
2) Place the receiver in a location open to the sky where you can
begin acquiring GPS satellites. The receiver should have a
relatively unobstructed view of the sky. When you travel to a new
area or if the unit has been off for several days, for instance when
you first use the receiver, it may take longer to determine a position fix.
Once you have turned the BrailleNote on and selected GPS from the
main menu and Start from the GPS menu, you should hear, "GPS
detected". That is the indication that your receiver is connected
properly. If you hear, "Turn on your GPS receiver or press V for
Virtual Mode.", check the steps above and try again. When you turn
the receiver off, wait 5 seconds or so and you will hear the Lose Fix
sound and KeySoft says: "Turn on your GPS receiver or press V for
Virtual Mode." This is one way to make sure that you have turned the
power off on your GPS receiver. You should turn the receiver power
off when not using the GPS program in order to conserve the
receiver's battery power.
6.2.1 Accuracy of GPS Announcements.
First, check your accuracy a couple times by pressing the GPS
Accuracy command, G. If you are tracking satellites when you press
G, you will hear:
"Quality Rating, Tracking X satellites"
Where the quality rating tells you the quality of the GPS connection
and X represents the number of satellites being tracked. If you are
tracking 3 satellites, the accuracy is marginal and is approximately
30 to 90 feet (10 to 30 meters). If you are tracking more than 3
satellites, the receiver uses the best 4 satellites available and
your accuracy may be 30 feet (10 meters) or better. It is not only
the number of satellites that determines position quality but also
the relative position of the satellites to each other. Because of
atmospheric anomalies, there are times when positions are inaccurate
for no obvious reason. If you are tracking less than 3 satellites
when you press G, you will hear:
"No Fix, Acquiring Satellites"
Another way to check how well you are tracking is with the Heading
function in the GPS program. Once you are heading in a given
direction of travel, press the Heading command Dot 5 repeatedly to
ensure that you are getting consistent compass heading information.
Try to move at least 2 miles per hour as slower speeds will result in
inaccurate relative heading information.
See BrailleNote GPS Concepts for more details on satellite and data accuracy.
6.3 Getting Oriented using GPS.
When you first come out of a building or subway, you have not yet
established a GPS direction of travel, and the receiver cannot
determine which direction you are facing until you start
moving. There are a couple of things you can do to get headed in the
proper direction:
You can start walking and get a compass direction from your
BrailleNote. If you do not intuitively know the cardinal compass
directions, having a talking or tactile compass can assist you in
heading directly to your destination as announced by the BrailleNote
GPS. Even if you do not start moving, the absolute position of your
destination as announced on the BrailleNote GPS should be reliable.
If you do not have a route recorded, you can still work your way to
your destination using the "getting warmer" method. Try to get the
destination to be announced ahead of you at the 11 o'clock, 12
o'clock or 1 o'clock positions. Once you get close to the
destination, the announcement starts to move away from the 12 o'clock
heading. When it gets to your 3 o'clock (right) or 9 o'clock (left)
position, it is time to make a 90-degree turn. This does not tell
you if there is a through street, but it gets you in the
vicinity. You also want the destination distance to steadily decline
as you move toward it. If it suddenly increases, double and triple
check the distance so you know you are not getting a bogus
reading. You may very well have veered off track but it is best to
make sure. Never trust the GPS information exclusively. Direction of
travel is not an issue in Virtual mode where your direction is
determined by the automatic route creation.
3. From the GPS-Talk User's list about WAAS
Chip Orange
Since someone asked me off list what was WAAS, here's a quick
explanation:
Take some permanent ground stations with GPS receivers and scatter them
around the area you want to cover (in this case North America). Program
each one of them with their exact latitude and longitude. Now, have
them constantly use their GPS receivers to compute their lat/lon, and
then compare it against what they *know* it to be. It's going to be off
by a little bit because of things like the weather, the ionosphere, etc.
They make adjustments to the mathematical algorithms used by the GPS
receivers so that the results come out to match their known position
exactly. They then send these adjustments to a master WAAS satellite.
It in turn rebroadcasts to all receivers the information from each
station, and says something like:
Hey, receivers in the San Francisco California area, you need to adjust
your calculations right now by x, y, and z.
These slight little adjustments make the difference between 15 meter
accuracy and 3 meter accuracy. It may take the WAAS satellite up to 5
minutes to run through its complete list of ground stations, so that's
why at first, you don't have a WAAS lock until your receiver has heard
its location.
EGNOS will work in a compatible way in Europe.
Unlike the other GPS satellites which constantly circle the Earth, the
master WAAS satellites are in geo synchronous orbit over the equator, so
you have to have a clear view to the south in order to pick up their
signal. If you happen to be on the north side of a mountain, this might
be a problem.
From GPS-Talk User's list, Don Haerr
WAAS is a typical government program that takes forever to implement.
People in the FAA make a career out of a single project. The schedule
says that they will be fully operational in 05, but no one really
believes it.
The reason why there are holes in the Midwest is that individual locales
are responsible for setting up there own surveyed differential GPS
sources to broadcast corrections to the WAAS satellites. This simply
hasn't happened there, although the FAA is talking about mandating it,
using air safety as an excuse.
Three meter accuracy is certainly achievable, but the real problem, as
you point out, is reception. The WAAS satellites are in geosynchronous
orbits and their signals are intended to be received by aircraft that
don't have to worry about ground obstructions. Here in the mountains,
as you can imagine, there are lots of blank spots, but I do know people
who get as little as two meter accuracy on good days when they happen to
get a couple of vehicles that have just been uploaded.
I don't think it's a general solution though. Things will get better
when the new civil signals show up in modernized GPS vehicles, but this
won't start happening until next year and it will be several years
before there are enough vehicles up there to make a difference.
Three new signals are planned. The first step is to turn on the same
code on the L2 frequency that is now broadcast on our L1 receivers.
Using the two frequencies, new receivers will be able to subtract out
the errors caused by the ionosphere which will take the typical ten
meter accuracy down to about five. This could happen on the whole
constellation by next year, if the Air Force gets their act together.
Current vehicles already support this capability, but the ground
doesn't. Pretty dumb, huh? The bad thing is that we'll all have to buy
new receivers to get this capability. The data interface for those
receivers shouldn't change though.
Another L2 Civil Signal (L2CS) is planned for modernized IIR and IIF
vehicles starting next year. This will provide more data to crunch and
will help some, but won't be fully operational for a long time.
For IIF vehicles, planned to be launched by Boeing starting in 06, the
Department of Transportation is designing an L5 frequency civil signal
that could help a whole lot, if they get it right.
We at Lockheed Martin are in the process of convincing the government,
and the current prime contractor, Boeing, to make accuracy improvements
on the ground that are long overdue. The war in Iraq got their
attention. I think we're almost there. Simple, fairly cheap ground
improvements can be made that could give the military an accuracy of
below a half a meter and this would have proportional affects on
Standard Positioning Service users like us. I'm not exactly sure what
we'll see though. When I know more, I'll let you all know.
Don
4. Excerpt from the Draft Chapter for an Adaptive Technology by Mike
May and Charles LaPierre
Chapter 9 Accessible GPS and Related Orientation Technologies
-Principles of GPS systems
What is the Global Positioning System (GPS)?
The Global Positioning System (GPS) is a worldwide radio-navigation
system formed from a constellation of 24 satellites and their ground
stations. (See figure 9.1) The Satellites orbit the earth at 10,900
miles in space, weigh 1900 lbs., are 17 ft in length, and last for
about 7.5 years before they need to be replaced. (See figure
9.2) The Ground Stations (also known as the "Control Segment")
check to make sure that the GPS satellites are functioning properly
and to keep track of their exact position in space. If there are any
discrepancies between the satellites and the ground stations, the
master ground station will transmit the corrections to the satellites
themselves. This will ensure that the GPS receivers have the correct
data from the satellites. (Trimble Navigation Limited, 2001)
The idea behind GPS is to use satellites in space as reference points
for locations here on earth. Picture yourself as a point on earth
with the satellites circling above you in space. The GPS software
measures the distance from the satellites to your GPS receiver. This
is calculated by measuring the time it takes for the signal to get
from the satellite to the receiver. By gathering data from at least
three satellites (also called trilateration, see figure 9.3), the GPS
receiver is able to calculate your position on earth and sends the
latitude and longitude coordinates to whatever device (including a
computer) that can make use of them. Latitude lines run horizontally
across the globe and Longitude run vertically. Every square foot
on the planet has a latitude and longitude and the GPS means this
intersecting point can be given a meaningful name. (See figure 9.4)
Accuracy of GPS
On average, commercial GPS receivers are accurate within 30
feet. So, instead of picturing your GPS position as a pinpoint,
picture it as a 30-foot bubble around your position. (See figure
9.5) However, there are various factors that can make the satellite
information more or less accurate.
For a GPS receiver to work properly, it needs to have a clear view of
the satellites. That means that GPS receivers do not work in places
where all the satellites can be blocked, such as indoors, in tunnels,
in subways, etc. You might also have poor GPS reception on streets
in big cities when some satellites are screened by skyscrapers (also
called urban canyons), areas surrounded by very tall mountains, etc.
Another situation where a GPS receiver might not be as accurate would
be when all the satellites are coming from the same direction. Due
to the vast distance between the satellites and the GPS receiver, the
satellites need different angles to decipher the precise location of
the receiver. (See figure 9.6) So, the receiver might be out in an
open valley, but if all the satellites are right above it, the
accuracy will diminish. (Dana, Peter H., 2001)
One other factor that could influence GPS accuracy is the need for
national security. The Department of Defense (DOD) developed GPS for
military purposes. Since this technology is available worldwide,
anyone can use GPS for location information. Originally, the
government implemented a system called Selective Availability (SA) to
intentionally scramble the signals from the satellites. This caused
inaccuracies of 100 meters. As of May 2, 2000, U.S. President
Clinton turned off SA but this decision could be reversed at any
time, (National Geodetic Survey), unlikely as the proliferation of
civilian applications permeate our society.
There are other factors that could affect GPS accuracy, but for
simplicity sake, we will stick to these three main factors mentioned above.
Two new satellite systems are being implemented to augment the
accuracy of GPS. WAAS, Wide Area Augmentation System, and EGNOS are
geosynchronous satellites, which work in conjunction with ground
stations to correct for some of the 30-foot average error in the GPS
system. These systems offer up to 3-meter accuracy when the signals
can be picked up. Unlike GPS, WAAS cannot always be picked up, even
when one is in the open. EGNOS is not operational as of this writing.
Both systems are slated to increase coverage as more stations are installed.
Additionally, a European satellite version of GPS called Galileo is
slated to be operational by 2009. This all adds up to better
accuracy, availability and reliability of global navigation systems.
A new term in fact is emerging to refer to these multiple systems,
GNSS, Global Navigation Satellite Systems.
-Application of GPS principles
From the above section we learned about the basics of how GPS
works. How do complex numbers and distance calculations turn into
helpful navigational information?
In GPS language, the latitude, 40 degrees 44 minutes 52.4 seconds
North, and longitude, 73 degrees 59 minutes 6.12 seconds West, means
that this point is currently 10,900 miles away from satellite A,
11,000 miles from satellite B, and 11,250 miles from satellite C.
Does that really help one navigate? Does one have a better idea of
where they are going with that information alone? Probably
not. However, with a GPS application and associated geographic
databases, that latitude and longitude means that point is the Empire
State Building in New York. Now that there is a name attached to
those coordinates, you know exactly where that point is located.
Over the past few years, the GPS commercial market has
exploded. People are using GPS in rental cars, on hiking trips, and
in many other recreational activities. Companies have been formed
whose business it is to put names on latitude and longitude
coordinates. These companies have built massive databases with
street names, addresses, business names, points of interest,
restaurants, underwater wrecks, and the list goes on. Anything,
which is stationary, is likely to be labelled. All of this extensive
data has been put into electronic form so that it can be converted
into various computer formats.
With this boon in electronic data, blind people no longer need be
limited to the 1% location information they could previously glean
from sighted people. The maps and points of interest are no longer
just a drawing on an inaccessible print map. This electronic data
can be converted into programs that work on computers designed for
blind or visually impaired individuals.
The blind traveller can now be a co-pilot in a car, not just a
passive passenger. They can keep the taxi driver honest; they can
enjoy hearing about the sites and businesses being passed while in a
car, bus, or even on foot. There is nothing more empowering for a
blind person than getting around effectively and location information
makes this much more possible for a lot more people.
5. Article about GPS, "GPS where is it taking you?"
http://www.GISdevelopment.net ---> GITA 2002 ---> Mobile - Taking it
to the street
GPS where is it taking you?
Joseph Mendoza
CDS/Muery Services
3411 Magic Dr.
San Antonio, TX 78229 U.S.A.
Abstract
Global Positioning System technology is moving at a breakneck pace.
As a result, it seems that everyone has a GPS receiver mounted to the
car dashboard, hiking backpack- and even the handlebar of the
paperboy's bicycle. As the cost-to-accuracy gap of GPS closes, what
impact will this have on industry grade equipment, methodologies, and cost?
This paper will address how GPS works and explain industry
terminology (geekspeak) such as PDOP and multi-pathing. It will
explain how accuracy levels are determined and the methodologies used
to achieve these, such as differential GPS versus corrected signal
post-processing applications.
Additionally, this paper will discuss how the latest advances in GPS
technology impact field data collection, outlining how GPS works and
the latest technology trends, as well as discussing when and if GPS
is appropriate. Further, a comparison of GPS methodologies and
accuracy targets will be included to best evaluate what method will
meet the requirements of the end-user without breaking the budget.
In short this paper will take you on a whirlwind tour of the GPS
world at its current state and what the not-too-distant future will
hold for you and your paperboy.
Introduction
Ever since the first man drew lines in the ground to represent
rivers, mountains and lakes, man has been intent on referencing
himself to the physical world he lives in. In time, the need, or
perhaps more to the point, the desire, to more accurately reference
himself to the physical world has driven methods and technology to
new levels. With the advent of the Global Positioning System (GPS),
man now has an accurate and powerful tool at his fingertips.
GPS products now seem to be in use everywhere in a variety of
applications as diverse as engineering and surveying to the leisure
activities of the weekend backpacker and fisherman. The GPS market is
certainly a growing and vast arena that seems to come up with new and
varied applications all the time. This prompts the question: where is
GPS taking us and how will it affect you? Before addressing that
question, let's look at the history and development of GPS. Note: In
an effort to succinctly explain GPS, many scientific details have
been left out of this paper. What remains is a general overview with
the aim to introduce you to the exciting and dare I say cool world of GPS.
Brief history, pre-space to satellites
As early as 1903, the Russian scientist Konstantin Tsiolkovskiy had
proven mathematically the feasibility of using the reactive force
that lifts a rocket to eject a vehicle into space above the pull of
the earth's gravity. Twenty years later, Romanian-born Hermann Oberth
had independently worked out similar formulas. Neither man built a
usable rocket to demonstrate the validity of his theories, nor had
they so much as mentioned an unmanned artificial satellite.
However the groundwork had been laid for just such work. (Green,
Constance McLaughlin, and Milton Lomask, 1970)
An American by the name of Robert Goddard had a similar vision, and
while engaging in postgraduate work at Princeton University before
World War I, Goddard demonstrated in the laboratory that rocketry
propulsion would function in a vacuum. By 1918 Goddard had
successfully developed a solid-fuel ballistic rocket, and by 1926 had
successfully launched a rocket propelled by gasoline and liquid
oxygen. In 1937 he launched a rocket that reached an altitude of
9,000 feet. Goddard was making great progress, but his work was not
followed except by a small community of rocket enthusiasts. (Green,
Constance McLaughlin, and Milton Lomask, 1970)
In 1943, the Nazi "buzz" bombs and the supersonic "Vengeance" missile
the "V-2s" that rained on London during 1944 and early 1945
awakened the entire world to the use of rockets as weapons. Soon a
good many physicists and military men began to study the work of
Robert Goddard with attention. (Green, Constance McLaughlin, and
Milton Lomask, 1970)
With the practical use of rockets now established, the post-war
nations of the United States and the Soviet Union soon turned their
attention to two fronts in rocketry: the development of
intercontinental ballistic missiles and the development of a rocket
capable of launching a satellite into space.
The Space Age
The Soviet Union launched the first man-made space vehicle, Sputnik
(meaning "space companion"), in 1957, and thereby launched us all
into the space age along with it. On January 31, 1958, the United
States followed suit with the launch of Vanguard 1. (Green, Constance
McLaughlin, and Milton Lomask, 1970)
Prior to the launch of the Russian satellite, scientists had
experimented with bouncing radio waves off of the moon. Therefore,
they were eager to study and experiment with a manmade satellite and
test their theories concerning tracking via radio waves. By studying
the orbit of Sputnik, scientists discovered that it could indeed be
tracked by its radio signal. This led to the concept that man could
also determine his position on the earth by reading the signal from a
satellite or space vehicle given that the precise orbit of the
satellite was known. Several programs then began to be discussed in
serious terms.
The U. S. Navy's NRL Naval Center for Space Technology (NCST)
conceived of the TIMATION (TIMe/navigATION) program in 1964. This
program was designed to provide the basis for a navigation system
with three-dimensional coverage (longitude, latitude, and altitude)
throughout the world. (U.S. Navy, Naval Research
Laboratory, 2001)
In 1973 the TIMATION program was merged with the Air Force's 621B
program to form the Navigation Signal Timing and Ranging Global
Positioning System or
NAVSTAR GPS program (originally named the Navigation Technology
program). The NAVSTAR GPS program is funded and controlled by the
U.S. Department of Defense
(DOD), and its primary function is to support military operations
throughout the world. The (NAVSTAR) GPS program was officially
declared fully operational
July 17, 1995. (U.S. Navy, Naval Research Laboratory, 2001)
The three major segments of GPS - Space, Control, and User
The Space Segment
An original constellation of 24 satellites in six orbital planes
(four in each plane) are used to send coded satellite signals that
can be processed in a GPS receiver, enabling the receiver to compute
position, velocity and time. The satellites are spaced 60 degrees
apart and are positioned at an altitude of 20,200 km (12,552 miles)
with a 55-degree inclination. In addition to the 24 satellites in the
constellation, three additional satellites are in orbit and will
eventually replace older space vehicles. (Dana, Peter H., 2001)
Control
The Control segment consists of five Monitor Stations (located in
Hawaii, Kwajalein, Ascension Island, Diego Garcia, and Colorado
Springs), three Ground Antennas, (located at Ascension Island, Diego
Garcia, and Kwajalein), and a Master Control Station (MCS) located at
Schriever AFB in Colorado. The monitor stations passively track all
satellites in view, accumulating ranging data. This data is processed
at the MCS and incorporated into satellite orbital models.
The updated orbital information, also called ephemeris data, is then
transmitted to each satellite via the Ground Antennas and is sent
with each satellite's navigation message. (NASA Jet Propulsion
Laboratory, 2001)
User Segment
The GPS User Segment consists of the GPS receivers and us, the user
community. The 24 satellites in their respective orbits provide the
user with five to eight satellites visible any where on the earth to
receive data. Satellite signal arrival times from at least four
satellites are processed to estimate four quantities, position in
three dimensions (X, Y, and Z) and GPS time (T). The receiver then
computes position dimensions in Earth-Centered, Earth-Fixed X, Y, Z
(ECEF XYZ) coordinates. (Dana, Peter H., 2001)
The computations are based on simple principles of velocity x travel
time = distance, which is somewhat like the old school math problem
"If a train leaves Chicago traveling at speed of 30 miles an hour and
travels for two hours, how far did it go?" (Trimble Navigation
Limited, 2001) The signals are then processed, not unlike
triangulation. One factor that complicates the situation is that the
signals are traveling at the speed of light. So the difference
between the arrival times of the signals is minute. Since the arrival
times of the satellite signals are such a critical factor in
calculating a position, each satellite is equipped with four atomic
clocks, two cesium and two rubidium. Satellite clocks are monitored
by Ground Control Stations and occasionally reset to maintain time to
within one-millisecond of GPS time. This information is then
transmitted to each satellite via the Ground Antennas and is sent
with each satellite's navigation message along with its ephemeris
data. (Dana, Peter H., 2001)
The Signal
The satellites transmit information via two radio waves that can be
picked up by GPS receivers. Each radio wave is modulated so that it
can carry specific information. The modulated signal resembles random
electrical noise, but since the signal is not random but coded (and
therefore follows a pattern), it is referred to as a pseudorandom
code. The radio waves, which carry the pseudorandom codes, are
distinguished by the designations of L1 and L2, and each carries
different information in its modulated code. L1 carries the Coarse
Acquisition (C/A) Code used by the civilian sector (free of charge)
and is also modulated to carry the Navigation Message and other
satellite system parameter information. L2 carries the Precise Code
(P-Code) used by the military.
The P-Code is encrypted and can only be received by specific
receivers equipped with key codes used to decipher the signal. (Dana,
Peter H., 2001)
The pseudorandom code for each satellite is distinct, which makes it
easy for GPS receivers to distinguish between one satellite and
another. In this way GPS receivers can tell exactly which satellites
make up a given configuration. This is important since the signals
are very weak. So a GPS receiver identifies one signal and, using
built in almanacs, actually searches for signals from the other
satellites it thinks should be in the configuration. Once it has
identified all of the satellites in the configuration it then begins
tracking their signals.
The GPS receiver then mimics or mirrors the pseudorandom code for
each of the satellites and compares the differences between its own
code and the one received.
It is able to do this because it knows the fluctuations in the
pseudorandom code. It then matches up one known point in the signal
received and its own and begins to make calculations.
To illustrate how this works, imagine that a satellite was playing
Iron Butterfly's "In-A-Gadda-Da- Vida" from space. At the exact same
time, you are sitting in a lounge chair also listening to
"In-A-Gadda-Da-Vida". As you listened to the version you are playing
against the one from space, you would notice that the version from
space was delayed slightly. This is because it takes some time to
travel the distance from the satellite in space to your lazy-boy. To
determine the distance, you could slow your version to match the one
from space (which is hard to do on a 45rpm) until they were
synchronized. Since the time shift between the two versions of
"In-A-Gadda- Da-Vida" is equal to the travel time of the satellites
version, we simply take the time shift between versions and multiply
it by the speed of light and presto, we determine the distance
traveled! (Trimble Navigation Limited, 2001) This same calculation is
then made for each signal received and used to pinpoint a location.
A word about Accuracy
Several factors will affect the accuracy of your readings. Visibility
or line of sight is crucial, since at least four satellites are
needed to accurately locate your position. Buildings, mountains and
even tree canopy can affect how many satellites you are "seeing" and
may prevent you from "seeing" enough satellites to use in deciphering
your location.
Since the pseudorandom signal sent by satellites resembles electrical
noise, receivers at times actually have trouble distinguishing the
signal from "true noise" in space caused by solar flares or other
naturally occurring events. Good receivers are better equipped to
decipher the noise and filter it out. Nevertheless it is a distorting
factor. (Dana, Peter H., 2001)
An event known as multi-pathing may give you false readings via
signal reflection. Multi-pathing occurs when a nearby object or
surface is reflecting or bouncing the satellite signal to your
receiver, making it think that it is the true line of sight reading.
The reflected signal is received and is computed as a real signal and
causes an effect similar to the ghosting of your TV screen. This
occurrence is not likely to happen on the open sea but may be
experienced in city locations where there are many surfaces.
Multi-pathing can be hard to detect or even avoid, so good receivers
are equipped to try and detect and then reject the reflected signal
when multi-pathing occurs.
Variances in the atmosphere may also cause distortions in your
readings. Changes in temperature, pressure, and humidity affect the
troposphere or the lower part of the atmosphere, which can delay
readings. Imagine a glass of water with a spoon sitting in it. The
portion of the spoon in the water appears distorted in relation to
the portion out of the water. This is because light is slowed ever so
slightly as it travels through the water distorting the image. This
same effect occurs as the signal sent by a satellite is slowed as it
passes through the water vapor in the air. In addition, delays can
occur in the ionosphere, which consists of charged air particles 50
to 500 km in the atmosphere. These delays, although slight, are
significant enough to effect calculating a good fixed coordinate.
A term that you may hear in reference to GPS accuracy is GDOP or
Geometric Precision of Dilution. GDOP is made up of other components
such as PDOP (Position
Dilution of Precision), HDOP (Horizontal Dilution of Precision), VDOP
(Vertical Dilution of Precision), and TDOP (Time Dilution of
Precision). Even though each of these can be calculated
independently, they all make for a good or bad GDOP reading. (Dana,
Peter H., 2001)
To illustrate, you may be receiving signals from four satellites that
all happen to be right on top of you or all in a straight line in
front of you on the horizon. (This configuration is not possible but
is used as an example to exaggerate the point). Therefore, even
though you have four readings, the configuration of the satellites
does not allow enough variance between the angles of the readings to
obtain a good PDOP, which would require a better spread in the
satellite configuration. Good receivers will automatically pick out
the best satellites from a given constellation makeup to give you the
best PDOP, which will make for a good GDOP.
Another factor that cannot be overlooked is Dick Clark's Law of Goofs
and Blunders. This law states that not even GPS is immune to the
occasional software glitch, hardware malfunction or good ole'
operator error, which might make for good video clips but bollixes
your GPS data. In a perfect world, coordinates can be fixed from just
three satellites. This would give us all the needed data to calculate
accurately any given position on the earth. As we have just
discussed, however, there are many factors that can delay, distort or
mirror signals received giving our GPS units fits trying to filter
out erroneous data. That is why signals from four satellites are
used. The extra measurement helps our receivers to verify against the
fourth signal how well it is computing our position. This increases
our chances of obtaining an accurate measurement in fixing our position.
Differential GPS
The purpose of Differential GPS is to correct errors that may creep
in due to numerous factors. It accomplishes this by taking satellite
readings at a known fixed location. It then takes where the satellite
tells it that it is located and compares that against where it knows
it is located and computes an error calculation. That data is then
passed onto the roving (or differential) receiver and used to correct
for errors. This type of DGPS is known as "real time" DGPS and
requires that the receiver be outfitted to receive and process this
information. How well this process works is dependent on the quality
of the receivers and the distance between the two points. The range
can be anywhere between 30 to 200 kms. The concept is predicated on
the fact that (to the degree possible) the atmospheric conditions are
alike at each location.
In instances where "real time" is not a critical factor, the fixed
location readings are collected and processed by computer against the
points taken by the roving receiver at a later time. This is known as
"post-processing" and can only be done if the fixed location is
taking readings at the exact time that the rover is also taking
readings. If a field technician is taking readings and the fixed
station happens to be down during that time, all of the readings
taken will be useless since there is no fixed location information to
be processed against the collected points.
The user and his uses
Military
Support of military operations is the first and primary function of
GPS as funded by the DOD. At one time, the DOD degraded GPS signals
received by non-military personnel, thereby reducing accuracy. This
effort was known as Selective Availability (SA) or anti-spoofing. The
primary intent behind the implementation of SA was the belief that
the signals could be easily used by enemy states against the US.
(U.S. Navy, Naval Research Laboratory, 2001) However, on May 1, 2000,
the President of the United States disabled SA with the intent of
never using it again. (Interagency GPS Executive Board (IGEB) 2001)
Of course, if circumstances warrant, it is conceivable that SA could
be reactivated.
Commercial
The commercial sector is laden with GPS applications, and GPS
receivers can be found in fire engines, police cars, airplanes,
helicopters, boats, construction equipment, movie cameras, fleet
vehicles, and farm equipment, just to name a few, and are used for
surveying, construction, mapping, agriculture, vehicle theft
tracking, digital information transfer security, environmental,
archeological and other uses. Our transportation infrastructure, for
instance, is heavily dependent upon GPS for tracking and navigation
of airplanes, marine traffic, and land vehicles such as trains.
Commercial Accuracy Targets
The accuracy levels of commercial applications vary, although the
disabling of SA has helped by making higher accuracy levels more
obtainable in general.
In comparison, the tracking of large ships in the ocean does not
warrant the same accuracy as does surveying in pins and markers for
an interstate bridge with costly pre-cast concrete sections.
Therefore, accuracy is dependent upon use, and there is a direct
correlation between level of accuracy and price.
The greater the accuracy required, the more expensive the equipment,
and also the more expensive the training and maintenance cost. Some
commercial receivers can be purchased for a couple of thousand
dollars with an accuracy from one to three meters (dependent upon
whether the points are using correction data), while survey grade
equipment may run $5,000 to $40,000 per receiver (and generally two
are required) and have an accuracy of just a few centimeters for DGPS
and less than a centimeter in static mode. (Dana, Peter H., 2001)
Survey grade equipment can achieve this type of accuracy by a method
called Carrier Phase Tracking. In the "In-A-Gadda-Da-Vida"
illustration earlier, we discussed how the pseudorandom code signal
from the satellite is matched to the code generated by the receiver
to determine time traveled and calculate distance. By using the
pseudorandom code in this manner, an accuracy of a few meters can be
achieved. By use of pseudo random code correction DGPS methods
(mapping grade), an accuracy of +-1 meter can be achieved. Survey
grade GPS projects usually carry an accuracy requirement of better
than a few centimeters.
In order to accomplish this high accuracy, two receivers are used in
the same manner as DGPS. But rather than comparing the pseudorandom
code, surveyors compare the carrier (radio) wave itself. This results
in higher accuracy since the carrier wave oscillates at a higher (or
faster) frequency than the pseudorandom code. This means that in the
same time frame, there are more points to match against the carrier
wave and less dead time between those points. By means of this
process, a higher accuracy can be achieved. This method is now
commonly used along with other standard survey methods for the
creation of landbase features with a high accuracy, such as those
used by taxation offices, city planning departments and other high
accuracy uses. (Dana, Peter H., 2001)
GPS and GIS
GIS data creation methods and software have significantly improved
over the last ten years. Years ago, the standard method of data input
and creation was a map or orthophoto registered to a digitizing
table. Today, data is often geo-referenced to a vector land base or
digital ortho, a method generally referred to as heads up digitizing.
The trend now is to incorporate GPS data collected by field crews
into the GIS system, which is facilitated by enhancements to GIS
software that make integration of GPS data much easier. This means
that many features are created in the field by technicians equipped
with GPS units. This data is then sent to in-house technicians and
made ready for delivery. This type of field data collection is used
to collect various types of data such as highway signage, electrical
structures, telephony equipment sites, and manholes. For capturing
features with limited attributes, a data cap is used to store the GPS
point and other related attributes. For features with a more robust
attribute requirement, a computer pen-based unit is coupled with the
GPS receiver and used to capture features and their associated
attributed information. This latter method has the advantage that
once a point is captured (assuming that you are using DGPS), it can
be viewed with its associated land base backdrop for reference. The
drawback is that a computer pen-based unit increases the cost to
equip each technician by several thousand dollars.
The additional cost that GPS equipment places on field data
collection can make it a cost prohibitive endeavor. In addition to
training costs and time, one also needs to consider the added effort
that GPS places on a field technician resulting in reduced
productivity, which may have an impact on the overall project schedule.
To Use GPS or Not?
Aside from these factors, which may be prohibitive, not all field
data collection projects necessarily need to collect GPS data. Some
companies simply do not have a need to place features with such
precision. In truth, the addition of precisely located data may
jumble an already working system. For instance, a company may be
using a vector land base built upon USGS quad maps with a relative
accuracy of +-50 feet (or an off-the-shelf land base) that has been
modified over time, making it difficult to upgrade to a more accurate
land base. The placement of highly accurate features into this system
may be confusing and difficult to work with. For instance, let's say
that a cable company has installed a new pedestal, and a real world
coordinate has been captured for the new feature. This pedestal is
now added to the GIS system and placed according to its real world
coordinate. The inconsistency of the land to the GPS-located pedestal
may make it appear that the pedestal is on the opposite side of the
street, or it may place it in an adjacent lot to the one in which it
actually resides. The confusion caused by its seemingly improper
placement may make it difficult to work with. In a situation like
this, the decision may be not to use GPS at all.
How Accurate it Accurate Enough?
When used, the requirements of GPS-collected data for a GIS system
vary. Generally, unless you are surveying, sub-meter and centimeter
accuracy is not warranted.
In most cases, one to three meters will suffice for most GIS
applications today. For instance, field inventory of streetlight
poles could easily be accomplished with an accuracy of one to three
meters, depending on the accuracy of the land base. It stands to
reason that a GPS point representing a streetlight can be navigated
to within a few meters and then should be visible to the technician
trying to locate it. Another factor to keep in mind is that the
symbology used in a GIS system is usually substantially larger in
scale than what is being represented and can change depending on the
scale viewed. Many streetlight and utility poles are modeled with a
symbol that is ten feet in diameter at its largest scale.
Additionally, a precise absolute point may look good in GIS, but a
series of precise absolute points may not. Therefore, precise GPS
data may be moved in a GIS for cartographic reasons, which erodes the
justification for a high accuracy reading in the first place.
Practically speaking, the occupation of a pole site for GPS purposes
is difficult at best. To truly occupy a point for a pole, you must be
directly above the site (which is very difficult to do with a 30'
pole). So you must stand next to (or rather "near" in order to get a
good GDOP) a pole to get a reading. This process in itself introduces
an error of a couple of feet, which can then be exacerbated if a QC
technician validating readings follows behind the next day and ends
up on the other side of the pole since a better GDOP reading is being
received there. Now there is a discrepancy of a meter and a half,
just to start with?All of which makes it seem that a one-meter
requirement may not be worth the trouble.
Regardless, GPS data integration into GIS systems is not a passing
fad. As land base and digital orthophoto precision increases, we will
see GPS data of increasing accuracies continue to be used in these
systems. To think that 1-3 meters accuracy is good enough is not
taking into account what history has taught us. Just a few years ago
+-30 meters seemed good enough, followed by +-10 meters then +-5
meters, and now we find this is not good enough. Since GPS accuracy
for GIS is based upon the final or intended use of the GIS
information, we will continue to see accuracies increase, as we
expect more and more from our GIS systems.
Recreational
The recreational GPS user community is growing with new services and
products being offered every day. Some of the common users are
backpackers, fishermen, hunters and outdoorsmen all of who are now
viewing a GPS receiver as standard equipment. Some vehicles now offer
GPS tracking capabilities in conjunction with onboard digital maps
that pinpoint your location and provide directions to your
destination. A newer service being offered combines cell phone and
GPS technology. This service tracks your vehicle so you can call for
directions to your destination even if you don't know where you
happen to be at the moment. It can also detect if your airbags have
been deployed or can dispatch emergency personnel as needed. If your
vehicle is stolen, the service can aid police in locating your vehicle quickly.
What is the future?
The use of GPS is limited only by the imagination of man himself.
Integration with existing devices is inevitable; perhaps one day soon
all cars will come standard with GPS receivers. Who knows, one day we
may have GPS nano-technology that you inject in your teenager, so you
know exactly where your kid is when it's ten o'clock. Integration of
GPS receivers into PDAs and laptops is already occurring.
It is a given that accuracy will continue to increase and prices will
continue to fall, at least for the recreational user. Industry grade
GPS units will continue to command a high price since these units are
usually cutting edge. If technology has taught us anything, it is
that if you want to be first, you are going to have to pay for it.
We can be certain that GPS technology will continue to work its way
into the average Joe's life. Which brings us to our initial question.
Where is GPS taking us? Your guess is as good as anyone's, but when
it comes to GPS one thing is for sure when we get there we'll know
exactly where we are!
References
List of 6 items
? Dana, Peter H., 2001, Global Positioning Systems Overview,
<http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html>http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html
? Green, Constance McLaughlin, and Milton Lomask. 1970, Vanguard: A
History. NASA SP-4202; rep. ed. Smithsonian Institution Press, 1971:
<http://www.hq.nasa.gov/office/pao/History/SP-4202/chapter1.html>www.hq.nasa.gov/office/pao/History/SP-4202/chapter1.html
? Interagency GPS Executive Board (IGEB) 2001:
<http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html>http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html
? NASA Jet Propulsion Laboratory Mission and Spacecraft Library 2001:
<http://msl.jpl.nasa.gov/Programs/gps.html>http://msl.jpl.nasa.gov/Programs/gps.html
? Trimble Navigation Limited, 2001
<http://www.trimble.com/>http://www.trimble.com
? U.S. Navy, Naval Research Laboratory 2001 - Spacecraft Engineering
Department (SED):
<http://code8200.nrl.navy.mil/nts.html>http://code8200.nrl.navy.mil/nts.html,
http://tycho.usno.navy.mil/gpsinfo.html#gpssa
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