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Old 06-08-2005, 03:18 PM
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Default The GPS System

By : Captain Rod & Susie Stebbins

This article is produced in cooperation with Magellan Systems Corporation, 960 Overland Court, San Dimas, CA 91773. Without this collaboration, this article might have gone on forever to try to explain the magic of satellite positioning, encrypting reasoning and communications. Their significant contributions in this venture are greatly appreciated.
The Owners :
The Global Positioning System (GPS) is a constellation of 24 satellites that orbit the earth twice a day, transmitting precise time and positioning information to anywhere on the globe, 24 hours a day. The system was designed and deployed by the U S Department of Defense to provide continuous, worldwide position and navigation data for the use of the United States and allied military forces.
Civilian Use of the System:
GPS's broad commercial applications were recognized early in the system's development, and it was decided to allow free access to GPS signals with certain constraints applied.
How it Works:
Each GPS satellite broadcasts two signals, PPS (Precise Positioning Service) and SPS (Standard Positioning Service). The PPS signal is an encrypted military-access code. The SPS signal is an unencrypted, spread-spectrum signal broadcast at 1575.42 MHz. Unlike signals from land-based navigation systems, the SPS signal is virtually resistant to multipath and night-time interference, and is unaffected by weather and electrical noise.
The SPS signal contains two types of orbit data, almanac and ephemeris. Almanac data contains the health and approximate location of every satellite in the system. A GPS receiver collects almanac data from any available satellite, then uses it to locate the satellites that should be visible at the receiver's location. Ephemeris data contains the precise orbital parameters of a specific satellite.
The GPS receivers listen to signals from either three or four satellites at a time and triangulate a position fix using the interval between the transmission and reception of the satellite signal. Any given receiver tracks more satellites than are actually needed for a position fix. The reason for this is that if one satellite becomes unavailable, the receiver knows exactly where to find the best possible replacement. Three satellites are required for two dimension positioning. Two dimension positioning reports position only. Four satellites are required for three-dimension positioning, that is to say position and elevation.
In general, an SPS receiver can provide position information with an error of less than 25 meters, and velocity information with an error of less that 5 meters per second. It is good to mention here that most "Chartmate" types are accurate only to 15 meters. It is because the system is so accurate, the US Government has activated what is known as Selective Availability (SA) to maintain optimum military effectiveness. Selective availability inserts random errors into the timing and ephemeris information broadcast by the satellites, which reduces GPS SPS code accuracy to between 25 and 100 meters.
For many applications, 100 meter accuracy is more than acceptable. For applications that require much greater accuracy, the effects of SA and environmentally produced errors can be overcome by using a technique called Differential GPS, which increases overall accuracy.
Differential GPS:
Differential GPS (DGPS) uses a GPS receiver at a fixed point whose position is known with submeter accuracy. This is the control unit. The receiver collects data from all visible satellites and computes predicted satellite ranges, which are compared with actual ranges. The difference is the satellite range error, which is then converted to correction signals for use by a roving receiver. The roving receiver would be to one on the system users boat.
It is assumed that this correction will be the same for other GPS receivers that in the same area and are using the same satellites for positioning. If the correction is communicated to other receivers in the area, usually by a beacon on the same site, the range error can be removed from satellite signals and precise fixes calculated by these receivers. It should be noted that not all data errors can be corrected in this way. Errors that are caused by receiver noise (which is inherent in any GPS receiver) and multipath problems cannot be eliminated with differential equipment. Multipath errors occur when the receiver's antenna "sees" the reflections of signals that have bounced off of surrounding objects.
Using DGPS to eliminate the effects of correctable errors requires that the user's GPS receiver be connected to a compatible Differential Beacon Receiver (DBR) and be within range of the broadcasting beacon. The DBR accepts and demodulates the broadcast corrections, which are then relayed to the GPS receiver. The GPS receiver applies the corrections to the navigation data it uses to compute a position solution, and then displays differentially corrected data. Care must be taken to ensure that the DGPS receiver and the GPS receiver are compatible for this procedure to be successful.
Monitoring and Controlling GPS:
The Global Positioning System is monitored and controlled by the US Air Force, which is responsible for updating and maintaining exact satellite position and signal data accuracy. It is also responsible for performing maintenance on the satellites, which may require a given satellite be taken "off-line." Since the system is subject to periodic updates and changes, the almanac data broadcast by the satellites is current only for a limited time, generally about six (6) months.
GPS Information Sources:
The needs of the worldwide civil GPS user community are served by the Civil GPS Information Center (GPSIC) located in Virginia. The GPSIC is operated and maintained by the United States Coast Guard for the US Department of Transportation. Its primary function is to provide information on the Global Positioning System and satellite status and to serve as a point of contact. The GPSIC has general GPS literature available free upon request. The Center also maintains up-to-date almanac data and Operational Advisory Broadcasts containing current constellation status and planned satellite outages.
There are three ways to quickly obtain current advisories and almanac data from the GPSIC and they are:
Recorded phone message can be obtained at (703) 313-5905.
Computer bulletin board is located at (703) 313-5910.
Live information is available by calling (703) 313-5900.
There are other sources for GPS information, ranging from free, governmentally produced literature to purchased professional literature and seminars. The geography department of your local college or the local office of the National Geodetic Survey may also be able to offer assistance.
Map Datum:
A map datum is a mathematical description of the earth or a part of the earth. It is used to correctly assign real-world coordinates to points on a map or a chart. Because the earth has a very irregular shape, taking accurate measurements of doing calculations on the earth's true surface is very difficult and complicated. A mathematically regular shape is much easier to deal with, if the shape accurately represents the earth's true shape. The most representative shape is an ellipsoid.
A map datum is a mathematical description of the earth or a part of the earth, and is based on the ellipsoid or the arc of an ellipsoid that most closely represents the area being described. In addition, the datum is centered at a specific location know as the datum origin. A datum may describe a small part of the earth, such as WGS84, depending on which ellipsoid or ellipsoidal arc is selected and where the datum origin is. Since datums use different ellipsoids and have different origins, the Latitude and Longitude coordinates of the same position differs from one datum to another. The difference may be slight or great, depending on the datum involved, but will affect the apparent accuracy of the positioning information provided by a GPS receiver.
Most GPS's and Chartmate type equipment use the WGS84 datum, which is the model of the earth that is the closest possible average of the planet as a whole. Which datum your charts are based on is usually found in the chart's legend. Occasionally, electronic charts do not include this information, which means that position coordinates determined with the Chartmate type equipment may not appear to agree with coordinates determined from a printed chart. When the variations are large it will be necessary to insert correction factors into the equipment. These correction factors will then be applied to position fixes before they are displayed.
Navigation Responsibilities:
It is important to remember and understand that aside from all of great help one gets from all of the electronic navigational equipment, it is still the final responsibility of the boat driver to know where he or she really is and where it is that he or she is going and how to get there. It is important to navigate with the assumptions that the electrical system on any boat may just decide to take a vacation too. There is little reason to rely on bargain equipment - check out what is available and purchase as much for features and brand reputation as for cost of the equipment. A few extra bucks will buy the best and the best does a better job.
Just for the record, a meter is just a bit longer than a yard which is three (3) feet. The following will allow you to calculate the meter explanations above to better understand what you might expect to encounter if your installation is sterile, properly isolated, grounded and your antenna is not impeded with other electronic interference (other equipment on the same boat and in the proximity of the DGPS and GPS antennas.
One Meter contains = 39.37 Inches.
One Meter contains = 3.281 Feet.
One Meter contains = 1.094 Yards.
For sources of GPS and Loran-C coordinates for fishing, diving, wrecks and reefs check out Weak Industries and for more writings by these authors check out Capt. Rod and Susie Stebbins
For additional information on GPS and other marine electronics, visit Sigma Marine.
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Old 06-08-2005, 03:25 PM
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Nice post Jim....
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Old 07-18-2008, 06:56 PM
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Default Re: The GPS System

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The System: The Global positioning System functions by using a number of satellites in orbit around the earth. These satellites in orbit output broadcast their position in orbit and a very accurate time value. This data is then received somewhere on the planet by a GPS receiver. With the information from 3 or more satellites, a unique position can be obtained. With 4 or more satellites being tracked, the position accuracy greatly increases and altitude can also be obtained.
Navigation: All GPS does is provide position and time. The GPS receiver can utilize a series of position and time values to derive other functions. Navigational information such as speed, distance, bearing, track and a host of others are obtained from this information. When a graphical display is utilized, this information can be provided to the user in an easy to understand format. Not only can the digital values be displayed, but a history of previous positions (Waypoints, Icons and Trails) can be referenced to make the current information for position and navigation more relevant.
Mapping: GPS does not necessarily mean that there is a map involved. Many current GPS products have some mapping background. This adds additional frames of reference to relate the position information. This can provide information that can be visible as well as not apparent to inspection at the time. Visibly verifiable information would be features like roads, intersections, shoreline and points of interest (POI's). Other factors that may be on the map that are not visible are items like contour lines for topographical or hydrographic mapping. These items can be referenced from the map but are not visible at the actual location. Mapping also allows for path planning. The shortest distance between two points may be a straight line. If an obstruction is in the way, a different route may not only prove faster but safer as well.
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Old 07-18-2008, 07:02 PM
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Default Re: The GPS System

GPS Tutorial
Introduction--What Is GPS?
The Global Positioning System (GPS) is a space age navigational system that can pinpoint your position anywhere on the globe, usually within a few yards or meters. This amazing technology is available to everyone, everywhere, day and night, and best of all, at no cost for use of the navigational data. GPS uses a constellation of 24 satellites in precise orbits approximately 11,000 miles above the earth. The satellites transmit data via high frequency radio waves back to Earth and, by locking onto these signals, a GPS receiver can process this data to triangulate its precise location on the globe.
GPS operates 24 hours a day, in all weather conditions, and can be used worldwide for precise navigation on land, on water and even in the air. Some of its many current applications include: boating, fishing, hunting, scouting on land or from the air, hiking, camping, biking, rafting, pack trips by horseback, hot air ballooning, general aviation, snowmobiling and skiing, search and rescue, emergency vehicle tracking, 4 wheeling, highway driving and a host of other outdoor activities where accurate positioning is required.
How GPS Determines Your Position
GPS uses satellite ranging to triangulate your position. In other words, the GPS unit simply measures the travel time of the signals transmitted from the satellites, then multiplies them by the speed of light to determine exactly how far the unit is from every satellite it's sampling.
By locking onto the signals from a minimum of three different satellites, a GPS receiver can calculate a 2D (two-dimensional) positional fix, consisting of your latitude and longitude. By locking onto a fourth satellite, the GPS can compute a 3D (three-dimensional) fix, calculating your altitude as well as your latitude/longitude position.
In order to do this Lowrance uses a 12 parallel channel receiver in all of its current products. Three of the channels lock on to satellites for triangulation. Another channel locks on to a fourth satellite for 3D navigation, which lets the unit calculate altitude in addition to latitude and longitude. These four channels continuously and simultaneously track the four satellites in the best geometrical positions relative to you. The additional eight channels track all other visible satellites, then add this data to the data from the original four satellites. The unit then over-resolves a solution, creating an accuracy-enhanced reading. The additional channels also ensure reliable, continuous and uninterrupted navigation, even in adverse conditions such as valleys or dense woods.
GPS Accuracy
GPS was conceived in the 1970s, and is controlled by the United States Department of Defense. Although GPS was initially envisioned for military use, the Government realized early on that there would be numerous civilian applications as well. Subsequently, the Department of Defense (DOD) created two transmission codes; the P code (Precision code) for military use, and the C/A code (Civilian Access code) for civilian use.
The highest accuracy levels were to be reserved for the military so as to prevent hostile enemy attacks against the U.S. using our own navigational system. However, once in operation, the civilian GPS receivers using the C/A 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 is called Selective Availability or S/A. This reduced the civilian GPS accuracy levels to being within 100 meters or less, 95% of the time. However, typical accuracy for most users averaged between 20 and 50 meters the majority of the time. You could easily see the effects of S/A on a GPS receiver when you were not moving. Typically, there would be random movements in speed, altitude and position readings, along with slow position "wandering" on the plotter trail. This was easily seen when you were on a .1 or .2 mile zoom range, and not moving. For example, while parked at the dock in your boat, you would see unexplainable changes in your digital speed readings up to a few miles per hour, even though you were not moving.

Plot of position accuracy using standard Lowrance GPS receiver (stationary).
Note the differences in scale.

5.5 hour period immediately prior to shutoff of selective availability
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8 hour period immediately after shutoff of selective availability
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Effective May 2, 2000 selective availability (S/A) has been eliminated. The United States Department of Defense now has the technology to localize the control system to deny GPS signals to selected areas. It is not often that your electronics products increase in value after you've purchased them. Now boaters, aviators, drivers, hikers, hunters and outdoor enthusiasts of all types can locate their position up to ten times more precisely (within 10 to 20 meters) and navigate their way through unfamiliar terrain. Anglers can now return to their favorite spot on a lake or river instead of just their favorite area. A GPS receiver like the GlobalMapĀ® 100 (with extensive mapping capabilities and Lowrance's optional IMS MapCreate™ Marine and Recreation CD-ROM) in combination with advanced technology of today's GPS management will take you anywhere you want to go.
The decision to allow civilians so much accuracy in location information was finally made because GPS is continually playing a more important role in the lives of people around the world - it's becoming a national utility. GPS is the global standard in navigation because it is completely free of charge to the public.
Differential GPS (DGPS)
Differential GPS, or DGPS, has been developed to improve GPS accuracy to within a few meters. DGPS was originally initiated by the U.S. Coast Guard to counter the accuracy degradation caused by Selective Availability. Even with S/A now eliminated, DGPS continues to be a key tool for highly precise navigation on land and sea. DGPS technology adds a land-based reference receiver – located at an accurately surveyed site – to the other GPS components. 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. This allows the station to 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. 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 and calculations.
The two most common sources of corrective DGPS signals currently are: (1) Coast Guard, land-based beacon transmitters, broadcasting the data at no charge to the public, covering all coastal areas and much of the inland USA as well; and (2) FM radio sub carrier transmissions available both in coastal and inland areas, but limited to paid subscribers. In order to receive DGPS correction data from Coast Guard beacon transmitters, a mobile GPS unit requires a separate beacon receiver. And to receive FM sub carrier DGPS signals from local subscriber radio stations, the GPS unit requires a separate FM receiver, normally the size of a pager. Naturally, your GPS unit must have the capability to both receive and process DGPS data.
Wide Area Augmentation System (WAAS)
GPS is plenty accurate for route navigation, but the U.S. Federal Aviation Administration has special need for aircraft traffic control that go beyond basic GPS. The FAA has a plan under way to boost GPS performance even further with its Wide Area Augmentation System, or WAAS. This GPS add-on will include a time control element that will help airliners fly closer together while avoiding collisions. In addition to carefully spacing airplanes along travel corridors, WAAS will eventually make instrument landings and takeoffs more accurate as it replaces existing aviation navigation systems.
Non aviators can use WAAS signals to make their GPS navigation even more accurate. However, WAAS has some limits you should know about.
First, the U.S. government has not completed construction of the WAAS system, so it is not yet fully operational. The ground stations are in place, but only a few of the needed WAAS satellites have been launched.
WAAS can boost the accuracy of land GPS navigation, but the system is designed for aircraft. The satellites are in a fixed orbit around the Equator, so they appear very low in the sky to someone on the ground in North America. Aircraft and vessels on open water can get consistently good WAAS reception, but terrain, foliage or even large man-made structures frequently block the WAAS signal from ground receivers.
You'll find that using your GPS receiver without WAAS is both easy and amazingly accurate. It's easily the most accurate method of electronic navigation available to the general public today. Remember, however, that this receiver is only a tool. Always have another method of navigation available, such as a map or chart and a compass.
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