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Johnny
Appleseed G P S - The Theory and Practice of GPS
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GPS is one of the most fantastic utilities
ever devised by man. GPS will figure in history alongside the development
of the sea-going chronometer. This device enabled seafarers to plot
their course to an accuracy that greatly encouraged maritime activity,
and led to the migration explosion of the nineteenth century. GPS
will effect mankind in the same way. There are myriad applications,
that will benefit us individually and collectively.
Latitude, Longitude, AMG etc etc etc:
I get many requests to explain the different numbers used by different GPS
at the same location. This is a valid and sensible question, and there are
easily understood answers. You may also wonder about terms such as UTM, GDA,
MGA, datum, position format, grid, and true north. My customers get free assistance
with these, and any other matter relating to their purchase, and the use of
the GPS.
What is GPS? (Explaining the system of
satellites and how it works)
Correction techniques for greater
accuracy (DGPS)
Elevation readings and GPS (How far are we above
sea level?)
The theory of positioning (How Sir Isaac Newton
and others made GPS possible)
Explanation of digital mapping forms
and functions in relation to GPS
Applications for GPS
What
is GPS?
The global positioning system is a satellite-based navigation
system consisting of a network of 24 orbiting satellites that are eleven
thousand nautical miles in space and in six different orbital paths. The
satellites are constantly moving, making two complete orbits around the Earth
in just under 24 hours. If you do the math, that's about 2.6 kilometers per
second. That's really moving!
The GPS satellites are referred to as NAVSTAR satellites. Of course, no GPS
introduction would be complete without learning the really neat stuff about
the satellites too!
The first GPS satellite was launched way back in February, 1978.
Each satellite weighs approximately 1 tonne and is about 5 metres across
with the solar panels extended.
Transmitter power is only 50 watts, or less!
Each satellite transmits on three frequencies. Civilian GPS uses the 'L1'
frequency of 1575.42 MHz.
Each satellite is expected to last approximately 10 years. Replacements are
constantly being built and launched into orbit. The GPS program is currently
funded with replacements through 2006.
The satellite orbits are roughly 25,000 kilometers from the earth's centre,
or 20,000 kms above the earth's surface.
The orbital paths of these satellites take them between roughly 60 degrees
North and 60 degrees South latitudes. What this means is you can receive
satellite signals anywhere in the world, at any time. As you move closer
to the poles (on your next North Pole or Antarctic expedition!), you will
still pick up the GPS satellites. They just won't be directly overhead anymore.
This may affect the satellite geometry and accuracy but only slightly.
One of the biggest benefits over previous land-based navigation systems is
GPS works in all weather conditions. No matter what your application is,
when you need it the most, when you're most likely to get lost, your GPS
receiver will keep right on working, showing right where you are!
 A
GPS satellite of Type 2R
How
did the technology evolve? You know from your history books that
Mr Marconi figured greatly in
the understanding of the electro-magnetic energy
we know as radio. This technology was applied
during the 1920's
by the establishment of radio stations, for which you needed
a receiver. The same applies for GPS- you only need a rather
special radio receiver. Significant advances in radio were bolstered
by large sums of money during and after the Second World War
(for eavesdroppping and communications necessities), and were
even more advanced by the need for communications with early
satellites and rockets, and general space exploration. The technology
to receive radio signals in a small hand-held, from 20,000kms
away, is indeed amazing.
So what information does a GPS satellite
transmit? The GPS signal contains
a 'pseudo-random code',
ephemeris (pronounced: ee-fem-er-iss)
and almanac data. The pseudo-random code identifies which satellite
is transmitting - in other words, an I.D. code. Ephemeris data
is constantly transmitted by each satellite and contains important
information such as status of the satellite (healthy or unhealthy),
current date, and time. Without this part of the message, your
GPS receiver would have no idea what the current time and date
are. This part of the signal is essential to determining a position,
as we'll see in a moment.
The almanac data tells the GPS receiver where each GPS satellite should be
at any time throughout the day. Each satellite transmits almanac data showing
the orbital information for that satellite and for every other satellite
in the system.
By now the overall picture of how GPS works should be getting much clearer.
(Clear as mud, right?) Each satellite transmits a message which essentially
says, "I'm satellite #X, my position is currently Y, and this message
was sent at time Z." Of course, this is a gross oversimplification,
but you get the idea. Your GPS receiver reads the message and saves the ephemeris
and almanac data for continual use. This information can also be used to
set (or correct) the clock within the GPS receiver.
Now, to determine your position the GPS receiver compares the time a signal
was transmitted by a satellite with the time it was received by the GPS receiver.
The time difference tells the GPS receiver how far away that particular satellite
is. If we add distance measurements from a few more satellites, we can triangulate
our position. This is exactly what a GPS receiver does. With a minimum of
three satellites, your GPS receiver can determine a latitude/longitude
position - what's called a 2D position fix. With four or more satellites,
a GPS receiver can determine a 3D position which includes latitude, longitude,
and altitude. By continuously updating your position, a GPS receiver can
also accurately provide speed and direction of travel (referred to as 'ground
speed' and 'ground track').
Accuracy
is a relative term of course. If you want to locate a fishing spot,
10 metres is probably fine. But if you want to determine a survey
boundary peg, we might need 2 cms. 10 metres, as it happens is fairly
typical of current GPS accuracy (since 1 May 2000). The first source
of position error used to be Selective Availability (or SA), but
as of 1 May 2000, this was deliberately cancelled. SA created inaccuracies
up to 100 metres in an intentionally-imposed degradation on the accuracy
of civilian GPS by the U.S. Department of Defense. The rationale
behind SA was to deny hostile military or terrorist organizations
the maximum accuracy benefits of GPS. Now that SA is gone, we can
look forward to more productive and safer use of GPS.
Other factors will effect accuracy, but may become significant only when looking
for accuracies better than 10-15 metres. These factors are satellite geometry
(relative positions of each satellite in the sky, units expressed as DOP),
multi-pathing (where satellite reception is blocked or reflected by buildings
etc), and propagation delay due to atmospheric effects. There will also be
internal clock errors. These latter errors will normally have no significance
for 10-15 metre users.
How accurate is GPS, really? A
typical civilian GPS receiver provides 10-15 metre accuracy, depending
on number of satellites available, and the geometry of those satellites.
More sophisticated GPS receivers, costing $5,000 or more, can not
by themselves, provide any better accuracies. To get within a centimeter
or two, they must use correctional information and computing, as
well as using more sophisticated radio reception techniques.
Similar correctional information is also available for a typical civilian
GPS receiver. Then the accuracy can be improved to
one or two metres (in some cases under
a metre!) through a process known as Differential GPS (DGPS).
DGPS employs a second receiver at a fixed location to compute corrections
to the GPS satellite measurements. How are these corrections provided to
your GPS receiver? There are a number of free and subscription services available
to provide DGPS corrections.
* The
Australian Maritime Safety Authority transmit DGPS corrections through
marine beacon stations along the Queensland coast (Byron Bay to Weipa), and
around Sydney, Melbourne, Adelaide, Perth, Port Hedland and Darwin. These
beacons operate in the 283.5 - 325.0 kHz frequency range and are free of
charge.
Your only
cost
to use this service is the purchase of a DGPS Beacon Receiver. This receiver
is then coupled to your GPS receiver via a three-wire connection, which relays
the corrections in a standard serial data format called 'RTCM SC-104.'
* WAAS provides the GPS receiver
with additional satellite ranging to achieve better accuracy and reliability.
This system is NOT available in Australia. It is limited to North America.
* Omnistar wide area satellite DGPS signals,
covering the whole of Australia and SE Asia. Again, a receiver is purchased,
but a license for the ongoing signal is required.
Elevation
Readings using GPS
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To combine true elevation readings,
and GPS, requires the use of something like the Garmin
eTrex Summit. , eTrex
Vista. or GPSMAP76S. These GPS
have a built in altimeter, which can give quite accurate
(within 3 metres) height readings.
There are two major factors
involved in elevation and GPS.
Firstly, what do you mean by elevation? And secondly, is a GPS derived
elevation, as good as a GPS derived horizontal position?
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1.
GPS primarily indicates a surface (horizontal) position
based on a mathematical model
representing the earth's near-spherical surface. Height
or elevation is a different kettle of fish.
GPS can give a distance from the centre of the earth, and then by using
the radius of the surface model (see above), give you an elevation
from the surface model. Let's call this the mathematical elevation.
Then you have to ask, does this represent a height above sea level?
The answer is no. It may do so in places, but only by accident.
There are tables of the differences
around the world, between the mathematical elevation and
sea level elevation. [The spherical (more accurately ellipsoidal)
models for GPS and sea level, are called the spheroid,
and the geoid, respectively]. These tables are the result
of observations taken over the last few centuries, by surveyors,
space scientists and geologists.
Geologists get involved in these observations,
because anomalies in gravity strengths often indicate mineralogy.
And gravity strengths relate to the behaviour of level
determination on the earth's surface.
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2. Because the position solution
found by GPS is a mathematical one, and the ranging from
the satellites is in the order of 20,000 kms, there is
an error bias in the direction of the earths centre. (Because
of intersecting lines that may not quite meet.) This of
course is the elevation solution.
So if we have an error of 10 metres
in the horizontal position, the error in the elevation
will be more like 20-30 metres.Your small standard GPS
unit usually displays elevation, but you must accept
it knowing the above limitations. I can say that it is reasonably sensible.
Around the coast of Australia, it will be somewhere around zero, give
or take 50 metres.
In Toowoomba, it will be about 600 metres. Elsewhere in the world,
it may show greater, or lesser discrepancy.
For a more technical explanation of
the differences between the GPS surface model (the spheroid),
and the sea level surface model (the geoid), you can visit
the Geoscience Australia website at www.auslig.gov.au
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Applications for GPS
So
what's the best GPS receiver for me? I believe you can answer that,
only when you understand some of the things a GPS can do for you.
I will attempt here to show you some. But first, just think of the
GPS as a calculator - it calculates distances and directions from
your current location.
* A GPS unit can record in memory, your current location (and 499 other locations)
in latitude and longitude, or in map grid format.
* Each location can be given a name or number, and time and date (automatically
if you wish).
* It can then calculate distance and bearing between any of those logged locations
(called waypoints in GPS-speak)
* It can guide you from and to any location, with a graphic highway symbol,
and tell you if you veer off path.
* It can automatically log your journey, and then guide you back over the same
trail.
* It can be used as a hand-held, in a boat, or can be mounted under the windscreen
of a car, or within the panel of a boat control station for practical route
guidance. It can be hooked up to a 12volt supply.
* It can store locations in routes, and the whole route can be retained and
recalled.
* You can download all of this information to a computer, and backload it to
any other GPS at any time.
* In use, the GPS will act as a compass, indicating your heading and bearing
(true north or magnetic), and will display your speed very accurately.
* Many GPS units also show sunrise and sunset, battery strength, satellite
positions and signal strengths, DOP, graphic displays and feature mapping of
waterways, roads, rivers and railways.
* There are a number GPS available that display a background map. This map
can be rather general (as in major roads, rivers etc), or it can be very specific,
such as marine charts, or with street level detail, and the ability to search
for an address. This type of mapping can be on a small handheld, or on a larger
in-car or in-boat GPS.
How
do you choose? Well, it depends which features are important for
you. Hopefully the above list will help you to decide that. Technically,
go for one that has 12 channels, because geometry and signal strength
will be better. You may also require an external antenna (for use
in a car or truck, some GPS do not have a facility for an external
antenna connection). Then, do you require a GPS with mapping capability?
This choice must be made from the start, because a non-map GPS cannot
be upgraded to a map GPS.
A number of issues come into play here:
What is the intended application? The most important issue is finding a GPS
suitable for your application. If your particular need is for a panel-mounted
GPS in your airplane, a handheld designed for the recreational boater is obviously
of little value! You can quickly narrow your choices down by identifying which
models are available for your application.
In some cases, you may still have a lot of choices. For example, if your intended
use is hiking or hunting, a GPS for outdoor recreation is suitable--but so
is a handheld GPS designed for boating or flying. In this case you may have
to examine specific features more closely. Unless you plan on flying too, all
the extra information of airports contained in the aviation handheld GPS probably
isn't worth the extra price. A marine GPS which uses cartridges to show navigation
markers and depth contours won't help out much on the trail either (unless
you also want to use the GPS on your yacht!).
In-car (city, or bush 4WD) may be one of your intended uses. You can now choose
GPS with background maps, and more detailed maps as an option, right down to
finding addresses. Some GPS can guide you street by street, with voice prompting,
to your intended destination.
What is the price range? Once you've narrowed the field, you'll
most likely still have several models over a range of prices from
which to choose. Examine each model closely. What do the higher-priced
models have that the lower-priced models do not? Do you need the
extra features or accessories that come with the higher-priced model
or is the lower-priced model sufficient to do the job?
Which model do you like the best? Choosing the right GPS receiver for you is
two parts rational planning and one part simple preference. If rational planning
still leaves you with two or three models to choose from, find a dealer for
these models and try operating each one. Sometimes the differences in operation
are dramatic. You may find one real easy to use and understand, while another
seems much more complicated and difficult to use. Choose the GPS receiver that
you LIKE best! You're more likely to still be happy with the decision you made
after one month or one year.
Written by Kimball Thurlow, with acknowledgement to Garmin International
for the basic framework of the document. December 1999. Updated May
2002.
1.
Sir Isaac Newton invents GPS
Article written by Kimball Thurlow, 11/2000, Copyright,
Johnny Appleseed GPS, Brisbane.
English mathematician, Sir Isaac Newton , speculated in the late
1600's that the earth was not a true sphere, and that the distance
from pole to pole, was less than the distance across the equator.
This estimation, which has become fact through further observation,
lays the basis for a true global positioning system. Newton was also
the bloke, you remember, who created the theory of gravity when the
apple fell from a tree. Simple really when you think about it afterward;
how else would you explain it. I mention Newton, because he was an
Englishman, and I am descended from the English. But there were others
of course. Newton was preceded by Ptolemy, an Egyptian, in the first
century AD. He simply expanded on prior thought from the Greeks,
but Ptolemy's are the earliest recorded works on geographical position.
The Byzantines and Arabs carried this knowledge through to the modern
era, and there were Chinese observations to add to these works.
Ptolemy was the first as far as we know, to describe position as
a latitude and longitude. In describing a sphere, and for that matter
a circle, we need both a radius and a centre point. So when Newton
speculated that the radius to the poles, was shorter than the radius
to the equator, he still needed to define the centre point. This,
and the exact shape and volume of the earth has exercised many minds.
When the first Sputnik and subsequent satellites were sent in to
orbit, scientists were able to measure their paths by range finding.
The path of each satellite, varied due to the effects of gravity.
From the observations, the centre of mass of the earth has been able
to be determined reasonably precisely in relation to a number of
points fixed on the earth's surface, and is accepted world wide in
the definition described as the World Geodetic System 1984 (or WGS84).
Simplistically, this, along with a radius, and a flattening factor
for the squashed polar axis, is the datum of our modern global positioning
system. From that centre point, the radius can define the path of
a satellite, or the approximate spherical surface of the earth.
2. Latitude and Longitude
Remember Ptolemy used a latitude and longitude format to describe
position. This format simply describes an angle back to the earth's
centre from your location. For Brisbane, draw a line to the centre
of the earth, and another line from the centre out to the equator.
The equator is accepted as a datum worldwide. So we have a Latitude,
south of the equator, example S27o 30'.
If you move north about 1.8 km, the latitude reading will change
to 27o 29'. That 1' difference (that is one minute of arc, an angular
measure, and nothing to do with time) represents 1.8 km (one nautical
mile) on the earths' surface. Handy to remember that.
Let us also draw a line from earth's centre to where the equator
meets a vertical line running from the poles through Greenwich, a
suburb in South East London. This is the Greenwich meridian, again
accepted worldwide as a datum for measurement of Longitude. The angle
to the Brisbane line is 153o 03' or thereabouts, depending on where
you are in Brisbane. This is E153o 03' Longitude.
Actually, geodisists (scientists who study the measurement of the
earth), and the global positioning system (GPS) use the earth's centre,
and three axes at right angles to describe a position on the earth's
surface. This positioning system is based on a linear measurement
of metres from the centre along each of the three axes, describing
a set of cartesian coordinates. Latitude and Longitude is a more
practical definition for everyday use, and is a derivative calculated
from the axis figures.
So the Latitude/Longitude position as described on your little hand-held
GPS unit has been made possible, only after long and thoughtful processes
spanning thousands of years.
If you intend to relate GPS position to a paper map (such as a street
directory, or topographic map), you should at least be aware of different
datum positions. Most maps produced prior to 1999, used the Australian
Geodetic Datum of either 1966 or 1984. By default, the GPS you buy
from an untrained sales person (especially if you don't specify your
intended use), will be set on the WGS84 datum. In this case, the
GPS position may differ by 200 metres from the map position. That
is a nuisance when you try to find a fishing spot on a small reef
only 20 metres wide!. And I have known people to miss a vehicle in
the bush, when in fact they were only a few hundred metres away!
All future map production will be on the Australian equivalent of
the WGS84 datum (what is called the GDA94). But the older maps will
remain on the newsagent and map-shop shelves for a long time yet.
They will still be useful, so long as you understand the differences.
At Johnny Appleseed GPS, we will be able to describe how to deal
with these anomalies at the time of purchase, so you get the best
from using your new GPS unit.
What is GPS? (Explaining the system of
satellites and how it works)
The theory of positioning (How Sir Isaac Newton
and others made GPS possible)
Applications for GPS
Explanation of digital mapping forms
and functions for GPS
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Johnny Appleseed GPS