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For a glossary of the majority of terms used in the GPS world, visit our GPS glossary page. Click HERE to go to the GPS Glossary.

Frequently Asked Questions:

 

What is GPS?

The Fundamental Components of GPS

GPS Limitations

How Does a GPS Receiver Determine Positions?

Differential GPS And The Base Station Concept

Base Stations - The Source for Reference Data for DGPS

What are Typical Applications of GPS

What Is GPS?

The Navigation Satellite Timing And Range Global Positioning System, or NAVSTAR GPS, is a satellite based radio-navigation system that is capable of providing extremely accurate worldwide, 24 hour, 3-dimensional locational data (latitude, longitude, and elevation). The system was designed and is maintained by the US Department of Defense (DOD) as an accurate, all weather, navigation system. Though designed as a military system, it is freely available with certain restrictions to civilians for positioning. The system has reached the Final Operational Capability (FOC) stage, meaning the system has a complete set of at least 24 satellites orbiting the earth in a carefully designed pattern. As of November 1995, there were 25 healthy satellites broadcasting signals enabling 3D positioning 24hours a day.

The Fundamental Components Of GPS

The NAVSTAR GPS has three basic segments; space, control, and user. The space segment consists of the orbiting satellites making up the constellation. This constellation is presently comprised of 25 satellites, each orbiting at an altitude of approximately 11,000 nautical miles, in one of six orbital planes inclined 55 degrees relative to the earth's equator. Each satellite broadcasts a unique "bar-code", known as Pseudo Random Noise (PRN) code, that enables GPS receivers to identify the satellites from which the signals came, and makes positioning possible.

The control segment, under DOD's direction, oversees the building, launching, orbital positioning, and performance monitoring. Monitoring and ground control stations, located around the globe near the equator, constantly monitor the performance of each satellite and the constellation as a whole. A master control station updates the information component of the GPS signal with satellite ephemeris data and other messages to the users. This information is then decoded by the receiver and used in the positioning process.

There are two classes of GPS service; the Precise Positioning Service (PPS) which is available only to users authorized by the military, and the Standard Positioning Service (SPS) which is available for civilian use. The civilian GPS user community has increased dramatically in recent years, due to the emergence of low cost portable GPS receivers and the ever expanding areas of applications in which GPS was found to be very useful. Some of these applications are: surveying, mapping, navigation and vehicle tracking. The number of civilian users now greatly outnumber military users.

GPS Limitations

Though GPS can provide worldwide, 3D positions, 24 hours a day, in any type of weather, the system does have some limitations. First, there must be a relatively clear "line of sight" between the receiver's antenna and several orbiting satellites. Anything shielding the antenna from a satellite can potentially weaken the satellite's signal to such a degree that it becomes too difficult to make reliable positioning. As a rule of thumb, an obstruction that can block sunlight can effectively block GPS signals.

The receiver must receive signals from at least four satellites in order to be able to make reliable position measurements. In addition, these satellites must be in a favorable geometrical arrangement. The four satellites used by the receiver for positioning must be fairly spread apart. In areas with a relatively open view of the sky, this will almost always be the case because the NAVSTAR GPS satellite constellation was strategically designed to provide at least four satellites with favorable geometry.

How Does A GPS Receiver Determine Positions?

The position of a point is determined by measuring distances from the receiver to at least 4 satellites. The GPS receiver "knows" where each of the satellites is at the instant in which the distance was measured. These distances will intersect only at one point, the position of the GPS receiver (antenna). How does the receiver "know" the position of the satellites? Well, this information comes from the broadcast ephemeris that are down-loaded when the GPS receiver is turned on. The GPS receiver performs the necessary mathematical calculations, then displays and/or stores the position, along with any other descriptive information entered by the operator from the keyboard.

The way in which a GPS receiver determines distances (called pseudo-ranges) to the satellites depends on the type of GPS receiver. Basically, there are two broad classes: carrier phase based and code based.

Differential GPS And The Base Station Concept

Differential GPS (DGPS) can be employed to eliminate the error introduced by SA and other systematic errors. Differential GPS requires the existence of a base station, which is simply a GPS receiver collecting measurements at a known x , y , z (latitude, longitude, and elevation). The base station's antenna location must be located precisely, using carrier phase GPS or other, traditional surveying techniques. The base station may store measurements for post processed DGPS or broadcast corrections over a radio frequency (for real-time DGPS), or both.

The assumption made with the base station concept is that errors affecting the measurements of a particular GPS receiver will equally affect other GPS receivers within a radius of 200-300 miles. If the differences between the base station's known location and the base station's locations as calculated by GPS can be determined, those differences can be applied to data collected simultaneously by receivers in the field. These differences can be applied in real-time if the GPS receiver is linked to a radio receiver designed to receive the broadcast corrections. This is especially applicable for accurate navigation. More often in GIS applications, these differences are applied in a post-processing step after the collected field data has been downloaded to a computer running a GPS processing software package. GPS processing software is typically integrated with GPS hardware and thus is provided by the receiver manufacturer. As a rule, post processed DGPS is considered slightly more accurate than real-time DGPS.

Base Stations - The Source For Reference Data For DGPS

There are several permanent GPS base stations currently up and running in New Jersey and in surrounding states that can provide the users of code based receivers with data necessary for differentially correcting positions. In addition, the US Coast Guard's Continuously Operating Reference Station (CORS) DGPS beacon located at Sandy Hook broadcasts real-time DGPS corrections. A separate radio receiver is required to receive the correction signal. Reference data collected at the station is also available over the Internet as hourly files stored in the RINEX 2 format. The files are available on-line for 21 days before being archived on CD-ROM. This station is part of a coastal network of stations the US Coast Guard is planning that will provide realtime corrections over a radio frequency. Neighboring stations are located at Montauk Point, Long Island, New York (to the North) and Cape Henlopen, Delaware (to the South).

What Are Typical Applications Of GPS ?

As mentioned, GPS was initially designed as a radio-navigation system for the military. Desert Storm was a proving ground for GPS under military conditions, and the system lived up to expectations. But with the technology becoming more affordable, there has been tremendous growth in civilian GPS activity over the last several years. GPS is currently used by a number of state agencies, county planning and health departments.

GPS has been widely recognized as an accurate, efficient method for collecting geographic coordinate data that can be used in a GIS. There are many applications where GPS can be used to generate coordinates for a GIS data layer. In the New Jersey Department of Environment Protection, as well as other state agencies, GPS is being employed in a wide array of applications.

In an effort to protect the state's water resources, GPS is being used to collect the coordinates for well heads as part of New Jersey's Well Head Protection Program. GPS could also be used to produce coordinates for potable surface water intakes, and reservoir boundaries.

To more effectively manage regulatory permits across the various environmental permitting programs, GPS is being used to collect coordinates for facilities that have permits. These include facilities that discharge to surface water, ground water, air, store hazardous waste onsite and/or have underground storage tanks. Future efforts should focus on obtaining the locations of the actual point discharges that may adversely impact the state's natural resources.

The environmental monitoring programs are using GPS to generate coordinates for monitoring stations throughout the state. The water monitoring programs have been determining coordinates of sampling stations on existing water quality monitoring networks and are planning to establish a new ambient network. The radiation protection programs have collected coordinates for the sampling stations used to monitor radiation levels at various distances from the state's two nuclear power plants.

Natural resource programs plan to use GPS in forest management applications including mapping the areas of particular forest tree types. The endangered species protection programs plan to collect endangered species locations as well as map critical habitats areas.

New Jersey plans to use GPS in emergency response applications. Should a major oil spill occur in New Jersey waters, coordinates for the spill location and aerial extent of the plume could be collected. In short order, an effective booming strategy could be developed to protect environmentally sensitive areas in the region of the spill. In the event of a major natural disaster, GPS will be used to assist in the damage assessment and inventory.

In surveying and mapping applications, activities that would normally take months now take only a few days utilizing GPS. Updating GIS data now can be done quickly, without manually digitizing from a series of maps that may not meet accuracy standards.

GPS is also being used quite extensively in the commercial shipping, fishing and recreational boating industries. Whether navigating through narrow shipping channels, to favorite fishing locales, or determining the most direct course from point A to point B, GPS is an affordable way to obtain accurate locational data.

Navigation for private and commercial aviation is a big market for GPS. There is a great deal of interest in using GPS in the future to fully automate the landings of aircraft, and to assist in collision avoidance in the air and on the ground.

Vehicle tracking has become a major application with GPS. A manager can track the locations of pick-up and delivery vehicles. Transportation utilities are testing GPS-based fleet management systems that will provide the capability to monitor on-time performance or breakdowns, and keep commuters informed. Transit authorities are using GPS for AVL (automatic vehicle location) to track the location of buses and to detect traffic problems.

The New Jersey Department of Transportation (NJDOT) is planning to use GPS to collect data on roadway feature locations for a roadway inventory. NJDOT will also be using GPS to improve its GIS map base.

New Jersey's Geodetic Survey Section is using GPS to develop a more dense geodetic control network for New Jersey.

Nearly 90% of control surveying for photogrammetry is now performed with GPS. It is clear that GPS is an exciting technology that will provide many users a useful locational tool. As GPS becomes less expensive and increasingly accepted, there is no doubt that many creative uses and applications will evolve.