GPS
Basics
What is GPS?
The
Global Positioning
System (GPS) is a satellite-based navigation
system made up of a network of 24 (or more) satellites. These
satellites were placed into orbit by the U.S. Department of Defense as
the system was originally intended for military applications. However,
in the 1980s, the government made the system available for civilian
use. GPS works in any weather conditions, anywhere in the world, 24
hours a day. There are no subscription fees or setup charges to use GPS.
The 24+ satellites that make up the GPS system
are orbiting the earth
about 12,000 miles above us. They are constantly moving, making two
complete orbits in less than 24 hours (the satellites are traveling at
speeds of roughly 7,000 miles an hour). These satellites are equipped
with atomic clocks and send out radio signals as to the exact time and
their location in space. The radio signals from the satellites are
picked up by the GPS receiver. Once the GPS receiver locks on to four
or more of these satellites, it can triangulate its location from the
known positions of the satellites. Essentially, the GPS receiver
compares the time a signal was transmitted by a satellite with the time
it was received. The time difference tells the GPS receiver how far
away the satellite is. Now, with distance measurements from four or
more more satellites, the receiver can determine the user's position.
This is a very simple explanation, but unless you are a surveyor or
engineer who needs to understand how to use GPS to locate within
fractions of an inch, this is all you really need to know.
GPS
Uses
Through
the use of these
receivers, you can navigate to a predetermined
location (or back to a starting point) without the use of maps or any
other equipment. When used with accurate maps like ones provided by the
USGS (United States Geological Service), you can navigate to identified
locations on maps or take readings from a location that you are at or
have been at and plot those locations on a map.
For Basic GPS use, you only need to understand
the most basic of the
receiver’s functions...i.e. - Set-up and initialization functions,
saving a waypoint, and using the GOTO feature. For this level of use,
you don’t need to understand the coordinate grid systems, datums or how
to use maps. Therefore, a new GPS user can make good use of the unit
with a minimal amount of experience by simply reading the manual and
working the keypad.
Those who need to (or want to) refer to maps
and use them to plot and
read position coordinates must understand at least one of the
coordinate grid systems. Once the grid system is understood and its
coordinate references can be identified on maps, it is a simple matter
to learn to use the tools and techniques to properly identify
coordinates.
Grid
Systems
While
the topic here is
GPS basics and not general mapping techniques,
the two are closely interrelated. Thus, a brief mention of the most
common grid systems is necessary. The two most common grid systems in
use in North America (and referenced on maps) are the Latitude
Longitude (Lat/Long) grid and the Universal Transverse Mercator (UTM)
grid. Both of these grid systems consist of reference points, units of
measurement, and some designation of direction to clearly identify a
position.
The Lat/Long grid has been in use for
centuries and most people have
heard of the basic terms of latitude or longitude.
The UTM grid is a metric grid system based on
60 grid zones around the
globe and a set of values in meters from reference points of the grid.
Despite some of the basic terminology being foreign to most, there are
some aspects to this grid that make it very simple and easy to use.
More information on both of these grid systems
(and their relationship
to GPS) can be found at
Maptools.com.
GPS
features and options
Like
any other form of
electronics, there are many options and prices.
The first consideration should be "How will the GPS equipment be
used?". The type of equipment needed all depends on its intended use.
It is as difficult to read a small handheld screen while driving down a
bumpy dirt road as it is to take a laptop computer on a hiking trip.
Like any other form of electronics, there are
many options and prices.
The first consideration should be "How will the GPS equipment be
used?". The type of equipment needed all depends on its intended use.
It is as difficult to read a small handheld screen while driving down a
bumpy dirt road as it is to take a laptop computer on a hiking trip.
Since modern GPS receivers share many of the
same features (regardless
of price), understanding all the features available will let you to
make a more informed purchase and allow you to use the equipment to its
full potential. Items bracketed with asterisks are features that are
either highly desirable (IMHO) and/or need to be considered during the
"How will the GPS be used?" phase.
** 12 Parallel Channels ** - Make sure
your receiver is capable of 12
Parallel Channels. Most units sold after 1997 are equipped with
this
feature, however, the first receivers available were all single
channel. Some of these older units have similar model numbers with
marketing stating they can scan 12 satellites. This is not the same as
a 12 Parallel unit that can process data from 12 satellites through 12
channels all at the same time. The older single channel equipment
processes information one satellite at a time through a single channel,
making them much slower.
Accuracy - As long as the antenna has
a clear view of the sky, accuracy
is consistent in most receivers. Regardless of the style or cost,
accuracy averages within 15 meters anywhere in the world. Accuracy can
be improved within three meters with WAAS.
Address Finder - Allows an exact
address to be located within a basemap
database.
Alarms - An alarm that notifies the
user of an approaching waypoint.
Text Alarms flash a message on the screen, audible alarms sound a
tone...some of the higher priced units even talk to you.
Altimeter - A 3-D, four-satellite fix
provides elevation information.
However, satellite based altimeters are not known for being highly
accurate...so some units provide a built in barometric altimeter for
accurate elevation readings independent of a satellite connection.
** Antenna ** - The options available
are built-in, detachable or
external. The antenna option is important because it is a determining
factor on how the equipment can be used. Receivers with built-in
antennas are more durable for hiking, but are restricted in their use.
Detachable antennas are ideal for receivers used in and out of
vehicles. External is for vehicle applications where equipment is
mounted with no clear view of the sky. This allows the GPS to be
mounted inside a vehicle.
Auto Routing - Provides turn-by-turn
directions to a waypoint.
Directions may be in the form of arrows and/or automated voice
commands. Usually requires basemaps and/or the ability to transfer maps
to the unit via a computer connection.
** Basemap ** - Most recent model
receivers include a map database
stored within their memory. Basemaps include general information on
cities, roadways and waterways. The maps are not always highly
detailed, but are still impressive, considering they typically include
such large geographic areas like North America or Europe. This
is
highly recommended. The additional cost is marginal and the increase in
the equipment’s usefulness is substantial.
Clock & Timer - Receivers provide
precise atomic time in either a
12 or 24 hour display. Various timer features include date, time
traveled and estimated time of arrival.
Compass Data - Receivers provide a
compass direction if the unit is
moving approximately ten MPH. Stationary, they will provide a compass
bearing from a current location to any other location. A receiver can
be programmed to display compass data in either true or magnetic north.
A pointer is also provided to help you maintain the correct bearing.
Some of the most recent units to hit the market include a built-in
digital compass to provide direction while the receiver is stationary.
** Computer Interface ** - Data in/out
capability allows the unit to
receive, (upload), data from a computer, or send, (download), data to a
computer. This information includes digital maps, track logs, waypoints
and routes.
Coordinates Displayed - Most receivers
provide the option to display
mapping coordinates in multiple international formats. The primary two
are latitude/longitude and UTM, Universal Transverse Mercator.
Cursor Arrow Data Field - Receivers
with a rocker keypad can scroll a
cursor arrow on a map page. Data displayed in a Data Field includes the
coordinates of the cursor arrow’s position, as well as the distance
from the current location.
DGPS or WAAS Ready - These receivers
are capable of accepting radio
signals that can increase accuracy to within three meters. These
accuracy-improving systems are primarily used in aviation and marine
applications.
** Memory ** - For receivers with a
basemap, memory is used to store
additional mapping data. GPS manufacturers provide greater detailed
maps on CD-ROM exclusively for their brand. Mapping detail, especially
topo contour lines use a great deal of memory. Two megabytes will allow
the storage of a few small areas. Eight megabytes may allow the storage
of the primary areas of a home state. Ideally, a receiver will have a
minimum of eight megabytes of memory. Some models use memory cards
allowing virtually unlimited storage with high capacity cards.
** Power Source ** - Handheld units run
on batteries. Vehicle mounted
units (usually) run on 12V. A unit that has the option using of a 12V
source allows you to use a cigar lighter power cable. Battery life is
important for extended use when no external power source is available.
Receivers are rated for battery life duration for both continuous use
and power saver modes.
** Rocker Keypad ** - Using a receiver
without a rocker keypad is like
using a computer without a mouse.
Routes - A series of waypoints listed
in sequence from start to finish.
Designed to guide to a destination, they can also be inverted or
reversed, to track back from the destination to the starting point.
Routes typically contain up to 30 waypoints...but the more waypoints a
route can contain, the longer and more complex those routes can be.
Satellite Status Page - Information
includes the number of satellites
being locked onto with a signal strength bar for each.
** Screen size ** - For visual ease of
operation, use a receiver with
the largest screen that can be realistically carried.
Sun Position - Provides sun and moon
positions including sunrise and
sunset.
Track Log - Plots an electronic
"breadcrumb" trail as a sequence of
dots or trackpoints, showing a path traveled.
Travel Data - Includes distance and
time to next waypoint, current
speed, average speed and trip odometer.
Water Resistance - Receivers are
rated for their resistance to water.
Water-resistant usually means the equipment can be splashed or briefly
dunked. Waterproof means the equipment can be submerged to a specific
depth for a specific amount of time before damage occurs. Regardless of
rating, use a watertight box or bag if used around water. Saltwater can
kill electronics instantly.
Waypoints - Specifically recorded
locations stored within a receiver’s
memory. Saved waypoints allow the return to exact locations. Most
modern receivers have the ability to store at least 300 waypoints.
GPS
Receiver Types
While
there can be some
'crossover' in regards to some specific models
(hybrids, if you will), GPS Receivers are grouped into the four
following basic categories.
Hand Held without Mapping - These basic
receivers are about the size of
a TV remote control, include a built in antenna and an approximate
two-inch view screen. They usually run off of batteries and some have
the ability to run from 12v power sources. They include many of the
features of more expensive units, but without a basemap database.
Hand Held with Mapping - These are the
same as the above units, except
they include a basemap database
Vehicle Mount - The term vehicle is
universal for practically anything:
ATV’s, cars, trucks, race cars, boats, planes, whatever. The units are
the size of a fish-finder, with a much easier to read four-inch plus
sized screen. Batteries are replaced by 12-volt wiring and antennas are
mounted externally.
GPS Receiver kits for PDA and Laptop
computer - Handheld computers can
be converted into GPS receivers through a combination of operating
software and an antenna. Handheld personal digital assistants “PDA’s,”
use an adapter sleeve with an antenna that plugs into an expansion
slot. Laptops use a dash mounted remote antenna.
GPS
Computer Interface
GPS
units that have
basemap data, are usually capable of interfacing
with a PC or laptop. Software can then be used to transfer data back
and forth between the GPS unit and the computer. The data can be maps
(or mapset info), waypoints, track logs, routes, etc. The software and
mapsets are usually proprietary to the manufacturer, so they cannot be
shared among the different brands. However, waypoints and certain
aspects of tracklogs & routes (since they are both just a
collection of waypoints) can be shared with between different GPS
brands, provided the same grid system is selected.
Common
GPS related Terms
and Acronyms
The
following is a
listing of common terms and acronyms used in relation to GPS and GPS
receivers.
Active Log - The segment
of a route currently being traveled.
Backtrack - Navigation of a route in
reverse order from the last position fix. Basically, the reverse of
navigating a route in normal sequence.
Bearing - For G.P.S. navigation,
it generally refers to the compass reading to navigate from one
position to another measured to the nearest degree. (In traditional
nautical navigation, it is referenced in degrees from quadrants of the
compass)
Coordinates - A set of numbers
that describes a given position for a given coordinate grid system.
Examples of coordinate grids are Latitude/Longitude and Universal
Transverse Mercator (UTM).
Course - The direction in
degrees between two waypoints or the course indicated by a G.P.S.
receiver when the "GOTO" function is activated.
COG - Course Over Ground: The
actual direction you (G.P.S. receiver) are traveling.
Datum - A reference system for
vertical and horizontal positions. Different datums have different
positions for the physical location of their origins, thus different
datums will represent coordinates in different positions. Differences
can be as much as a mile. All reliable maps that show coordinate
systems provide Datum information.
Declination - The angular
difference between True North and some other reference for north such
as Grid North or Magnetic North.
Differential G.P.S. - A
means of compensating for Selected Availability error in G.P.S.
locating using radio transmissions.
Elevation - The distance above
mean sea level usually in meters or feet.
Equator - The Latitude reference
point for the Lat/Long grid system. Other positions of latitude are
referenced as degrees of North Latitude if they are north of the
equator and degrees of South Latitude if they are south of the equator.
Also see Latitude.
ETA - Estimated Time of Arrival:
The estimated time of your arrival computed at your present speed
toward the destination.
ETE - Estimated Time En route:
The projected travel time it will take to arrive at your destination or
waypoint.
GIS - Geographic Information
System: A category of computer programs and applications that are used
to organize, analyze and display geographic information.
GMT - Greenwich Mean Time: Time
at 0° or the Prime Meridian passing through Greenwich England. Also
known as Universal Time Coordinated (UTC) and, in aviation, Zulu Time.
G.P.S. - Global Positioning
System: A generic term for satellite based positioning system.
Grid - A system of horizontal
and vertical lines used to chart specific position coordinates i.e. the
Latitude/Longitude Grid system.
Grid North - The direction that
north/south lines of a grid point.
Grid Zone - One of the
60 zones covered by the Universal Transverse Mercator (UTM ) grid
system of navigation.
Heading - The direction that you
are facing or traveling see Bearing.
Initialization - The initial
orienting process that a G.P.S. receiver does when it is first turned
on. If it is the first time the receiver has been turned on or if the
receiver is more than a few hundred miles from the last location it was
used, it will take some time or help from the operator by inputting or
selecting from a menu the approximate location.
Landmark - A specific
location or identifiable natural or man-made geographic feature. Also
referred to as a waypoint in G.P.S. navigation.
Latitude - The angular distance
north or south of the equator when using the Latitude/Longitude grid
system. These latitude lines are parallel to the equator and often
referred to as "Parallels".
Legend - Information on a map
for such things as scale, datum, and other details.
MGRS - Military Grid Reference
System: A metric grid system that is a subset of the UTM system. It
uses letter pairs to represent 10km squares.
Magnetic Declination - The
difference between True North and the direction that a magnetic compass
points for a given location. Magnetic declination varies from place to
place and can change as much as a degree in a year.
Magnetic North - The direction
that a magnetic compass will point. Can be 10°, 15° or even
more different than True North.
Mean Sea Level - The average
level of the ocean.
Meridian - A line of longitude
going north and south from pole to pole measured in degrees from the
Prime Meridian or 0°. See Longitude.
MOB - Man Over Board: A feature
in G.P.S. receivers to quickly mark a given location as you are
traveling. Some receivers then activate a GOTO navigation function to
that waypoint.
NAD27 - North American Datum of
1927: The datum used on most large scale USGS topographic maps.
NAD83 - North American Datum of
1983: A newer datum than the NAD27. Almost identical to WGS84.
NAVSTAR - The name of the United
States Department of Defense satellite navigation system.
Northing - A term used to denote
positions north of the origin in the UTM coordinate grid system.
Parallel - A line of latitude.
See latitude.
Position Fix - The calculated
position of the current location by a G.P.S. receiver.
Prime Meridian - The reference
line for Longitude in the Lat/Long grid system. All other meridians of
longitude are designated in degrees East or West of the Prime Meridian
up to 189°. The Prime Meridian is 0° Longitude.
Route - A series of progressive
waypoints stored in a G.P.S. receiver which can be activated and
navigated.
Scale - The ratio between actual
distance and that same distance represented in a map.
Selected Availability -
The intentional error introduced in G.P.S. signals by the government to
degrade accuracy.
SOG - Speed Over Ground: The
speed the G.P.S. receiver is traveling.
Topographic Map - A map that
gives not only locations of roads, landmarks and other features, but
also gives contour lines at different elevations to show valleys, hills
and other topography of the area.
True North - The geographic
north of the globe and represented on the Lat/Long grid by the lines of
longitude or meridians.
USGS - United States Geologic
Survey: A part of the U.S. Department of Interior and the primary
mapping agency for the U.S. government.
UTC - Universal Time
Coordinated: The time on which all G.P.S. signals are synchronized..
The same as Greenwich Mean Time.
UTM - Universal Transverse
Mercator: A metric grid system consisting of 60 zones to cover the
earth’s surface.
VMG - Velocity Made Good: The
speed at which you are progressing toward the destination or the GOTO
waypoint.
Waypoint - A location designated
by a set of coordinates and stored in a G.P.S. receiver to be later
used as a GOTO destination or as part of a route.
XTE - Cross Track Error: The
amount deviated to the left or right from the true course.
