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lecture12 - Lecture Notes 12 Lecture 12 Goals Understand...

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Lecture Notes 12 Lecture 12 Goals Understand the GPS System EECS 455 (Univ. of Michigan) Fall 2012 October 16, 2012 1 / 62
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Lecture Notes 12 Lecture 11: Global Positioning System (Location, Location, Location) Uses 24 satellites 20,200km (12,000 miles) with half day orbit Uses one-way ranging from GPS Uses two frequencies: L1 (1575.42 MHz), L2 (1227.6 MHz) Using the range measurements from 4 satellites can estimate 3 positions (latitude, longitude, altitude) and a correction to the user’s clock. Transmits data regarding satellite location to enable users to do ranging. EECS 455 (Univ. of Michigan) Fall 2012 October 16, 2012 2 / 62
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Lecture Notes 12 GPS Components Control Segment Ground system Uploads corrections to satellite position and time Space Segment 24 satellites Low earth orbit; period 12 hours Each satellite contains accurate clocks (2 cesium clocks, 2 rubidium clocks) Each transmits a CDMA signal Data transmitted is location of satellite, time stamp User Segment Receiver tracks the range to various satellites Contains a clock Determines position, time EECS 455 (Univ. of Michigan) Fall 2012 October 16, 2012 3 / 62
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Lecture Notes 12 GPS The GPS user equipment determines the range to at least four satellites Using these four values the GPS unit determines the users clock bias (error) and the location (x,y,z) Differential GPS uses ground stations to transmit correction errors due to atmospheric effects at known locations. EECS 455 (Univ. of Michigan) Fall 2012 October 16, 2012 4 / 62
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Lecture Notes 12 GPS Two frequencies: L1=1575.42 MHz=154 × 10.23 MHz L2=1227.60 MHz=120 × 10.23 MHz Two different spreading codes: Coarse aquistion code C/A, Precision code P. C/A (coarse acquisition) code (Gold code) operates at 1.023 MHz and modulates the in-phase carrier. P code operates at 10.23 MHz and modulates the quadrature-phase carrier. Two bands can be used to measure the ionospheric delay. P code encrypted for military. EECS 455 (Univ. of Michigan) Fall 2012 October 16, 2012 5 / 62
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Lecture Notes 12 GPS Transmitter (L1 band) Data 50bps C/A Code 1.023Mcps P Code 10.23Mcps 2 P c cos ( 2 π f 1 t ) - radicalbig 2 P p sin ( 2 π f 1 t ) EECS 455 (Univ. of Michigan) Fall 2012 October 16, 2012 6 / 62
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Lecture Notes 12 GPS Transmitter (L2 band) Data 50bps P Code 10.23Mcps radicalbig 2 P p cos ( 2 π f 2 t ) EECS 455 (Univ. of Michigan) Fall 2012 October 16, 2012 7 / 62
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Lecture Notes 12 C/A Gold Code Generator 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 EECS 455 (Univ. of Michigan) Fall 2012 October 16, 2012 8 / 62
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Lecture Notes 12 C/A code (Gold Code) Generator Notes Initial state of shift registers is the all one state.
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