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Determining X, Y, Z and T coordinates with GPS satell itesX = latitudeY = longitudeZ = altitudeT = time

A minimum offour (4) orbital satellites is necessary to determine 3D position andtime. Three (3) satellites are necessary if you are only wishing to calculate your2D position.

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Control Segment Tracking Stations for GPSThere are five Monitor Control Stations located on the surface of the Earth. The

Master Control Station is located in Colorado Springs, home of the USAF Academyand near NORAD, reflecting the DOD influence in the birth of GPS. Note how theremaining control stations are all located in the tropics on remote islands in eachrespective ocean. This enables maximum monitoring ability 24/7/365 of the GPSorbital satellites.

Ephemeris DataThe following chart displays a truncated list of ephemeris data. Ephemeris data isupdated every ten minutes. You should always wait a few minutes after turningon your GPS so that it may collect the most recent ephemeris data.

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Standard Orbital Tracks of GPS Satell ites in 3D and 2D

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Dilution of Precision (DOP)Good satellite geometry is essential to ensuring accurate readings of position anddate collection. An ideal DOP will be having ample satellites from which to readthat are both high and low in the sky, with 45 angles considered to be optimal.The following charts display both good and bad DOP values. FYI: VDOP =Vertical DOP, HDOP = Horizontal DOP, GDOP = Geometric DOP, PDOP= Percentage DOP

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Determining PDOPAn ideal DOP value will be low (PDOP

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The greatest total number of satellites is found between 12:00AM to 12:30AM(0000h to 0030h) and from 11:15PM to 12:00AM (2315h to 0000h). There is alsogood coverage from 7:00AM to 9:15AM (0700h to 0915h). The greater thenumber of satellite, generally the lower the PDOP value, but this is not always thecase. You may have cases where half a dozen satellites may be clustered in onearea of the sky, giving good satellite coverage but poor PDOP. This will oftenoccur at very high latitudes (ex. Greenland or Antarctica) or in equatorial regions.

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The lowest PDOP values are found from 12:00AM to 1:00AM (0000h to 0100h) andfrom 10:15PM to 12:00AM (2215h to 0000h). There are also low PDOP valuesfrom 7:00AM to 930AM (0700h to 0930h). These times are also the times when

you have your maximum satellite coverage. Note that your highest PDOP values(2:30AM or 0230h) coincide with your lowest satellite coverage.

Your choices for optimal coverage and low PDOP are going to be just aftermidnight, a couple of hours after sunrise, and the hour just before midnight. Youmay also calibrate your software package develop filters that will automaticallyeliminate times of day in which the satellite coverage is too low and/or the PDOPvalues are too high, as seen in the diagram below. The filter has been set toeliminate all times in which satellite coverage is below 4 and to eliminate all timeswhen PDOP values are above 6. This filter does not display the most optimaltimes to conduct GPS readings, but it does eliminate the worst times and gives theuser a much larger window of opportunity to conduct fieldwork.

Important these filters do not take into account terrain features or man-madeobstructions. Good satellite coverage and low PDOP values can changedramatically depending upon site-specific features on the surface of the Earth (i.e.tall buildings, mountains, steep gorges, etc.)

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