GPS Basics
Introduction
Lewis and Clark would stop their travels for a day or more at a time just
to use celestial navigation techniques to find and map their location
on their trek to our western shores of the United
States. Today, it is possible to know
where you are almost instantaneously at any hour of the day. A small
handheld unit called a Global Positioning System (GPS) Receiver can
now give you information about your location and give you your position
accurate within feet. All you need is one of these reasonably priced
units and a clear unobstructed view of the sky.
GPS satellites now circle the earth sending radio signals back to the
surface. These satellites enable you to determine your position anywhere
on earth at any time during the day and are not affected by bad weather.
You may not want to throw away your map and compass, but a GPS receiver
provides you with a tool that you will not want to give up once you
have mastered its use. A portable GPS receiver allows you to mark your
starting point, track your route, and even point to your destination
point. Move expensive units even provide you with maps to help your
navigation.
That's all the good stuff. But you should realize that the GPS system
does provide some limitations. In fact, manufactures of GPS receivers
are quick to warn owners of new GPS units that they should not "rely"
on the GPS receiver for navigation. One limitation to the correct functioning
of the unit is that a GPS unit needs to receive signals from four satellites
to function correction. In urban areas, signals may be received from
seven or more satellites, but there are some areas of the world that
only receive the four signals. That means that you will be subject to
intermittent outages. More of a problem is that GPS signals cannot be
received through the walls of buildings or through heavy vegetation
in the outdoors. For example, deep under a forest canopy you may have
difficulty using your GPS receiver. Rocks and tall buildings provide
similar problems. And you also need to remember that your receiver is
a electrical/mechanical device that is subject to malfunction and probably
even more important stops with dead batteries. So the prudent personal
will have a backup system for navigation and not totally rely on the
GPS
In the early days...
This navigation "thing" is traced back to about 2000 years ago in China.
Natural magnet were used as direction finding devices called
"sinans." Although we thing of the needle of a compass always
pointing north, the early name "sinan" meant "south-pointing
ladle." According to ancient records, natural magnets were employed in China
as direction-finding devices. The early compass in the Han Dynasty consisted
of a bronze on which 24 directions were carved and a rod made from a natural
magnet. The compass is thought to have come to Europe
around 1200 AD. Europeans used a thin piece of magnetite hung on a string.
Magnetite was given the name lodestone that means "stone that
leads." The compass was just the beginning of navigation. It could tell
someone their orientation, which direction is north, but it could not provide
a person's location.
The stars could help with navigation and the astrolabe, the quadrant and
the sextant became the next real advances in navigation. These devices made
it easy to determine latitude or the distance above or below the equator. The
real difficulty was to determine the distance east or west, what we call
longitude. In the mid 1700's the chronometer enter the scene. This device
enable the navigator to determine the local time by the sun and compare it to
the time in Greenwich. The
difference could then be translated in the distance from Greenwich
or longitude. The main limitation was that weather could make it difficult to
determine local time and still left some limitations to reliable navigation
on a continual basis.
Electronic Signals Through the Air
Real advances in navigation occurred in the early 1900's with the use of
radio signals to determine positions. Early systems used the sending of radio
waves and measuring the differences in times sent and received. With two
towers, and signal could be sent to one tower and then a signal would be sent
to the other. By using a simple formula, the distance between the two towers
could then be computed. The problem with two towers, however, is the same
with early navigation by the stars. One dimension of the position could be
determined or the x-axis, but the position could not be determined on the
y-axis. If three radio towers were used, it was possible to determine a two
dimensional positional could be determined through a process of
triangulation.
Advancing through the years, satellites become the source of the radio
waves that can be used for navigation. Triangulation then becomes the
distance between three satellites reflected in their spherical signals.
Where the three signals intersect, the location is identified in three
dimensions, latitude, longitude, and altitude. There is a twist, however.
For a GPS receiver to calculate its position a fourth satellite must
be used to synchronize the time between the satellites different atomic
clocks and the receivers less accurate quartz timepiece. If there are
only three satellites available, only a two dimensional position can
be determined. Also, for a GPS receiver to function properly, it must
know the location of satellites. It gets this information from an almanac
that is transmitted from one of the satellites and then stored in the
receiver. The first time you use the receiver it will have to calculate
its position called Time To First Fix (TTFF). It can store this information
for about six months unless it is moved more than 300 miles while it
is off. It this course, you will again have to wait for TTFF.
Development
The GPS was developed to serve the military need to increase the
accuracy of missiles. The Air Force worked on the first system with
other branches of the military soon joining with a system called Navstar,
a name that did not live long. Eighteen satellites are needed to cover
the whole earth, but up to 29 satellites are kept in orbit at 12,000
miles above the earth to allow the system to be maintained. Five ground
stations are also part of the system. Satellites transmit low-power
radio signals with frequencies in the UHF band. A concern of the developers
of the system was that anyone in the world could access the signals
and actually use the system against the United
States. To discourage this from happening
two different signals were beamed from the satellites. Coarse Acquisition
(CA) code that was purposely only accurate to about 15 meters or about
49 feet. The second code beamed to the earth, Precision (P) code needs
complex military decoders, but is accurate to 1 meter or a little over
three feet. CA code was also purposely inconsistent or follows a policy
of Selective Availability (SA). This random inaccuracy would originally
make the code inaccurate up to 100 meters, but because of civilian complaints,
is now accurate to about 15 meters or a little over 49 feet.
It has been determined by the Federal Aviation Administration that
the GPS cannot be used for aviation so a new differential GPS call WAAS
has been developed. This provides more accuracy than a receiver of a
basic GPS signal, but you do have to have a receiver that is WAAS enabled.
This gives you accuracy to about 3 meters or a little less than 10 feet.
Unfortunately, WAAS signals are easily affected by terrain and vegetation
obstacles and are most effective in flat open spaces.
Map Datum
Every map is drawn from a standard reference point called a datum. It is not an actual map that is build into
a receiver. A series of lines on the
map called a grid describes a location in reference to the datum point. Several grids can exist on a map, but there
will only be one datum point. There
are more than 100 datums available to choose from, the two most common datum
used in North American are the North American Datum 1927 (NAD27) and the
World Geodetic System 1984 (WGS 84).
You need to be sure to set your receive to the correct datum before you enter
any coordinate into your receiver. You
should not have to ever change this setting unless you leave the United
States or use a chart of paper map
that specifies a different datum in its legend.
Coordinates
Maps have grid patterns to enable the map reader to be able to uniquely
identify every point on the map. A combination of letters and/or numbers
are used to identify map locations. Your GPS receiver should support
the Universal Transverse Mercator (UTM) an latitude/longitude grids
because they can work together to cover every location in the world.
There are other grids used in other countries and even the United States,
but these two are the most commonly used. The best receivers allow you
switch from one grid to another, but you will probably have to make
the seldom make switch unless you are traveling extensively and using
a variety of maps with different coordinates.
Waypoint
Receivers require a name for every coordinate. This name of the coordinates
are called a waypoint or a landmark. Navigation with a GPS receiver
begins when you enter the waypoint into the memory of your receiver.
The receiver than takes your current position and calculates the distance
and direction to the destination waypoint. Many GPS receivers allow
you to enter a short unique name for a waypoint and some specific descriptive
data. Unfortunately, even if the unit has enough memory available, choosing
letter descriptions with the receiver controls is awkward without an
input keyboard.
Route
The path that you will travel can be described by a list of sequential
waypoints or a "route." The route enables you to be guided
from the first point on your route to each sequential point on your
path until you reach your final destination. Many receivers have a "goto"
function that takes you directly to your designation point, but it takes
the shortest route or a straight line. The "route" function
enables you to select points along a route to your destination that
bypass trees, hills, or other obstacles that might be in a straight-line
path. Along when you are following a route with your GPS, it takes you
from one point to the next without you having to shift directional settings.
Your receiver probably has an automatic function that enable you to
reverse you route to make a return trip along your route.
UTM Grid
All recent USGS topographic maps include UTM grid marks in the margins
of the map. The advantage of the UTM grid system is that it provides
a constant distance relationship anywhere on the map. With latitude
and longitude, the grid has angular coordinates and the distance covered
by a degree of longitude differs as you move towards the poles and only
equals the distance covered by a degree of latitude at the equator.
With the UTM system, the coordinate numbering system is always tied
directly to a distance measuring system. The UTM grid is a projection
of the curved surface of the earth onto flat sheets of paper and as
a result there are some inaccuracies. Map corrections can be made to
adjust to true north, but this should not be necessary for the casual
user. These inaccuracies are usually not large enough to make a difference
for those using handheld receivers for recreational purposes.
The numbers along the top of the map are call eastings and provide
the position relative to east and west. The numbers along the vertical
sides of the map are call northings and provide a north-south position.
The easting number increases as you are going east. A full coordinate
will be written as: 394000mE.
The next full coordinate to the east will be 395000mE.
There are 1000 meters or 1 km between 94 and 95. The three numbers before
the "m" stand for meters. To compute the E-W difference between
two coordinates you would simply subtract. For example 395150mE
is 300 meters west of 395450mE
or 1300 meters west of 396450mE.
Northing numbers increase as you go north. A full northing coordinate
is 3346000mN. Distances
can be computed with northings just as with eastings with 1000 being
between each full coordinate and the last three numbers representing
meters.
UTM coordinates fall within 60 different zones throughout the world,
each 6 degrees wide. The zone will be printed on the map and it will
be included in a complete coordinate. For example, the coordinates above
come from a map in Zone 15 so the complete coordinate will be written
15 395000mE. The second
complete coordinate will have 15 N 3346000mN
to indicate N (orth) or S (outh) of the Equator or U (military/distance
from equator). For recreational use, this zone information is not necessary
for plotting points on a map that will already be positioned in a specific
zone and will have the zone labeled on the map.
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