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T GPS Fundamentals Your location is: 37 o 23.323’ N 122 o 02.162’ W

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T Global Positioning System Your location is: 37 o 23.323’ N 122 o 02.162’ W

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T Satellite Signal Structure CarrierL1L2 Frequency1575.42 MHz1227.60 MHz Wavelength 19cm24cm Code Modulation C/A-code- P(Y)-codeP(Y)-codeNAVDATA C/A - Coarse Acquisition Code P - Precise Code (Y-Code when encrypted) NAVDATA - Satellite health, satellite clock corrections, and ephemeris parameters.

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T Satellite Signal Structure Carrier C/A-code P-code NAVDATA Received Signal

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T GPS Observables Autonomous Position Relative Position What is Measured? Code RangesCarrier Phases

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T Code Range Results Autonomous Position +/- 100 m (330 ft) error (horizontal) +/- 156 m (515 ft) error (vertical)

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T Carrier Phase Results Measured Reduced Baseline or Vector (cm precision) Azi = 212 o 42’ 49.8244” Dist = 557.05307 m Ell Ht = 4.8751 m X = -408.251 m Y = -84.830 m Z = -369.413 m OR

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T ECEF Coordinate System +Z -Y +X X Y Z ECEF X = -2691542.5437 m Y = -4301026.4260 m Z = 3851926.3688 m

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T Reference Ellipsoid a b a = semi-major axis b = semi-minor axis b a H WGS-84 Ellipsoid a = 6378137.000000 m b = 6356752.314245 m 1/f = 298.2572235630

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T ECEF and WGS-84 +Z -Y +X ECEF X = -2691542.5437 m Y = -4301026.4260 m Z = 3851926.3688 m X Y Z b a H WGS-84 = 37 o 23’ 26.38035” N = 122 o 02’ 16.62574” W H = -5.4083 m

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T GPS Heights vs. Elevations h = Orthometric Height H = Ellipsoid Height N = Geoid Height h = H - N N N N h h h H H H Earth’s Surface Ellipsoid Geoid

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T Errors Sources in GPS Selective Availability (SA) and Anti-Spoofing (AS) Multipath Ionospheric Noise Human Error

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T Selective Availability (SA) Clock dither Ephemeris error Anti-Spoofing (AS) Encryption of the P-code signal SA and AS 100 meters ????????

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T Multipath

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T Ionospheric Effects Ionosphere < 10 km > 10 km

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T Human Error

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T Survey Requirements 10:30 10:35

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T GPS by Trilateration D Distance D = Speed of Light x Time

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T Summary There are three segments to GPS. All must be working to use GPS. There are 2 GPS signals, L1 and L2. Code Range measurements result in autonomous positions with several meter precision. Carrier Phase measurements result in relative positions with centimeter precision.

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T Summary (cont.) GPS results are in reference to an ECEF coordinate system and the WGS-84 ellipsoid. Errors in GPS can be minimized by planning and utilizing proper surveying techniques. At least 4 SVs are required to determine a position or survey with GPS. At least 2 receivers are required to survey with GPS.

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T Questions?

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How Global Positioning Devices (GPS) work

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