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The Global Positioning System (GPS)

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 :

T he 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.

Hand Held GPS

Civilian Use of the System:

G PS'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:

E ach 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.

Accuracy:

I n 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. (Editor's Note: Selective Availability was turned off by the government, but could be reinstated at any time without notice for national security.)

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.

USCG and WAASDGPS

 The following information was provided to Weak Industries, Inc., via Sigma Marine Supply, Tarpon Springs, Florida in response to questions posed to the United States Coast Guard in May 2001. We greatly appreciate Sigma's permission to reprint this vital information. 

"FW Your questions on WAASDGPS"

"On August 24, 2000, the Federal Aviation Administration (FAA) announced that their space-based, L-Band Wide Area Augmentation System (WAAS) became available for use by "some aviation and al non-aviation" users. The FAA announcement has prompted numerous inquiries to the Coast Guard regarding the maritime use of WAAS and the status of the Coast Guard DGPS system. The following Questions and Answers are directed at helping to clarify the status of these two systems for the mariner.

1) Why did the Government design and build two different GPS augmentation systems?

a. The 1994 National Telecommunications and Information Administration (NTIA) Technical Report to DOT on a National Approach to Augmented GPS Services studied the necessity of expanded government efforts in providing DGPS services. Its goal was to recommend the optimum integrated system to meet aviation and terrestrial navigation needs. A variety of systems we being proposed at the time. The study concluded that a combination of two systems, the FAA's Wide/Local Area Augmentation System (WAAS/LAAS and the USCG's DGPS system, was the optimum mix. This integrated system, consisting of the L-band line-of-sight WAAS for aviation users, and the terrain-following medium frequency DGPS for maritime and terrestrial users, meets the vast majority of the nation's precise navigation and positioning needs.

2) Is WAAS currently certified for maritime navigation?

a. NO. WAAS is not yet fully operational and is currently in a testing status, undergoing further development. It is not certified for use as a safety of life navigation system in the maritime navigation environment. WAAS may be used, with caution, in the maritime environment to improve overall situational awareness, but it should not be relied upon for safety-critical maritime navigation. The Maritime DGPS Service, on the other hand, is fully operational and meets all the standards for the harbor entrance and approach phases of navigation.

3) After WAAS reaches initial operating capability (IOC) in a few years, will it be suitable for maritime navigation?

a. WAAS is not optimized for surface (maritime and terrestrial) use, rather, it is designed primarily for aviation use. It is intended to eventually support aeronautical enroute through precision approach air navigation. The current WAAS test signals are transmitted by two-geo-stationary satellites on a line-of-sight, L-band radio frequency. This means that if anything obstructs the view of the portion of the sky where the satellite is, the WAAS signal will be blocked. Since geo-stationary satellites are positioned over the equator, the farther north users are, the lower the geo-stationary satellites are in the sky - increasing the likelihood of an obstruction. In contrast, the medium frequency (285-325 kHz) radio beacon-based Maritime DGPS Service is optimized for surface (maritime and terrestrial) applications because it's ground wave signals "hug the earth" and wrap around objects. This means that the Coast Guard DGPS system is well suited for the maritime environment (down in the "ground clutter") where a geo-stationary satellite can be clocked by terrain, harbor equipment and other man-made and natural objects.

4) Can the Coast Guard's DGPS system be used by aviation?

a. That's up to the FAA. However, the Coast Guard's system was designed with the surface (maritime and terrestrial) user in mind. It was neither designed not intended to meet aviation requirements. Although aviation users could potentially get some modest benefit from the Coast Guard's DGPS for applications such as surface traffic management at airports of General Aviation, it could not attain the type and level of aeronautical service which WAAS and LAAS are designed, without significant re-engineering.

5) Is the Coast Guard DGPS system a "transient technology" that is here today but will be gone tomorrow?

a. NO. DGPS has already been adopted globally as an international maritime standard established by the 1994 International Telecommunications Union document ITU-R-M.823. It meets IMO Resolution A 815(19) standards for navigation in harbor entrances and approaches. Over 40 nations have fully embraced this robust technology and are implementing DGPS services identical to our own.

6) Which system is more accurate, WAAS or DGPS?

a. On the average, WAAS and DGPS accuracy are virtually the same, although DGPS accuracy is better when the user is near a DGPS transmitting site. The WAAS architecture is designed to provide uniform seven (7) meter accuracy (95% of the time) regardless of the location of the receiver - within the WAAS service area. DGPS is designed to provide better that ten (10) meter navigation service (95% of the time), but typically provides better than one (1) meter horizontal positioning accuracy (95% of the time) when the user is less than one hundred (100) nautical miles from the DGPS transmitting site. Accuracy degrades at a rate of approximately 1 meter per hundred nautical miles as the user moves away from the transmitting site. A total of 56 maritime DGPS sites provide coastal coverage of the continental United States, the Great Lakes, Puerto Rice, portions of Alaska and Hawaii, and portions of the Mississippi River Basin.

Conclusions : Once WAAS becomes fully operational, the combination of Coast Guard and FAA systems is expected to provide a robust, complimentary service to all modes of transportation. We look forward to the day that industry provides the public with a fully integrated receiver, one that uses all available radio navigation systems to provide unprecedented accuracy, integrity, and availability.

 Despite the differences between DGPS and WAAS, it should always be kept in mind that both services ultimately rely upon a single navigation system - GPS - which is vulnerable to interruption at any time. This lends additional credence to the recommended practice of using all available means of navigation and not relying upon any single system. Remember, prudent mariners will always keep looking out the window!" 

Differential GPS:

D ifferential 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:

T he 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:

T he 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:

I t 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 go back to the Southern Charm Catalog.

©Rod and Susie Stebbins, Weak Industries, Inc.

 


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