GPS receivers that support maps come with a basemap of the region the GPS receiver was sold in (such as North America or Europe) that shows city locations, highways, major roads, bodies of water, and other features. Precisely what the basemaps display varies by manufacturer and model. Although base- maps do provide general information, some GPS receiver users want more detailed maps that show city streets, topographic features, marine navigation aids, or places outside the United States.

 

How long building a map takes depends on the size of the area that you select, how much map detail you want to include, and how fast your PC is. This can range from a minute or less for small areas (such as a metropolitan area) to five or ten minutes for a large map (such as one that includes many different states). How much time it takes to upload a map into a GPS receiver also depends on the size of the area you select and how the receiver stores maps. If you’re uploading a large map from a PC via a serial cable, it can take hours to transfer the map between a PC and your GPS receiver. GPS receivers that support Universal Serial Bus (USB) communications are much faster.

 

 

For GPS receivers that use SD or MMC memory for storage, after the map has been created and saved to the memory card, it’s just a matter of inserting the card into the receiver. After you purchase GPS receiver map software, be sure to check the manufacturer’s Web site every now and then to see whether updated releases of the PC software are available. You may be able to download upgraded versions of the program with bug fixes and enhanced features. Keep in mind that when you download the program, updated map data doesn’t come with it. Some GPS receiver manufacturers use different methods for stemming software and map piracy. Some map products (notably nautical charts) have multiple regions stored on CD-ROM, and you need to purchase an unlock code for each region you want to access. In addition, programs commonly link the serial number of a GPS receiver to a map, meaning that the map will work only with the GPS receiver that the map was originally uploaded to.

 

For the most part, GPS manufacturers have a lock on the market when it comes to maps that can be uploaded to their receivers. GPS receiver owners must use proprietary maps distributed by the manufacturers. However, a small group of technically adept GPS and map enthusiasts have found ways around this map monopoly.  At the present, creating your own do-it-yourself GPS receiver maps is a somewhat complicated process, but a number of tutorials are available on the Internetcturers have a lock on the market when it comes to maps that can be uploaded to their receivers. GPS receiver owners must use proprietary maps distributed by the manufacturers. However, a small group of technically adept GPS and map enthusiasts have found ways around this map monopoly.  At the present, creating your own do-it-yourself GPS receiver maps is a somewhat complicated process, but a number of tutorials are available on the Internet.

 

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GPS might be divided up in the following way:

 

 

The first major component of GPS is Earth itself: its mass and its surface, and the space immediately above. The mass of the Earth holds the satellites in orbit. From the point of view of physics, each satellite is trying to fly by the Earth at four kilometers per second. The Earth’s gravity pulls on the satellite vertically so it falls. The trajectory of its fall is a track that is parallel to the curve of the Earth’s surface. The surface of the Earth is studded with little “monuments”– carefully positioned metal or stone markers–whose coordinates are known quite accurately. These lie in the “numerical graticule” which we all agree forms the basis for geographic position. Measurements in the units of the graticule, and based on the positions of the monuments, allow us to determine the position of any object we choose on the surface of the Earth.

 

 

The United States GPS design calls for a total of at least 24 and up to 32 solar powered radio transmitters, forming a constellation such that several are “visible” from any point on Earth at any given time. The first one was launched on February 22, 1978. In mid-1994 all 24 were broadcasting. The minimum “constellation” of 24 includes three “spares.” As many as 28 have been up and working at one time. The NAVSTAR satellites are neither polar nor equatorial, but slice the Earth’s latitudes at about 55°, executing a single revolution every 12 hours. Further, although each satellite is in a 12 hour orbit, an observer on Earth will see it rise and set about 4 minutes earlier each day.

 

 

Ground-Based StationsWhile the GPS satellites are free from drag by the atmosphere, their tracks are influenced by the gravitational effects of the moon and sun, and by the solar wind. Further, they are crammed with electronics. Thus, both their tracks and their innards require monitoring. This is accomplished by four ground-based stations near the equator, located on Ascension Island in the South Atlantic, at Diego Garcia in the Indian Ocean, and on Kwajalein Atoll, and in Hawaii, both in the Pacific, plus the master control station (MCS) at Schriever (formerly Falcon) Air Force Base near Colorado Springs, Colorado. A sixth station is planned to begin operation at Cape Canaveral, Florida. Each satellite passes over at least one monitoring station twice a day. Information developed by the monitoring station is transmitted back to the satellite, which in turn rebroadcasts it to GPS receivers ( electronic tracker ).ReceiversThis is the part of the system with which you will become most familiar. In its most basic form, the satellite receiver consists of• an antenna (whose position the receiver reports),• electronics to receive the satellite signals,• a microcomputer to process the data that determines the antenna position, and to record position values,• controls to provide user input to the receiver, and• a screen to display information.More elaborate units have computer memory to store position data points and the velocity of the antenna. This information may be uploaded into a personal computer or workstation, and then installed in GIS software database. Another elaboration on the basic GPS unit is the ability to receive data from and transmit data to other GPS Tracking Device –a technique called “realtime differential GPS” that may be used to considerably increase the accuracy of position finding. 

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While this is a text on how to use GPS in GIS–and hence is primarily concerned with positional issues, it would not be complete without mentioning what may, for the average person, be the most important facet of GPS: providing Earth with a universal, exceedingly accurate time source. Allowing any person or piece of equipment to know the exact time has tremendous implications for things we depend on every day (like getting information across the Internet, like synchronizing the electric power grid and the telephone network). Further, human knowledge is enhanced by research projects that depend on knowing the exact time in different parts of the world. For example, it is now possible to track seismic waves created by earthquakes, from one side of the earth, through its center, to the other side, since the exact time may be known worldwide.

 

 

The subject of this blog is the use of GPS as a method of collecting locational data for Geographic Information Systems (GIS). The appropriateness of this seems obvious, but let’s explore some of the main reasons for making GPS a primary source of data for GIS:

 

• Availability: In 1995, the U.S. Department of Defense (DoD) declared NAVSTAR to have “final operational capability.” Deciphered, this means that the DoD has committed itself to maintaining NAVSTAR’s capability for civilians at a level specified by law, for the foreseeable future, at least in times of peace. Therefore, those with personal GPS tracking devices may locate their positions anywhere on the Earth.

 

• Accuracy: GPS tracker allows the user to know position information with remarkable accuracy. A receiver operating by itself, can let you locate yourself within 10 to 20 meters of the true position. (And later you will learn how to get accuracies of 2 to 5 meters.) At least two factors promote such accuracy:

First, with GPS, we work with primary data sources. Consider one alternative to using GPS to generate spatial data: the digitizer. A digitizer is essentially an electronic drawing table, wherewith an operator traces lines or enters points by “pointing”–with “crosshairs” embedded in a clear plastic “puck”–at features on a map. One could consider that the ground-based portion of a GPS system and a digitizer are analogous: the Earth’s surface is the digitizing table, and the GPS receiver antenna plays the part of the cross-hairs, tracing along, for example, a road. But data generation with GPS takes place by recording the position on the most fundamental entity available: the Earth itself, rather than a map or photograph of a part of the Earth that was derived through a process involving perhaps several transformations.

 

Secondly, GPS itself has high inherent accuracy. The precision of a digitizer may be 0.1 millimeters (mm). On a map of scale 1:24,000, this translates into 2.4 meters (m) on the ground. A distance of 2.4 m is comparable to the accuracy one might expect of the properly corrected data from a medium-quality GPS Tracking Device . It would be hard to get this out of the digitizing process. A secondary road on our map might be represented by a line five times as wide as the precision of the digitizer (0.5 mm wide), giving a distance on the ground of 12 m, or about 40 feet.

 

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Because the scope of GPS research and application development is so broad and conducted by researchers all over the globe, it is impossible to give a comprehensive listing. Therefore, merely demonstrates the extraordinarily rapid development of the GPS Solution . GPS made its debut in surveying and geodesy with a big bang. During the summer of 1982, the testing of the Macrometer receiver, developed by C. C. Counselman at M.I.T., verified a GPS surveying accuracy of 1–2 parts per million (ppm) of the station separation. Baselines were measured repeatedly using several hours of observations to study this new surveying technique and to gain initial experience with GPS. During 1983 a thirty (plus)-station first-order network densification in the Eifel region of Germany was observed (Bock et al., 1985). This project was a joint effort by the State Surveying Office of North Rhein-Westfalia, a private U.S. firm, and scientists from M.I.T. In early 1984, the geodetic network densification of Montgomery County (Pennsylvania) was completed. The sole guidance of this project rested with a private  GPS surveying firm (Collins and Leick, 1985). Also in 1984, GPS was used at Stanford University for a high-precision GPS engineering survey to support construction for extending the Stanford Linear Accelerator (SLAC). Terrestrial observations (angles and distances) were combined with GPS vectors.

 
The Stanford project yielded a truly millimeter-accurate GPS network, thus demonstrating, among other things, the high quality of the Macrometer antenna. This accuracy could be verified through comparison with the alignment laser at the accelerator, which reproduces a straight line within one-tenth of a millimeter (Ruland and Leick, 1985). Therefore, by the middle of 1984, 1–2 ppm GPS surveying had been demonstrated beyond any doubt. No visibility was required between the stations. Data processing could be done on a microcomputer. Hands-on experience was sufficient to acquire most of the skills needed to process the data—i.e., first order geodetic network densification suddenly became within the capability of individual surveyors. President Reagan offered GPS free of charge for civilian aircraft navigation in 1983 once the system became fully operational. This announcement was made after the Soviet downing of the Korean Air flight 007 over the Korea Eastern Sea. This announcement can be viewed as the beginning of sharing arrangements of GPS for military and Tracking Devices For People. 
Engelis et al. (1985) computed accurate geoid undulation differences for the Eifel network, demonstrating how GPS results can be combined with orthometric heights, as well as what it takes to carry out such combinations accurately. New receivers became available—e.g., the dual-frequency P-code receiver TI-4100 from Texas Instruments—which was developed with the support of several federal agencies. Ladd et al. (1985) reported on a survey using codeless dual-frequency receivers and claimed 1 ppm in all three components of a vector in as little as 15 minutes of observation time. Thus, the move toward rapid static surveying had begun. Around 1985, kinematic GPS became available. Kinematic GPS refers to ambiguity-fixed solutions that yield centimeter (and better) relative accuracy for a moving antenna. The only constraint on the path of the moving antenna is visibility of the same four (at least) satellites at both receivers. Remondi introduced the antenna swapping technique for the rapid initialization of ambiguities. Antenna swapping made kinematic positioning in surveying more efficient. 

Finally, during 1998 and 1999, major decisions were announced regarding the modernization of GPS. In 2000, SA was set to zero as per Presidential Directive. When active, SA entails an intentional falsification of the satellite clock (SA-dither) and of the broadcast satellite ephemeris (SA-epsilon); when active it is effectively an intentional denial to civilian users of the full capability of GPS ( GPS Tracking Device ).

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The elevation or altitude calculated by a GPS receiver from satellite data isn’t very accurate. Because of this, some GPS units have altimeters, which provide the elevation, ascent/descent rates, change in elevation over distance or time, and the change of barometric pressure over time. (The rough-and-ready rule is that if barometric pressure is falling, bad weather is on the way; if it’s rising, clear weather is coming.) Calibrated and used correctly, barometric altimeters can be accurate within 10 feet of the actual elevation. Knowing your altitude is useful if you have something to reference it to, such as a topographic map. Altimeters are useful for hiking or in the mountains.

 

On GPS units with an electronic altimeter/barometer, calibrating the altimeter to ensure accuracy is important. To do so, visit a physical location with a known elevation and enter the elevation according to the directions in your user’s guide. Airports are good places to calibrate your altimeter or get an initial base reading; their elevation is posted for pilots to calibrate their airplanes’ altimeters. If you’re relying on the altimeter/barometer for recreational use, I recommend calibrating it before you head out on a trip.

 

 

Some GPS receivers have features that allow you to increase the accuracy of your location by using radio signals not associated with the GPS satellites. If you see that a GPS receiver supports WAAS or Differential GPS, it has the potential to provide you with more accurate location data.

 

 

WAAS 
Wide Area Augmentation System (WAAS) combines satellites and ground stations for position accuracy of better than three meters. Vertical accuracy is also improved to three to seven meters. WAAS is a Federal Aviation Administration (FAA) system, so GPS can be used for airplane flight approaches. The system has a series of ground-reference stations throughout the United States. These monitor GPS satellite data and then send the data to two master stations — one on the west coast and the other on the east coast. These master stations create a GPS message that corrects for position inaccuracies caused by satellite orbital drift and atmospheric conditions. The corrected messages are sent to non-NAVSTAR satellites in stationary orbit over the equator. The satellites then broadcast the data to GPS receivers that are WAAS-enabled. 
GPS units that support WAAS have a built-in receiver to process the WAAS signals. You don’t need more hardware. Some GPS receivers support turning WAAS on and off. If WAAS is on, battery life is shorter (although not as significantly as it is when using the backlight). In fact, on these models, you can’t use WAAS if the receiver’s battery-saver mode is activated. Whether you turn WAAS on or off depends on your needs. Unless you need a higher level of accuracy, you can leave WAAS turned off if your GPS receiver supports toggling it on and off. WAAS is ideally suited for aviation as well as for open land and marine use. The system may not, however, provide any benefits in areas where trees or mountains obstruct the view of the horizon. 

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