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Point averaging of the position components, before and after S/A is turned off
 Dr. M. H. Refan Shahid Rajaee Teachers Training University (SRTTU) Lavizan, Tehran 16788, Iran refan@srttu.edu  Dr. K. Mohammadi Iran University of Science and Technology (IUST) Narmak, Tehran 16844, Iran mohammadi@iust.ac.ir
Abstract A Low Cost GPS engine was used in this research. First of all a suitable hardware was designed and used to collect and save the receiver’s output data, with S/A on, by a special software. This was also repeated with S/A off. Another software was also written to draw variations in position measurement errors in two different conditions mentioned above. Finally, a simple post processing technique, point averaging was used, and the data were averaged in various periods of time for S/A on and off. The decrease in measurement errors was proportionate to the duration of averaging. Introduction During past several years, the main problem in improving of the positioning measuring accuracy was Selective Availability (S/A) error. Which was produced and fed into GPS system by U.S. Department of Defense (DoD) in order to degrade the achievable navigation accuracy when non-military GPS receivers are used. In addition to S/A, there is some other error sources that cause the position and time measuring from GPS receivers to be inaccurate [1,2].
Because of above mentioned error sources, all GPS collected points have a certain number of errors. This means that the location we grant will not reflect the real location. Therefore, users who wish to increase the accuracy of their GPS receiver must take steps to minimize the errors. Post processing techniques offers GPS users an affordable and relatively easy mean for correcting GPS errors. The simplest post processing technique is Point Averaging, which relies on simple arithmetic to correct GPS errors. So, if we collected many points and then averaged them together, we could feel confident that the majority of points were fairly close to the real location and the effects of all outliners would be minimal [4,5].  GPS Receiver There are a wide variety of GPS receiver products in today’s market. Some are complete systems containing keyboards and LCD displays such as hand held receivers, and some are available as electronic boards, called Original Equipment Manufacturer (OEM) products. We used the Rockwell “ Microtracker LP (MLP)” receiver, which is a low cost single board, single frequency, C/A code, five parallel-channel, GPS engine suitable for integration into a wide variety OEM products. Two serial data ports are provided with the MLP. One is used to output position components, velocity, time Information, and status information to the OEM’s application software input initialization data, and commands are received over the same serial data port. A second serial port is dedicated to DGPS1 correction input. Since the output data of the receiver are available in NMEA2 and Binary protocols, and the Binary protocol provides more detailed information, the receiver was initialized to communicate Binary protocols. There are several Binary messages provided by MLP. One famous and general purpose of these messages is message No. 103, which is available on the first output port as default, when we configure the receiver in Binary mode. We designed and implemented a hardware in order to setup the receiver and connect it to a PC. Figure 1 shows the hardware structure [6,7]. 
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