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 Sometimes when the GPS unit is not turned on for a length of time, the almanac can get outdated or "cold".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).

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.

GPS is a satellite-based navigation system that works by receiving navigation messages from satellites and calculating locations. 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.

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.

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.

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

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.

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.

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.

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.

NAVSTAR is an acronym for Navigation Satellite Timing and Ranging, a name given to the GPS satellite system by the US Government.

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.

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.

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.

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.

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."

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.

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.

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