GPS stands for Global Positioning System, and is a satellite based navigation system. The system consists of satellites, operated by the US Department of Defense, which orbit the Earth and transmit coded signals with information on satellite positions and time. Earth-based GPS receivers obtaining signals from no fewer than four satellites can calculate accurate horizontal and vertical position. Four GPS satellite signals are needed to compute positions in three dimensions and correct the time offset in the receiver clock.
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A base station is a GPS receiver that collects GPS measurements at a known location. Its main components are an antenna, a GPS receiver, and a device to which the GPS data is logged - most often a personal computer. The antenna's location is determined very accurately (to within a few centimeters or less) by surveying methods. A base station provides reference data that can be used to increase the accuracy of GPS data collected in the field.
Under normal circumstances a GPS receiver should be able to determine a fix for a user's location to within 10 meters. Very often, the results are in the 5 to 7 meter range. To consistently achieve better than 5 meter accuracy, GPS measurements from a base station are needed to differentially correct GPS measurements determined in the field.
Several sources of error related to timing, satellite orbits, and the atmosphere affect GPS measurements. The GPS measurements collected by a base station are used to model these error sources at any given time. Since the measurements at the base station are collected at a known location, these sources of error can be determined and used for improving or correcting field data that was/is collected at the same time. The differential correction can be performed on field data after the data has been downloaded to a PC running GPS data processing software (post-process correction), or performed in real time while the field data is being collected. Generally the post-process correction solution is more accurate, and is favored over the real time solution when requirements dictate achieving higher accuracy. Real time correction still typically provides better than 5 meter accuracy, but users might not always be able to receive the correction signals in all environments. Naturally, the real time solution has advantages when using GPS for navigation.
Base stations collect and store GPS measurements for later retrieval by users. Some base stations also transmit correction data that are used for real time differential GPS. Many newer GPS receivers have WAAS (Wide Area Augmentation System) correction capabilities. WAAS is a satellite based real time correction solution that was initially designed to aid aircraft navigation.
Your best option is to acquire base data from one of the National Geodetic Survey (NGS) Continuously Operating Reference Stations (CORS). The closest stations are located at Trenton, NJ; Sandy Hook, NJ; Newark, NJ; Warminster, PA; West Chester, PA; Dover, DE; Reedy Point, DE; Palisades, NY; and East Moriches, NY. Base data from these sites can be downloaded over the Web by accessing the NOAA CORS site.
Among other things, each file consists of GPS positions, range measurements (pseudoranges to all satellites in view), and carrier phase data collected over the course of one hour. The files are compressed to an executable file. Depending on your GPS processing software, you may have to uncompress the files. To do this, double click on the file's icon. This should create a decompressed version of the file. The decompressed file will have the same filename as the compressed file, but with an .ssf extension.
Files can be accessed via the internet at a base station maintained by the New Jersey Department of Transportation in Trenton, NJ.
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