Chapter
1
What is GPS?
The
History of GPS
GPS his its roots, ultimately, in the struggle of humans to correctly
identify where we have been, are, and will be. The result of this struggle was
to begin building maps. The earliest maps were simple mental pictures of the
terrain around a group’s camp or hunting area. The first maps produced date to
more than 5,000 years ago, and are found on clay tablets from the middle east.
The Greeks and Romans began surveying for roads and new settlements using
methods that differ little up to last century.
Then, in the 18th century, the French began using triangulation to create
maps of their country. Later, this was combined with trilateration to map entire
continents and were referenced to stars. These maps, were often hundreds of
meters off even for the most painstaking surveys conducted.
These problems of using line of sight surveying techniques were first
addressed during and after WWII. The idea of using radio waves to locate and
track ships was introduced in an attempt to coordinate forces during the war
effort. This gave birth to the precursors of GPS, the HIRAN, OMEGA and LORAN
systems, each of which had major problems. The main problem was that each of
these systems were ground base, and to be used successfully, one needed direct
line of sight with a transmitter (OMEGA does provide global coverage but with
low accuracy and LORAN has accuracy of 450m but only covers 10% of the planet).
The next step was to take the above ideas into space. The first space
based navigation system was called TRANSIT, it is still in use today. However,
this system used Doppler shift to calculate position, and accuracy was only
attainable if the receiver sat in one position for a couple of hours, and thus
was only good for large-scale navigation.
TRANSIT had some majors problems that GPS was specifically designed to
overcome. These included the large gaps in coverage time due to TRANSIT only
having 6 satellites in near circular polar orbits, which meant only a few
minutes of coverage for every 90 minutes. Also, this earlier system did not give
accurate measurements when the receiver only sat still for a few minutes, it
needed much longer to calculate positions.
Today, GPS is a fully functioning worldwide system used for everything
from real-time navigation to mapping your hometown! To further appreciate the
time and resources that built this system, let’s take a look at the
specifications.
Actually, today’s GPS system grew out of two US military projects run
during the 1970s. These resulted in the first generation of satellites being
launched during the late 70s. The 1980s saw another wave of satellites being
built and the US government guaranteeing the ability of civilian use. The system
reached Initial Operation Capability (IOC) in 1993 with 24 first generation
(Block I) satellites deployed, and in 1995 reached Full Operation Capability
(FOC) with 24 second generation (Block II) satellites deployed.
So, what exactly is GPS?
Many of you have probably asked or been asked this question more than
once. Well, we are going to tell you in simple terms. GPS is:
-a space based
positioning system used for navigation, tracking and mapping
-it is designed and maintained by the US Military
-truly global in scope
-available every hour of every day all year long, and is cheap to use!
Now that we have a basic idea of what GPS is, and why it was developed,
let’s look at how it actually works. Basically, what are the actual components
(human and machine) that allow this amazing utility to do what it does.
The
Science of GPS
First, the information that GPS gives us is a unique set of coordinates
for a point or feature (these features could be anything from roads to habitat
boundaries). These coordinates are usually referred to as GPS addressing. That
means, that it is possible for any point on the planet to have a unique address,
a unique set of coordinates that is accurate enough to be used again and again
in the future. The possibilities are endless!
The process that GPS uses to give us coordinates can be broken down into 5 basic
parts, they are:
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1. Receiver uses triangulation based on signals from the satellites to
figure positions.
2. To do this, the GPS measures the distance from satellites by measuring
travel time of a radio message.
3. To
measure the travel time, the satellites use nuclear clocks to accurately
measure time.
4. Then, the travel time is used to calculate distance from the receiver,
which gives the satellites position in space.
5. Techniques to compensate for time delay while radio signal travels
through the atmosphere. |
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We will now examine this process in more detail.
Satellite Ranging
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The
basic idea behind GPS is to measure our distance from a group of
satellites in space, this is accomplished by simply measuring the amount
of time that it takes for a radio signal to reach us. There are sizable
complications in this system to work out, mainly compensating for errors
caused by travel of the signals through the atmosphere, but we will
explore this later.
So, we know that we are 10,000km from a satellite. That means that
we are somewhere on the surface of an imaginary sphere with a diameter of
10,000km surrounding the satellite. Now the receiver figures its distance
from another satellite, and the resulting sphere will form a circle where
it intersects the other sphere. We can do this with a third circle and it
will narrow down the number of intersections to two points. These two
points are technically all we need to know to accurately figure our place
on the surface of the planet as only one of the points will be on the
surface while the other point will be either out in space or outside of
our current region (pre-set into the GPS unit when it is initialized).
However, most receivers will just use the point formed from a forth
intersecting sphere to give us one point, and since trigonometry tells us
that we need four ranges to unambiguously locate ourselves in three
dimensions, it is best to take readings with that many satellites.
That’s all, the receiver uses satellites as reference points for
triangulating position. Now we can examine some of the problems that make
this easy to understand process more complicated in the real world. |
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Measuring
Distance to a Satellite
You are already familiar with the concept behind this aspect of GPS,
it’s the simple time x velocity = distance equation we all learned in primary
school. Now, there are two main problems with this; we need very accurate
clocks, and a way to compensate for the distortion caused by the radio signals
traveling through the atmosphere.
First, the problem of accurate clocks has been solved by the advent of
other technologies (like your quartz wristwatch). The signal from a satellite
directly overhead will reach a receiver in about 6/100ths of a second, but
luckily the Trimble GPS Receivers that you use are able to measure time to a
nanosecond (.000000001 of a second). However, this still doesn’t completely
eliminate the possibilities of error. After all, if the satellite and receiver
were out of sync by only 1/10th of a second, our distance measurement would be
off by 1,860 miles! The clocks on the satellites are accurate, each satellite
has four $NZ200,000 atomic clocks onboard. Unfortunately, that puts the
availability of having an atomic clock in our handheld units a little out of
most people’s reach.
There is a solution to the problem, and this is where the trigonometry
comes in. When we add a forth satellite, the use of a simple trigonometric
equation will eliminate any possible clock error that occurs, and each receiver
that we use has this equation hardwired into it!
So, now that we know we can measure the distance accurately, we need to
know where the satellite was when it sent the signal. This is accomplished by a
complicated “psuedo-random” code that is generated by both the receiver and
satellites. It is the same idea as if you stood at one end of a field and a
friend stood at the other end, and then you both started counting. When you said
“three,” you might only then hear your friend say “one.” This way, you
could tell how long ago the number was said (or in the case of GPS, the signal
left the satellite), and use that to calculate the time difference, and use that
to calculate the exact position. This is combined with another code that is
constantly monitored by the US Department of Defense which has true location
information encoded into it.
As you can see, the idea is simple, but the implementation of these
theories is a bit complex. However, they have been well thought out and
completely overcome, making GPS a wonderfully accurate utility for us to use in
measuring distance.
Atmospheric
Disturbances to the Signals
The radio signals that are transmited by the satellites are not extremely
powerful, for instance, they cannot penetrate buildings like FM or AM signals.
Also, the waves slow down ever so slightly when they pass through the
atmosphere, which means that our distance equation (time x velocity = distance)
may not accurately reveal the true distance.
Now, we could try to predict the average delay caused by the changing
atmosphere, but each day is obviously not average. The other way is to use a
very sophisticated GPS unit that can compare the signals from two satellites to
calculate the variation caused by the upper atmosphere (the ionosphere), and
some of the units that you use may very well have this capability.
The resulting variation caused when the signal passes through weather is
harder to correct, but it usually causes less that 2-3 meters of error. That
amount of error is fine for mapping large areas (where is a city at) or general
points (where is a car park). However, this amount of error is unsatisfactory
for many types of data collection, and can be corrected (this will be covered in
the DGPS chapter later).
Angles
Matter
Another aspect of the GPS system that can cause errors is the angles
of the satellites in relation to the horizon.
The lower the angle, the more atmosphere the signal has to travel through,
and the worse the error created. Telling the GPS unit to only use signals from
satellites above a certain angle can also compensate for this.
GPS
Segments
Okay, now that we understand the basics of GPS, let’s explore the
infrastructure of GPS, these three parts are the Control, Space and User
Segments.
The Control Segment is that part of GPS that is ground based and monitors
the satellites. It was constructed and is maintained by the US Department of
Defense. It monitors the change in the satellites orbits (ephemeris) and
transmits signals to correct for it. There is one master control base in
Colorado Springs, Colorado, USA, and five monitoring stations located in Hawaii,
Ascension Island, Diego Garcia and Kwajalein.
The Space Segment is composed of the 24 satellites that orbit the planet
at 55 degrees across the equator twice a day.
The satellites are called NAVSTAR, were constructed by Rockwell
International, have thrusters for course correction and weigh approximately 1900
lbs.
The final segment is the User Segment, that’s us! Actually, its us and
the US military. The fact that this system was designed for military
applications has serious affects on its use. In the beginning, the US military
introduced an artificial error into the signals, and this selective availability
could throw measurements off by as much as 70 meters.
However, this selective availability was turned off in 2000, and is only
briefly turned on now when the US goes to war. Currently, the plan is to have
selective availability turned off completely by 2006.
Geodesy
Geodesy is the science of creating models that closely represent the size
and contour of the Earth’s surface. These most common aspect of Geodesy that
we will be working with are coordinates. There are two main types of coordinate
systems that we work with, the regular 2D coordinates that we are al familiar
with (X & Y, Latitude and Longitude, UTM, etc.) and 3D (X Y & Z,
Lat/Long/HAE, etc.).
The user, when operating the GPS unit, selects a specific type of
geodetic coordinate systems. Different coordinate systems work better for
different parts of the world. The world standard for GPS is the WGS 84
projection adopted in 1984.
A
remark about altitude measurement in GPS is probably in order at this point. In
Geodesy, there are two main ways that we measure altitude, both mean using a
model that represents the surface of the Earth. The first one is a Geoid, a
surface of equal gravitational pull best fitting the average sea level over the
earth surface. The second is an Ellipsoid, which is a smooth mathematical model
of the Earth’s surface. WGS 84 uses the ellipsoid to take Earth fixed
measurements referred to as HAE (height above ellipsoid).
A
Quick Note about Errors
Even though the technology behind GPS has been designed with an enormous
amount of resources and ingenuity, errors can still sneak in. However, the same
ingenuity that went into designing the above system also helps us to overcome
these problems. The next chapter will address both sources of error and ways to
correct for it.