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Boating Nautical File

GPS - Global Positioning System

GPS Overview
What is GPS ?
Triangulation Basics
GPS Accuracy
What is WGS 84 and AGD 66 ?
GPS Models
GPS Glossary
References Used

GPS Overview

Boys will have their toys !! Especially when technology can make life so much easier. As mentioned in fishSA's Hot Spots, one method of determining where you are over water is thru the "line of sight" method, using land based marks as reference points. Though this method is usefull, it does leave a margin of error due to human failings in sight and judgement.

Once considered a luxuruy, but now an intergral part of the boater's arsenal to fishing, hand-helpd GPS units are the norm. The variety available, its scope and functionality, and its price now allows the general boater to consider this device as a tool and a great asset, when on water.

The Global Positioning System (GPS) is an orbit-based satellite navigational system conceived in the 1970s. Since then, it's been fully developed and controlled by the United States Department of Defense (DoD). GPS was originally intended for military use, but from the outset the United States Government realized that the system could meet numerous vital civilian needs as well.

The United States Government has reserved the highest level accuracies for the military, but has developed a special Civillian Access (C/A) code which makes the GPS system available to everyone, everwhere, day-and-night.

What is GPS ?

GPS is a technology developed primarily through the work of the United States Department of Defense. The US military's goal for GPS was to accurately navigate weapons from mobile platforms to prime targets in "unfriendly" countries in times of conflict. However, GPS has now expanded its value to non-military applications, such as open water navigation, search and rescue operations, precision agriculture, geographic surveys, and countless other uses.

Twenty-four satellites orbit about 11,000 miles above the Earth, transmitting orbital position information, current time, and error corrections that need to be taken in consideration. Back on Earth, your GPS receiver interprets the signals broadcasted by at least three satellites to "triangulate" a general position. Triangulation requires the receiver to use the clock reading, location, and orbit of each satellite and mathematically determine its relative location. A fourth satellite is used for time corrections between the GPS receiver and satellite's clocks.

Triangulation Basics

Three GPS satellites must be able to transmit their position, time, almanac, and ephemeris (satellite orbit) correction data to your geographical area.

Your GPS receiver interprets this information, and determines the distance between you and each satellite. The distance is basically calculated by multiplying the speed of the travelling signal with the amount of time it took to reach the receiver from the satellite:

Distance = Velocity * Time

The velocity in this case is roughly the speed of light, or 186,000 miles per second in a vacuum (the errors from transmitting this signal through an atmosphere is described futher on).

Time with "atomic clock" precision is provided through the satellites' transmission, and your receiver maintains and corrects its own clock using the orbiting satellites as a reference. This accuracy is crucial as a slight error of only a fraction of a second can give you readings miles away from your actual location.

Now that your receiver knows its distance and has orbital map, or ephemeris, of each satellite, it can use this information to plot a radius, with the satellite as the center. It knows that the distance is "X" miles from a particular satellite, so a line can be "drawn" from it's location towards Earth. Using the same method for all three satellites, your receiver can find a point that intersects with all three radii. This point is your GPS receiver's location and is usually updated at least once every second, depending on the receiver model.

Even with three satellites transmitting extremely accurate readings to your GPS receiver, your calculated position may not truly represent your actual location. In fact, sometimes you may find that your GPS receiver says that you are at "point A", when in reality, you are actually standing over 300 ft (100 meters) away! Where does this kind of error come from?

Errors can occur from several different sources. Some are due to the "physics" involved in signal transmission and the Earth, while others are intentionally provided, compliments of the US Department of Defense. Some of these errors include:

The satellite's signal may bounce off particles in the atmosphere, which creates a slight time delay to the receiver. Since the receiver is comparing the time stamp provided by the satellite with its own record of time, any slight delays will add an error to the triangulation result.
Signals may also bounce off of buildings, mountains, and other imposing objects before reaching your receiver, increasing the amount of travel time.
The US Department of Defense may turn on "Selective Availability", which is an intentional degrading of the GPS signal. By altering the time stamp or ephemeris readings from the GPS satellites, triangulation results become inaccurate. This method was intended to prevent "unfriendly" countries from using the US DoD's technology to precisely target United States interests.

GPS Accuracy

Selective Availablity (S/A)

Because GPS was designed as a military navigational system, the US Department of Defense created two transmission codes; the "P" code (Precision Code) for US military use, and the "C/A" code (Civilian Access code) for civilian use. The highest accuracy levels were to be reserved for the US military so as to prevent hostile enemy attacks against the U. S. using the DoD's own navigational system. However, once in operation, the civilian code proved to be more accurate than the DoD had intended. Consequently, the military developed a system for randomly degrading the accuracy of the signals being transmitted to civilian GPS receivers. This intentional degradation in accuracy, controlled by the US DoD, was named "Selective Availability" or "S/A".

It should be emphasized that all brands of civilian GPS receivers are equally affected by Selective Availability: no manufacturer has an advantage. The US government has stated that with S/A turned on, civilian GPS accuracy levels will consistently be 100 meters or less, 95% of the time. The other 5% of the time, the accuracy will be 300 meters or less. However, typical accuracy for most users averages between 20 and 50 meters the majority of the time.

S/A is a random error, intentionally input by the military. It can be low or high, and vary in degree at any given moment. When on, this deliberate accuracy degradation can represent up to 100 meters. With S/A off, civilian GPS accuracy levels increased to 15 meters or less. Even with Selective Availability on, most GPS users experience typical accuracy much closer than 100 meters.

You can easily see the effects of S/A on a GPS receiver when you are not moving: Typically, there will be random movements in speed, altitude and position readings, along with jitter in the plotter trail. For example, while parked at the dock in your boat, you may see unexplainable changes in your digital speed readings up to a few miles per hour, even though you're not moving. That's S/A at work.

Differential GPS (DGPS)

Although there's a strong possibility that S/A will be turned off completely in the future, Differential GPS, or DGPS, has been developed to improve GPS accuracy to within a few meters. Originally initiated by the U. S. Coast Guard, DGPS adds a land-based reference receiver located at an accurately surveyed site. Since this non-moving DGPS reference station knows where the satellites are located in space at any given moment as well as its own exact location, the station can compute theoretical distance and signal travel times between itself and each satellite. When those theoretical measurements are compared to actual satellite transmissions, any differences represent the "error" in the satellite's signal caused by S/A. All the DGPS reference station has to do is transmit the error factors to your DGPS receiver, which gives the information to the GPS receiver so it can use the data to correct its own measurements.

What is WGS84 and AGD66

WGS 84

World Geodetic System 1984 (WGS 84) is an earth fixed global reference frame, including an earth model. It is defined by a set of primary and secondary parameters:

the primary parameters define the shape of an earth ellipsoid, its angular velocity, and the earth mass which is included in the ellipsoid reference
the secondary parameters define a detailed gravity model of the earth.

AGD 66

At present, there are two geodetic coordinate sets in common use throughout Australia. Both have been computed on the Australian Geodetic Datum (a local datum). They are referred to as:

The Australian Geodetic Datum 1966 (AGD 66)
The Australian Geodetic Datum 1984 (AGD 84)

The coordinate sets have been derived using the same spheroid (the Australian National Spheroid) and the same origin coordinate values (see Insert 4). They differ because the AGD84 values were computed using a much improved data set. Effectively, AGD84 is an updated, but distinctly different, version of AGD66. Care should be taken not to mix the two sets of coordinates during mapping operations.

In 1982 a new national adjustment, referred to as the Geodetic Model of Australia 1982 (GMA82), was performed using all data previously included in the 1966 adjustment as well as additional, modern terrestrial and space-based observations. This new adjustment also used the gazetted Australian Geodetic Datum. The coordinate set resulting from this adjustment was accepted by the National Mapping Council in 1984 and is known as the Australian Geodetic Datum 1984 (AGD 84).

The Geocentric Datum of Australia (GDA)

By the year 2000, Australia will adopt a geocentric datum as its principle national datum. Its coordinate set will be referred to as:

The Geocentric Datum of Australia 1994 (GDA 94)

Its adoption will allow closer integration with international coordinate frameworks and navigation systems. In particular, the datum will coincide almost exactly with the datum used to support the Global Positioning System (WGS 84). This will allow GPS-derived coordinates to be used directly with GDA 94 in almost all circumstances.

All Australian data sets, including revised hard copy maps, will be progressively converted to the new coordinate system. GDA 94 coordinates have already been computed for every trig station in the Australian Primary Geodetic Network. The computations involved a much larger data set than was available at the time of AGD 84.

GPS Models

Listed below are three of the most popular brands of hand-held GPS units available here in Adelaide. All offer the same basic functional features, however the Garmin brand seems to be the most sought after, due to the large variety of models available, as well as their fast rate of acquisition time to establish a link to each of the GPS satellites.

Magellan GPS 2000XL

Delivers a 12-parallel-channel GPS with superior tracking that locks onto satellites amazingly fast and keeps a tight lock under the toughest conditons--even in dense cover!
Equally impressive, the GPS 2000 XL will last 24 continuous hours on 4AA batteries, and the 10-oz. unit fits easily in your pocket.
Five navigation screens show your position, direction, speed and progress, or use the plotter screen for the big picture.
Built tough for outdoor use with wraparound rubber armoring and scratch-proof display.
Plus it's resistant to shock and is absolutely waterproof.
Features:

200 landmarks
5 routes with 20 reversible legs
"Auto Backtrack" feature
Real-time track plotter
Sunrise/sunset lunar calculations
Re-settable trip odometer
Landmark projection upload/download capability
72 plus 1 user defined map daturns Latitude/longitude, UTM, OSGB, Finnish, Irish, Swedish, and Swiss grid coordinate systems
Position averaging to compensate for military degradation of satellite signal
6.6"x2.3"x1.3" Weight 10 oz.

Garmin GPS II

Features:

Two-way screen display for vertical or horizontal orientation.
External antenna
Zoom
Compass or graphic highway steering navigation screens
Auto start/stop trip timer
Differential ready MultiTrac8 receiver tracks and uses up to 8 satellites
Backlit 2.2"Wx1.5"H LCD
Moving map plotting
Dedicated zoom keys from .2-360 miles
TracBack
250 waypoints, 20 reversible routes, MOB
Coordinates: Lat./long., UTM/UPS, plus 7 grids, including Maidenhead
106 map datums
EZinit feature allows fast "point-and-shoot" initialization
NMEA 183 and RS-232 DGPS correction
2.32"Hx5"Lx1.62"W
Uses 4AA batteries for up to 20 hours, or 10-36V DC.
Includes wrist strap, user's manual and hook 'n' loop vehicle mount.

Garmin� GPS 12XL

Features:

Environmentally tough compact case built like Garmin's military models
External antenna capability
20 reversible routes of 30 waypoints each
TracBack navigation automatically turns your track log into a route back home
9 proximity waypoint alarms
Over 1,000 track log points
User programmable map datum and UTM grid format
4AA batteries, users's manual, carrying case, wrist strap
Up to 12 hours of battery life
5.8"Hx2.1"W1.2"D

Garmin� GPS 12 and GPS III

Small, lightweight, 12-parallel-channel handheld receivers track up to 12 satellites for precise location information.
Their waterproof construction with electro-luminescent display means you can use them anytime--rain or shine, day or night.
Comprehensive graphic icons provide visual reminders of the type of waypoints you've marked (camp, fishing spot, treestand and more).
Features:

500 user waypoints
20 reversible routes of 30 points each
MOB function
Position averaging to compensate for military degradation
TracBack function with over 1000 track log points to lead you back to your origination
Lithium battery backup
Proximity waypoint warnings
Trip computer with avg. speed, trip timer and max. speed
Over 100 map datums, including one user datum, lat./long., UTM/UPS and 7 grids including Maidenhead
Differential ready
NMEA and RS-232 DGPS interface
Uses 4AA batteries or optional DC power supply (sold separately)
Accurate to 49'.
GPS III also features

a Moving GPS Map for a detailed display of state and country boundaries, lakes, rivers, railroads, cities, coastline,
State/interstate highways and local thoroughfares.
Horizontal/vertical viewing, clock/arrival/course error warnings and trip odometer.

Eagle Explorer

Features:

12-parallel channel hand-held GPS receiver.
Internal 10 year battery back-up
4 screen positions:

Plotter screen, shows current position with coordinates in latitude/longitude, distance-to-go, speed-over-ground, bearing and course-over-ground
Position screen, displays present position, time-to-waypoint, cross track error and waypoint selected
"Window" function provides information via 10 multiple-data split-panel screens
Plotter screen displays navigational data, course line, present position, waypoint and displays your track history

100 x 65 pixel ULTRAVISION display screen
Memory for up to 200 waypoints and 20 routes plus 500 event marker icons
Back-lighted display
Operates on 4 AA batteries (batteries not included) or external power source.
Full 1-Year Manufacturer's Warranty
6-3/4"Hx2-1/4"Wx1-5/8"D.

Eagle View

Loaded with leading-edge technology the Eagle View sports a wide, high-contrast ULTRAVISION display screen, a twelve-parallel-channel receiver for the most accurate, instantaneous and reliable position updates possible.
On screen menus make operation a snap, and a convenient "Windows" feature groups different kinds of information into easy-to-read panels.
Features:

Exceptionally fast satellite lock-on
Position updates every second
Displays position in latitude/longitude
Advanced plotter graphics with ranges from 1/10 to 4000 nautical miles
"Menu" key offers easy-to-use on-screen feature selections
Low-profile GPS antenna and mounting hardware included
Back-lighted screen and keyboard
Full 1-Year Manufacturer's Warranty.

GPS Glossary

Acquisition Time:
Amount of time required for a GPS unit to lock onto 3 satellites to provide a "2D View" of present position.

Bearing (BRG):
The precise compass direction (in degrees) from your present position to the next waypoint. (Readings are selectable in either degrees magnetic or true north).

Cold Start:
The process of powering up a new GPS receiver for the first time and having it search out and lock onto the satellites by itself, without the benefit of initialization data. This procedure is slower and may require up to 15 minutes for initial satellite acquisition only.

Course Over Ground (COG):
The current direction (in degrees) that you are actually traveling. (Again selectable in degrees magnetic or true north).

Cross Track Error (XTE):
Digital reading on GPS steering screens that indicates precisely how far off you are, to the right or left of the center of the course.

Differential GPS (DGPS):
A system devised initially by the U.S. Coast Guard to improve GPS accuracy levels to within 5 meters. It employs a land-based, fixed position, DGPS reference receiver to first calculate the Selective Availability errors, then transmit the necessary correction factors to mobile GPS receivers in the area. DGPS systems do require an added beacon receiver to communicate with the standard GPS unit.

Differential Global Positioning System. DGPS utilises private satellites and/or ground stations which are precisely located to augment the GPS satellites, thus providing an extremely accurate GPS calculation.

Dilution of Precision:
The error in your GPS calculation caused by poor geometry of GPS satellites.

Distance To Go (DTG):
Digital readout (selectable in miles, nautical miles or kilometers) displayed only when navigating to a waypoint. It simply indicates the remaining distance from your present position to the next waypoint.

Dry Nitrogen Filled:
A special process in which a unit is sealed and filled with dry nitrogen to help prevent fogging and internal corrosion.

Ephemeris Error:
An innaccurate prediction of the satellite's orbit will cause an error in your position calculation, and is refered to as Ephermeris Error

Event Market/Icon:
A graphic symbol that can be placed on the plotter screen (and saved in memory) to represent some special event or area of interest to the GPS user.

GGA:
An NMEA sentence in latitude, longitude, ant altitude

GLL:
An NMEA sentence in latitude and longitude.

GLONASS:
Russian satellite system with a comparable number of satellites to the American GPS system.

GNSS:
Global Navigation Satellite Systems. GPS system in this category.

HDOP:
Horizontal Dilution of Precision. The horizontal component of the error caused by poor geometric position of GPS satellites, which results is a less precise reading from your GPS receiver.

Initialization:
Refers to the simple procedure of telling a new GPS receiver "where it is", when it is turned on for the first time. Information required includes: approximate present position in latitude/longitude coordinates; and the current local time and date.

Ionosphere:
An area of the atmosphere approximately 40 miles thick, which contains charged particles.

Maximum Latency:
Setting the Maximum Latency, specifies the maximum age of the Differential GPS signal that is acceptable. This allows you to eliminate the inaccuracy caused by old readings.

Multiplexing Receiver:
GPS receiver that rotates a small number of channels to multiple satellites in order to provide current positioning data. Typically, multiplexing receivers require more time for satellite acquisition and lock on, and are not as accurate as parallel channel receivers. Multiplexing receivers are also more prone to lose satellite fix in dense woods compared to parallel channel GPS receivers.

Navigation:
Navigation is the act or practice of locating a position and plotting a course.

NAVSTAR:
NAVSTAR is a constellation of 24 satellites, launched by the American Department of Defence. They are the satellites used by most GPS receivers to calculate your position.

NMEA 0183 v2:
The US national standard for navigational related information, which FieldWorker uses to receive GPS information from GPS receivers. Any receiver which transmits in this protocol can be utilised by FieldWorker.

Parallel Channel Receiver:
GPS receiver that simultaneously tracks multiple satellites to provide the fastest, most reliable and accurate navigational data, under the most adverse conditions.

Pixel:
A picture element; the "building blocks" of a liquid crystal display (LCD). The greater the number of vertical and horizontal pixels, the better the screen resolution and detail.

Plotter Display:
Provides an overhead "bird's eye" view of your current position relative to the waypoints and event marker/icons you have saved. A dotted line marks the shortest route to the chosen waypoint, and a recorded plot trail displays the path you have taken so far.

Position Display:
ne of the primary navigational data screens that emphasize the present position latitude/longitude coordinates, as well as other helpful navigational information.

Present Position:
Current location on the face of the Earth, in terms of the specific latitude/longitude coordinates, displayed in degrees/minutes/and thousandths of a minute.

Programmable Windows:
The ability to customize existing split panel window groupings with specific combinations of navigational data.

Relative Bearing
The bearing of an object relative to you, with 0� directly in front of you. If you are facing East, with an object to your South, it would have a relative bearing of 90�. An object to the North would have a bearing of 270�.

Route:
Consists of two or more waypoints combined in a course of travel. It provides the automatic capability to navigate through several waypoints, without having to reprogram the unit after arriving at each one. Once programmed into the GPS unit, the route provides the option of navigating forward through the waypoints or in reverse order.

Satellite Status Display:
An information screen that shows technical data about each satellite in view. Information includes receiver channel numbers; actual satellite I.D. numbers; status of satellite tracking (T) or searching (S); satellite elevations and azimuths; signal to noise ratios (SNR) (the higher the number, the better); and dilution of precision ratings (GDOP is most important; the smaller the number, the better potential accuracy).

Savable Plot Trails:
The capability to save your actual plot trail crated on the plotter screen, thereby enabling the GPS user to either backtrack the course immediately, or save and retrace the trip at a later time.

Selective Availability (S/A):
The system used by the U.S. Department of Defense to intentionally degrade the accuracy of satellite GPS signals being transmitted to civilian GPS receivers. All brands of civilian GPS receivers are equally affected by S/A. With random S/A on , the government has guaranteed that civilian GPS accuracy levels will consistently be 100 meters or less, 95% of the time. If S/A is turned off, those accuracy levels will improve to 10 to 15 meters consistently.

Speed Over Ground (SOG):
Digital reading that indicates your current ground speed. (Selectable in miles per hour, knots or kilometers per hour).

Steering Screen:
Shows a graphic "highway view" of the GPS user's course over ground. Provides helpful instructions as to how far off course, which direction to steer, right or left, to make corrections, and displays related navigational data pertaining to the waypoint.

Straight Line Navigation:
The standard method of navigation used by recreational GPS products. When commanded to "navigate to a waypoint", the unit draws a straight, dotted line from the present position to the selected waypoint. It's the shortest, most direct route to the destination. Caution: Straight line navigation does not take into account any obstacles in the path; interim waypoints may be required to navigate safely around obstacles.

Time To Go (TTG):
Digital reading showing the time remaining from your current position to the next waypoint. This function takes into account your Distance To Go (DTG), and your Velocity Made Good (VMG) to give you as closely as possible the amount of time left to reach your waypoint. Displayed in hours, minutes and seconds, it will continue counting down until your waypoint is reached.

True and Magnetic North:
True north is the top of the world, where all lines of longitude converge. Magnetic north is the location our compasses point to; it lies several hundred miles to the south of true north, at a location in Canada.

True Bearing:
The bearing of an object relative to true North. If you are facing East with an object to your South, it has a true bearing of 180�.

Velocity Made Good (VMG):
Digital speed reading (similar to SOG) that compensates for progress being made toward a waypoint. For example, when traveling directly on course toward a waypoint, the SOG and VMG readings may match. However, when traveling off course, the VMG reading will typically be slower than the SOG. VMG is a true indication of the speed being made to selected waypoint.

Waypoint:
Location, spot or destination (latitude/longitude) that can be stored in memory to be recalled and used at a later time for navigation purposes. Simply think of it as an "electronic address".

References Used