Of michigan 1 0 1 fall 2012 2 time 3 4 5 6 october

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Unformatted text preview: 12 2 time 3 4 5 6 October 16, 2012 7 20 / 62 Lecture Notes 12 GPS C/A code notes 20 repeats of Gold code per data bit Width of receiver output “blip” is twice the chip duration (2Tc = .977517µs). Data transmitted contains information about satellite location and correction of the satellite clock. EECS 455 (Univ. of Michigan) Fall 2012 October 16, 2012 21 / 62 Lecture Notes 12 GPS P code Operates at 10.23 M chips/second (Mcps) P code is exclusive or (⊕) of two sequences (length 15,345,000 chips and 15,345,037 chips) Codes are reset every week (period =1 week) Each satellite has a separate code obtained by delaying one of the two sequences by different amounts EECS 455 (Univ. of Michigan) Fall 2012 October 16, 2012 22 / 62 Lecture Notes 12 GPS Timing Time divided into 1024 weeks (repeats every 19 years, 36 weeks). Zero time is midnight Saturday January 5, 1980 or 12:00am Sunday Jan 6, 1980, August 22, 1999 12:00am,... Weeks are divided into 1.5 second “X1” epochs. X1 epoch occurs every 1.5 seconds (403,200 X1 epochs every week). Z count is the number of X1 epochs since zero GPS time (max=412,876,800=228.62) 10 bits for the week number (0 to 1023). 19 bits for the number of X1 epochs since start of time of week (TOW) count. 17 MSB are included in the HOW message indicating number of 6 second subframes since the beginning of the week. EECS 455 (Univ. of Michigan) Fall 2012 October 16, 2012 23 / 62 Lecture Notes 12 GPS Data GPS Data is a 50bps stream Same information on the C/A code and P(Y) code. Synchronous with the C/A code epochs Data are formated into 30 bit words (0.6 seconds) 10 words is a subframe (6 seconds, 300 bits) 5 subframes is a frame (30 seconds, 1500 bits) 25 frames or pages (12.5 minutes) for complete set of data EECS 455 (Univ. of Michigan) Fall 2012 October 16, 2012 24 / 62 Lecture Notes 12 GPS Data Word 1 and 2 of all subframes contain a preamble for synchronization and a handover word (HOW). First subframe describes the satellite clock information relative to GPS time Ephemeris data giving the satellites position (updated every 2 hours) Almanac data is the coarse orbit and status information for each satellite in the constellation. Repeats every 12.5 minutes EECS 455 (Univ. of Michi...
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