Click any of the questions below to find out more information about the topics.
Beginner Information
GPS (Global Positioning System) is a navigation technology that provides precise time and location anywhere, anytime and under any atmospheric conditions, by using the NAVSTAR satellites. The 24 satellites that make up the GPS space segment are orbiting the earth about 12,000 miles above us. They are constantly moving, making two complete orbits in less than 24 hours. These satellites are travelling at speeds of roughly 7,000 miles an hour. GPS satellites are powered by solar energy. They have backup batteries onboard to keep them running in the event of a solar eclipse, when there's no solar power. Small rocket boosters on each satellite keep them flying in the correct path. Here are some other interesting facts about the GPS satellites (also called NAVSTAR, the official U.S. Department of Defense name for GPS): The first GPS satellite was launched in 1978. A full constellation of 24 satellites was achieved in 1994. Each satellite is built to last about 10 years. Replacements are constantly being built and launched into orbit. A GPS satellite weighs approximately 2,000 pounds and is about 17 feet across with the solar panels extended. Transmitter power is only 50 watts or less.
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 is a satellite-based navigation system that works by receiving navigation messages from satellites and calculating locations.
Each GPS satellite transmits data that indicates its location and the current time. All GPS satellites synchronize operations so that these repeating signals are transmitted at the same instant. The signals, moving at the speed of light, arrive at a GPS receiver at slightly different times because some satellites are further away than others. The distance to the GPS satellites can be determined by estimating the amount of time it takes for their signals to reach the receiver. When the receiver estimates the distance to at least four GPS satellites, it can calculate its position in three dimensions.
There are at least 24 operational GPS satellites at all times plus a number of spares. The satellites, operated by the U.S. Department of Defence, orbit with a period of 12 hours (two orbits per day) at a height of about 11,500 miles travelling at near 2,000mph. Ground stations are used to precisely track each satellite's orbit.
GPS receivers locate the satellites transmitting the incoming signals and use CDMA (Code Division Multi Access) method to identify individual codes. This then means the GPS system is able to identify each satellite's unique ID to calculate precise location and navigational data. 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. Here are the steps:
1. All satellites have clocks set to exactly the same time
2. All satellites know their exact position from data sent to them from the systems controllers
3. Each satellite transmits its position and a time signal
4. The signals travel to the receiver delayed by distance traveled
5. The differences in distance traveled mark each satellite appear to have a different time
6. The receiver calculates its own position.
- How many GPS Satellites orbit the earth ?
There are just under 30 navigational satellites orbit the Earth and more may be added in the future. Each satellite makes one Earth orbit every 12 hours. The satellite's orbit repeats almost the same ground track (as the earth turns beneath them) once each day. The orbit altitude is such that the satellites repeat the same track and configuration over any point approximately each 24 hours (4 minutes earlier each day). There are six orbital planes (with nominally four Space Vehicles in each), equally spaced (60 degrees apart), and inclined at about fifty-five degrees with respect to the equatorial plane. This constellation provides the user with between five and eight Space Vehicles visible from any point on the earth.
- How powerful is the GPS Signal ?
Civilian GPS currently uses channel L1. To give you some idea of where the L1 signal is on the radio dial, your favorite FM radio station broadcasts on a frequency somewhere between 88 and 108 MHz. The GPS signal is transmitted on the L1 frequency of 1575.42 MHz in the UHF band. The satellite signals are also very low power signals, on the order of 20-50 watts. Your local FM radio station is around 100,000 watts. Imagine trying to listen to a 50 watt radio station transmitting from 12,000 miles away. That's why it's imperative to have a clear view of the sky when using your GPS.
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.
NAVSTAR is an acronym for Navigation Satellite Timing and Ranging, a name given to the GPS satellite system by the US Government.
GPS satellites transmit two low power radio signals, designated L1 and L2. Civilian GPS uses the L1 frequency of 1575.42 MHz in the UHF band. The signals travel by line of sight, meaning they will pass through clouds, glass and plastic but will not go through most solid objects such as buildings and mountains. A GPS signal contains three different bits of information a pseudorandom code, ephemeris data and almanac data. The pseudorandom code is simply an I.D. code that identifies which satellite is transmitting information. You can view this number on your Garmin GPS unit's satellite page, as it identifies which satellites it's receiving. Ephemeris data, which is constantly transmitted by each satellite, contains important information about the status of the satellite (healthy or unhealthy), current date and time. This part of the signal is essential for determining a position. The almanac data tells the GPS receiver where each GPS satellite should be at any time throughout the day. Each satellite transmits almanac data showing the orbital information for that satellite and for every other satellite in the system. Factors that can degrade the GPS signal and thus affect accuracy include the following: Ionosphere and troposphere delays. The satellite signal slows as it passes through the atmosphere. The GPS system uses a built-in model that calculates an average amount of delay to partially correct for this type of error. Signal multipath. This occurs when the GPS signal is reflected off objects such as tall buildings or large rock surfaces before it reaches the receiver. This increases the travel time of the signal, thereby causing errors. Receiver clock errors. A receiver's built-in clock is not as accurate as the atomic clocks onboard the GPS satellites. Therefore, it may have very slight timing errors. Orbital errors. Also known as ephemeris errors, these are inaccuracies of the satellite's reported location. Number of satellites visible. The more satellites a GPS receiver can "see," the better the accuracy. Buildings, terrain, electronic interference, or sometimes even dense foliage can block signal reception, causing position errors or possibly no position reading at all. GPS units typically will not work indoors, underwater or underground. Satellite geometry/shading. This refers to the relative position of the satellites at any given time. Ideal satellite geometry exists when the satellites are located at wide angles relative to each other. Poor geometry results when the satellites are located in a line or in a tight grouping. Intentional degradation of the satellite signal. Selective Availability (SA) is an intentional degradation of the signal once imposed by the U.S. Department of Defense. SA was intended to prevent military adversaries from using the highly accurate GPS signals. The government turned off SA in May 2000, which significantly improved the accuracy of civilian GPS receivers.
- Who owns the GPS Satellites ?
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.
- What is TTFF or Acquisition Time ?
The time it takes for a GPS receiver to acquire satellite signals and determine the initial position. You need at least 3 satellite fixes for the GPS Receiver to be able to triangulate it's position, but most will prefer to function on 4-5 or above.
- What should be the average TTFF ?
TTFF's can vary from depending how long the GPS has been off, to local surroundings, interference and ionosphere interference. Please note the times listed below are only as a guideline and should not be guaranteed. Fixes can differ depending on how much Almanac or Ephemeris data is already recorded as current data within the GPS Receiver. Times can also be affected by localised interference, multipath errors (rebounded signals) or external interference like ionosphere interference or solar flares.
- Cold Fix
- Usually a cold fix is considered when the GPS has been powered off for more than 4 hours, but less than 2 months. Average times can be around 30 seconds up to 5 minutes, but the average most people experience is around 1 minute.
- Warm Fix
- Usually a warm fix is considered when the GPS has been powered off for anything from 20 minutes up until 2 hours. Average times can be expected to be around 20 seconds to 1 minute
- Hot Fix
- Usually a hot fix is considered when the GPS has been powered off for under 20 minutes. Average times can be expected to be around 5-30 seconds.
- Sources of GPS signal errors ?
Factors that can degrade the GPS signal and thus affect accuracy include the following:
Ionosphere and troposphere delays The satellite signal slows as it passes through the atmosphere. The GPS system uses a built-in model that calculates an average amount of delay to partially correct for this type of error.
Signal multipath This occurs when the GPS signal is reflected off objects such as tall buildings or large rock surfaces before it reaches the receiver. This increases the travel time of the signal, thereby causing errors.
Receiver clock errors A receiver's built-in clock is not as accurate as the atomic clocks onboard the GPS satellites. Therefore, it may have very slight timing errors.
Orbital errors Also known as ephemeris errors, these are inaccuracies of the satellite's reported location.
Number of satellites visible The more satellites a GPS receiver can "see," the better the accuracy. Buildings, terrain, electronic interference, or sometimes even dense foliage can block signal reception, causing position errors or possibly no position reading at all. GPS units typically will not work indoors, underwater or underground.
Satellite geometry/shading This refers to the relative position of the satellites at any given time. Ideal satellite geometry exits when the satellites are located at wide angles relative to each other. Poor geometry results when the satellites are located in a line or in a tight grouping.
- Intentional degradation of the satellite signal Selective Availability (SA) is an intentional degradation of the signal once imposed by the U.S. Department of Defence. SA was intended to prevent military adversaries from using the highly accurate GPS signals. The government turned off SA in May 2000, which significantly improved the accuracy of civilian GPS receivers.
- 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.
- 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.
The state also 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.
The state'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.
Accuracy and Datums
Many things can affect how accurate your GPS receiver is. The atmosphere, the ionosphere and the position of your receiver could all affect the GPS accuracy. Also any buildings, natural structures or heavy foliage that obstruct the GPS view (line of sight) of the sky may decrease the position accuracy. The GPS accuracy will also depend on the level of clearance with the US DOD (Department Of Defence). There are currently two available radio signals that receivers can use: the Standard Positioning Service (SPS) for civilians and the Precise Positioning Service (PPS) for military and authorized personnel. The DOD had been known to occasionally jam the GPS signals for civilians on a short-term basis, but it is believed that this tactic is no longer employed. In general civilian (not military) GPS 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. The US Government had originally activated what was 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. Luckily for us, Selective Availability (SA) was switched off on May 2nd 2000.
- What is Selective Availability ?
In general civilian (not military) GPS 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. The US Government had originally activated what was 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. Luckily for us, Selective Availability (SA) was switched off on May 2nd 2000.
- GPS Availability - March 21, 2003
The United States Government recognizes that GPS plays a key role around the world as part of the global information infrastructure and takes seriously the responsibility to provide the best possible service to civil and commercial users worldwide. This is as true in times of conflict as it is in times of peace. The U.S. Government also maintains the capability to prevent hostile use of GPS and its augmentations while retaining a military advantage in a theater of operations without disrupting or degrading civilian uses outside the theater of operations. We believe we can ensure that GPS continues to be available as an invaluable global utility at all times, while at the same time, protecting U.S. and coalition security requirements. Check GPS STATUS & OUTAGE INFORMATION
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.
When you get a GPS navigation signal, how do you know you can trust it? asks Laurent Gauthier, the EGNOS project manager at the European Space Agency. EGNOS will tell you whether you can trust the signal. It will tell you that you are at a particular spot with a high degree of certainty and definitely within an area enclosed by a circle with the spot at the centre. In effect, it will give you your position and say by how much it could be out. EGNOS is Europe's first venture into satellite navigation. It will augment the two military satellite navigation systems now operating, the US GPS and Russian GLONASS systems, and make them suitable for safety critical applications such as flying aircraft or navigating ships through narrow channels. Consisting of three geostationary satellites and a network of ground stations, EGNOS will achieve its aim by transmitting a signal containing information on the reliability and accuracy of the positioning signals sent out by GPS and GLONASS. It will allow users in Europe and beyond to determine their position to within 5 m compared with about 20 m at present. EGNOS is a joint project of the European Space Agency (ESA), the European Commission (EC) and Eurocontrol, the European Organisation for the Safety of Air Navigation. It is Europe's contribution to the first stage of the global navigation satellite system (GNSS) and is a precursor to Galileo, the full global satellite navigation system under development in Europe. EGNOS will become fully operational in 2004. In the meantime, a test signal, broadcast by two Inmarsat satellites, allows potential users to acquaint themselves with the facility and test its usefulness.
The Wide Area Augmentation System (WAAS) is a GPS-based navigation and landing system that provides precision guidance to aircraft at thousands of airports and airstrips where there is currently no precision landing capability. Systems such as WAAS are known as satellite-based augmentation systems (SBAS). WAAS is designed to improve the accuracy and ensure the integrity of information coming from GPS satellites. The FAA is using WAAS to provide a Lateral Navigation/Vertical Navigation (LNAV/VNAV) capability with commissioning in 2003. Concurrently, the FAA will evaluate the approach to achieve Global Navigation Satellite System (GNSS) Landing System (GLS) capability in later years. WAAS testing in September 2002 confirmed accuracy performance of 1 2 meters horizontal and 2 3 meters vertical throughout the majority of the continental U.S. and portions of Alaska.
WAAS stands for Wide Area Augmentation System. WAAS is a system of satellites and ground stations that provide GPS signal corrections, giving you even better position accuracy. A WAAS-capable receiver can give you a position accuracy of better than three meters, 95 percent of the time. And you don't have to purchase additional receiving equipment or pay service fees to utilize WAAS. WAAS consists of approximately 25 ground reference stations positioned across the United States that monitor GPS satellite data. Two master stations, located on either coast, collect data from the reference stations and create a GPS correction message. This correction accounts for GPS satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere. The corrected differential message is then broadcast through one of two geostationary satellites, or satellites with a fixed position over the equator. The information is compatible with the basic GPS signal structure, which means any WAAS-enabled GPS receiver can read the signal.
Currently, WAAS satellite coverage is only available in North America. There are no ground reference stations in South America, so even though GPS users there can receive WAAS, the signal has not been corrected and thus would not improve the accuracy of their unit. For some users in the U.S., the position of the satellites over the equator makes it difficult to receive the signals when trees or mountains obstruct the view of the horizon. WAAS signal reception is ideal for open land and marine applications. WAAS provides extended coverage both inland and offshore compared to the land-based DGPS (differential GPS) system. Another benefit of WAAS is that it does not require additional receiving equipment while DGPS does.
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.
- 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, postprocessed 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).
The European Union is in the initial stages of planning and development of an alternative GPS constellation. This is because the European nations want to have a system that is autonomous and not controlled by the US Military. The service is to be called GALILEO and is not anticipated to be operational before 2008. Billions of Euros are being budgeted for the building of this system. It is uncertain whether current GPS units would be able to receive the signals from this service. Bickering between the US and Europe about compatibility, frequency ranges and channel spacing seems certain to continue for some time. The higher accuracy services from this source will be chargeable but there should be a free consumer signal.
This is the Russian GPS constellation. The signals cannot be picked up on ordinary GPS units and the receivers are expensive. Due to budgetary restraints the system has few satellites at the moment and is not relevant to the consumer. Certain academics and scientists use the GPS/GLONASS capable receivers for complex applications that require higher accuracy in the vertical plane.
GPS Terminology
Waypoints are locations or landmarks worth recording and storing in your GPS. These are locations you may later want to return to. They may be check points on a route or significant ground features. (e.g., camp, the truck, a fork in a trail, where Charlie buried his treasure).
Waypoints may be defined and stored in the unit manually, by taking coordinates for the waypoint from a map or other reference. This can be done before ever leaving home. Or more usually, waypoints may be entered directly by taking a reading with the unit at the location itself, giving it a name, and then saving the point. Waypoints may also be put into the unit by referencing another waypoint already stored, giving the reference waypoint, and entering the distance and compass bearing to the new waypoint.
- Difference between Bearing and Heading
'Bearing' is the direction you are aiming at while 'heading' is the direction you are actually going. Sometimes they are the same but sometimes you can't head directly where you want to go because of fences, wind, road, and other reasons. Heading is sometimes called 'track' so there are three words in use to describe these things. When flying you can actually point the plane in one direction while flying in another (due to wind) and neither may actually be toward the final destination (due to mountains).
A route is a series of waypoints entered in the order that you want to navigate them.
The number of channels used by your GPS receiver is directly related to the number of satellite transmissions it can interpret at once. For example, if you have an 8 Channel receiver, then you can access eight different satellites at once. A 12 Channel receiver can interpret signals from twelve satellites and a 16 Channel receiver can interpret signals from sixteen satellites at once. This is important if accuracy and consistency is a prime concern. The average GPS Receiver covers 12 satellites, but newer GPS Receivers are coming onto the market that support 16 or 20. In most cases you can only see around 8-10 satellites at anyone time, sometimes 12 so having a 16 or 20 satellite receiver will not really increase performance at this time.
It is a very precise clock carried by each of the GPS satellites. These clocks are accurate to within 1 second in every million years.
Technical Information
The receiver stores data about where the satellites are located at any given time. This data is called the almanac. Sometimes when the GPS unit is not turned on for a length of time, the almanac can get outdated or "cold". When the GPS receiver is "cold", it could take longer to acquire satellites. A receiver is considered "warm" when the data has been collected from the satellites within the last four to six hours. When you're looking for a GPS receiver to purchase, you may see cold, and warm acquisition times. If the time it takes the GPS unit to lock on to the signals and calculate a position is important to you, be sure to check the acquisition times. A full set of alamanc data can take up to 12.5 minutes in a completely cold setup (factory setup).
The GPS picks up two types of data almanac and ephemeris. Almanac data gives positional information for the satellites. The data is continously transmitted and stored in the memory of the GPS receiver so it knows the orbits of the satellites and where each satellite is supposed to be. The almanac data is periodically updated with new information as the satellites move around. Any satellite can travel slightly out of orbit, so the ground monitor stations keep track of the satellite orbits, altitude, location and speed. The ground stations send the orbital data to the master control station which in turn sends corrected data up to the satellites. This corrected and exact position data is called the Ephemeris data (pronounced i-'fe-me-res). This data is valid for about four to six hours and is transmitted in the coded information to the GPS Receiver. If you have a full set of Almanac data and just need a complete set of Ephemeris data, you will find that this will take a minimum of 30 seconds to download (eg what you would usually see from a Cold TTFF).
- What is NMEA and NMEA 0183xx
NMEA stands for National Marine Electronics Association, a US standards committee that defines data message structure, contents and protocols to allow the GPS receiver to communicate with other pieces of electronic equipment. NMEA 0183 is a standard data communication protocol used by GPS receivers.
SiRF comes as the main chipset in GPS chips in PC and Pocket PC based GPS Receivers which drives how you receive signals from the satellite. Whether it tries to look at all 12 in view satellites, or whether it discards the weakest signals and goes for the stronger signals. The SiRF chipset has gone through a number of incarnations. These are mainly SiRFstar, SiRFstar I, SiRFstar II, SiRFstar IIe, SiRFstar IIe/LP, SiRFstar IIt, recently SiRFLoc and SiRFXTrac were added.
SiRF is both a chipset and a protocol, allowing you to talk directly to the chipset itself giving you greater control over the GPS with features such as trickle mode for Low Power consumption.
- SiRFstar IIe
- The standard in performance and flexibility for high volume GPS chip sets. The SiRFstarIIe was the first product to use the SiRFstarII architecture - setting the standard for GPS performance. It features 1920 time/frequency search bins, integrated WAAS, EGNOS, DGPS, and an ARM CPU integrated into the digital chip.
This benefits users with fast, accurate GPS that is flexible enough to integrate user tasks on to the processor. This flexible platform supports a wide variety of consumer GPS devices.- SiRFstar IIe/LP
- The new lower power standard for high volume GPS chip sets. If you're manufacturing or designing a device using GPS with battery power, the SiRFstarIIe/LP provides an innovative way to add location awareness to your product. Drawing only 60mA in full power and 20mA in TricklePower, the SiRFstarIIe/LP is one of the lowest power full-feature GPS chipsets on the market. The chipset also has an ARM CPU with 40 MIPS of total processing power. This can be used as the engine to drive your product reducing component cost by removing the extra CPU. The SiRFstarIIe/LP family is comprised of the GRF2i/LP low-power front end, the GSP2e/LP highly integrated digital section, and the GSW2 software, which provides a path for new features and continuous GPS improvement.
- SiRFstar IIt
- The host-based GPS chip set solution. SiRFstarIIt makes it easy and economical to add high-performance SiRFstarII technology to systems that are based on many popular processors and operating systems. The SiRFstarIIt solution allows GPS function to be added at minimal cost in components and circuit board area, by sharing the host system's processor and memory resources. The chip set acquires and tracks satellites, then sends raw measurements to the host processor running the SiRFNav software in parallel with the host applications. The SiRFNav software module computes position time and velocity. For enhanced accuracy and navigation reliability, WAAS corrections can be applied to GPS measurements or GPS can be combined with dead reckoning data and processed by SiRFDRive software. The chip set consists of the GSP2t highly integrated digital chip, GRF2i integrated RF receiver and the SiRFNav software. The two chips are packaged as 9mm 48-pin LQFP packages and require minimal external components thanks to their high level of integration. SiRFNav software is designed for easy porting to host systems.
- SiRFLoc
- The worlds first A-GPS Multimode technology enabling E-911 and LBS platforms. With the potential of Location Based Services (LBS) and the requirement to meet the enhanced 911 (E-911) mandate, the importance of implementing a location solution is on the increase - for service providers and handset manufacturers alike. Designed to provide the most flexible and effective location solution available for wireless applications, SiRFLoc is the worlds first multimode Aided GPS (A-GPS) solution.
- SiRFXTrac
- Autonomous high-sensitivity software designed to extend the range of GPS. Complimenting SiRF's existing product line of GPS solutions, SiRFXTrac is a high sensitivity GPS software solution. SiRFXTrac extends the operating range in which GPS can be used - dramatically increasing the versatility of GPS-enabled products such as personal digital assistants (PDA's), automotive navigation solutions, AVL applications, and other location aware consumer devices. If loaded with SiRFXTrac high sensitivity software, GPS-enabled mobile consumer devices will be able to continue operating in far more locations than ever before possible.
Just like SiRF is a chipset that uses the NMEA comlpiant protocol, Evermore also is a chipset designed by another company.
XTrac is a firmware on the SiRF chipset that will boost the sensitivity of a GPS receiver. It does so by acquiring more signal from weaker satellites before it calculates your position. For example, a normal GPS will acquire signals from 4 satellites with the strongest signals to calculate your position. In the XTrac mode, the GPS will acquire signals from 2 more weaker satellites (total 6 satellites) before outputting a position. Thus, the better sensitivity and accuracy. However, this comes at the expense of a lag/delay because the GPS is waiting to acquire and process those additional signals. Therefore, sometimes the user will experience a delay in position and will cause the miss of a turn or exit. The normal GPS is recommend for regular in car navigation use in suburban area with good view of the sky. In more harsh conditions, such as major metropolitan and urban areas like New York, London or in a area where there is a lot of foliage, XTrac would help to acquire and maintain signals where as the normal mode might have lost the signals completely. Originally the CF GPS are sold either as normal OR with XTrac. Because when using XTrac it's essentially looking to obtain data from weaker satellites rather than the stronger satellites, the average TTFF could be longer as it's searching out the weaker signal. SiRF also say in their October 2002 Press Release "The SiRFXTrac software enables the highly popular SiRFStarIIe/LP chipset to acquire, and continue tracking GPS signals at far lower signal levels than is currently possible with competitive autonomous GPS solutions. For the user, this means that GPS can now be used in environments previously deemed inaccessible environments such as urban canyons, parking garages, dense foliage, multi-level freeways, and, in some cases, indoors. By expanding the number of areas in which GPS can get a position fix, SiRFXTrac will improve existing location-based applications and enable new ones that have been impractical until now."
SiRF XTrac is an autonomous high-sensitivity software designed to extend the range of GPS. Complimenting SiRF's existing product line of GPS solutions, SiRFXTrac is a high sensitivity GPS software solution. SiRFXTrac extends the operating range in which GPS can be used - dramatically increasing the versatility of GPS-enabled products such as personal digital assistants (PDA's), automotive navigation solutions, AVL applications, and other location aware consumer devices. If loaded with SiRFXTrac high sensitivity software, GPS-enabled mobile consumer devices will be able to continue operating in far more locations than ever before possible.
The XTrac software/firmware basically acquires a fix on the stronger satellites first, but then continues to attack the weaker satellite signals to build a bigger picture, and works hard at bringing in a fix for the weaker satellites. This means that TTFF time can be extended in some cases or continuous, and due to this you may experience a brief lag on a map depending on the software you use when you are using your GPS Receiver in XTrac mode. But why bring in more satellites that are weak ? Well if you're in heavy foliage or in a covered car park, you will probably not be able to receive a fix. If XTrac can sniff out the weaker signals and over time give you a better fix, then when you are under cover you may still be able to receive a fix. It's all about sensitivity.
Possible perceived problems when using XTrac GPS Receivers:-
- When using GPS in-car
- If you are using GPS for in-car satellite navigation, you may notice some lag when you accelerate or decelerate. When you come to stop at a traffic light or roundabout, you may see that your positions shows you slightly past the junction or it may continue for a short distance. This is normal because the signal you are receiving isn't always 100% accurate, and you may experience this 'lag' and doesn't mean that the GPS Receiver is faulty.
- Snap to road technology
- If you are using GPS for in-car satellite navigation and the application you use utilises a snap to road feature (commonly used), then sometimes if the positional data is quite bad, you may be snapped/positioned onto the wrong road. Once you start driving for a few hundred yards, the software should be able to recognise that you are not on the same road as it thinks and put you back onto the correct road. This also may then mean you receive unnecessary re-routing information. This is completely normal for XTrac receivers.
Bluetooth GPS
- Bluetooth Passkeys / Bonding
To successfully connect a Pocket PC, Palm or PC to a Bluetooth GPS Receiver, many require you create a bond between the two devices by pairing them together using a security key or passkey. This although it is a security measure to stop others using your device, is a fairly weak form of security, but you will need to enter the passkey when bonding. Some devices may not require a passkey or may require one of the following passkeys to be used.
- Delorme Earthmate
- Passkey 0000
- Emtac
- Passkey 0183
- Deluo Blackbox
- Passkey 0000
- Falcom GPS
- Passkey 0183
- Fortuna Clip-On
- Passkey 0000
- Fortuna GPSMart
- Passkey 0000
- GlobalSat
- Passkey 2003
- Haicom BT-401
- Passkey 0183
- Holux GR-230
- Passkey 6268
- HP iPAQ Bluetooth
- Passkey 2003
- Leadtek 9537
- Passkey 0000
- Navman 4100
- Passkey 0000
- Navman 4400
- Passkey 0000
- Pharos BT GPS
- Passkey 0000
- Pretec Bluetooth GPS
- Passkey 0000
- Rikaline X7
- Passkey 1234
- RoyalTek RBT 3000
- Passkey 0000
- San Jose (SANAV) BT-48
- Passkey 0000
- Socket
- Passkey 0183
- SysOnChip
- Passkey 0000
- TeleType
- Passkey 0183
- TomTom
- Passkey 0000
- TripNav TN-206
- Passkey 2003
- Using third party Mains/Car chargers
We do not recommend using a third party mains or car charger unless it is rated identically to the original charger or unless the manufacturer agrees. There have been a number of failing Bluetooth GPS Receivers on the market due to people using their Pocket PC chargers because either the voltage or ampage (not both) match that of the Bluetooth Receiver, and over a moderate period of time will eventually burn out the circuits inside the GPS, so we recommend checking with the manufacturer first, or check to see if we sell a charger specifically for the Bluetooth GPS Receiver.
GPS Fun
Nasa has an online tracking system where you can track GPS Satellites orbitting the world along with other satellites and see exactly where they are in space in relation to their flight path.
Tracking Santa can be done around Christmas time and is a great way for young children to get introduced to GPS.
http://www.eaze.net/~citius/santa.htm
http://www.travelbygps.com/special/santa/santa2003.php
Maps
1. Can I upload all types of MapSource maps to a Garmin GPS ?
Yes
2. Can I upload all types of MetroGuide maps to a Garmin GPS ?
Yes
3. Can I upload Magellan MapSend/DataSend maps to a Garmin GPS ?
No
4. Can I upload Magellan MapSend/DataSend to all Magellan GPS ?
Yes
5. Can I upload City Navigator Maps for USA & Canada to a Garmin GPS ?
Yes, but automatic routing will not work.
6. Can I upload any other types of maps to a Garmin, or Magellan GPS ?
No, for Garmin handhelds you need to use Garmin MapSource CD's and for Magellan you need to use Magellan MapSend/DataSend CD's.
7. I thought there was an application called GPSmapper that will allow you to upload custom maps to a Garmin or Magellan handheld ?
Yes there is, and it is available at this website, but using this can corrupt the Garmin and Magellan handheld GPS's and if you do use this you will automatically invalidate your warranty!