Android devices use the same global positioning technology as Google Maps and most third-party GPS tools do. This allows users to locate themselves on a map, find and navigate to destinations via detailed directions, and search maps using a number of different methods. This article will teach you how to use GPS in Android with Google Maps.
There are many sources of possible errors that will degrade the accuracy of positions computed by a GPS receiver. The travel time of GPS satellite signals can be altered by atmospheric effects; when a GPS signal passes through the ionosphere and troposphere it is refracted, causing the speed of the signal to be different from the speed of a GPS signal in space. Sunspot activity also causes interference with GPS signals. Another source of error is measurement noise, or distortion of the signal caused by electrical interference or errors inherent in the GPS receiver itself. Errors in the ephemeris data (the information about satellite orbits) will also cause errors in computed positions, because the satellites weren't really where the GPS receiver "thought" they were (based on the information it received) when it computed the positions. Small variations in the atomic clocks (clock drift) on board the satellites can translate to large position errors; a clock error of 1 nanosecond translates to 1 foot or .3 meters user error on the ground. Multipath effects arise when signals transmitted from the satellites bounce off a reflective surface before getting to the receiver antenna. When this happens, the receiver gets the signal in straight line path as well as delayed path (multiple paths). The effect is similar to a ghost or double image on a TV set.
The base station, depending upon how it is configured, can correct roving GPS receiver data in one (or both) of two ways. 1) In the first method, called real-time differential correction or real-time differential GPS (DGPS), the base station transmits (usually via radio link) error correction messages to other GPS receivers in the local area. In this case, the positions you read on your GPS receiver while you are out collecting data, are the corrected positions. 2) The second method, called post-processed differential correction, is performed on a computer after the roving receiver data are collected. While you are out in the field collecting data, the positions you read on your roving GPS receiver are uncorrected. It is not until you take your rover files back to the office and process them using differential correction software and data from the base station file, that you get corrected positions. The base station file contains information about the timing errors. This information allows the differential correction software to apply error corrections to the roving receiver file during processing. Since the base and rover receivers have to "see" the same set of satellites at the same time, the base file has to start before the rover file starts, and end after the rover file ends (a base station is normally set up to track all satellites in view, insuring that it will "see" at least the four satellites that the roving receiver is using to compute positions). Post-processed differential correction, then, requires both base and rover receivers that are capable of collecting and storing files. Most recreational grade receivers cannot collect and store files that can be differentially corrected.
You can set up your own local base station or share a base station with other GPS users in your area. If you are using post-processed differential correction, the base station can usually serve users in an area with about a 2 to 300 mile radius (the further the roving receiver is from the base station, the less accurate the corrections). If you are using real-time differential correction, you must establish radio links, and your coverage area is limited by the strength of the radio transmissions. If you plan to set up your own base station, make sure the manufacturer can supply all the necessary components including base and rover receivers, radios (if using real-time correction) and differential correction software (if using post-processed correction).
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