12. 540 Principles of the Global Positioning System Lecture 07 Prof. Thomas Herring 02/27/02 12.540Lec07
02/27/02 12.540 Lec 07 1 12.540 Principles of the Global Positioning System Lecture 07 Prof. Thomas Herring
Today's Lecture EXamine specifics of GPS signals Multiple modulations on same carrier signal Structure of signals GPS receiver operation and satellite acquistion 02/27/02 12.540Lec07
02/27/02 12.540 Lec 07 2 Today’s Lecture • Examine specifics of GPS signals • Multiple modulations on same carrier signal • Structure of signals • GPS receiver operation and satellite acquistion
Specifics of GPs signal GPS transmits at two frequencies L11575.42MHz(2×77×1023MHz) L2122760MHz(2X60X10.23MHz) Wavelengths L1-190 mm: L2 -244 mm Codes Course acquisition code( C/A) Chip rate(rate at which phase might change)1.023 MHZ Precise positioning code(P code)10.23 MHz Y-code(Antispoofing code) also 10.23 MHZ derived by multiplying P-code by-20KHz code(highly classified 02/27/02 12.540Lec07
02/27/02 12.540 Lec 07 3 Specifics of GPS signal • GPS transmits at two frequencies – L1 1575.42 MHz (2x77x10.23 MHz) – L2 1227.60 MHz (2x60x10.23MHz) – Wavelengths L1 ~190 mm; L2 ~244 mm • Codes: – Course acquisition code (C/A) Chip rate (rate at which phase might change) 1.023 MHz – Precise positioning code (P code) 10.23 MHz – Y-code (Antispoofing code) also 10.23 MHz derived by multiplying P-code by ~20KHz code (highly classified)
Specifics of GPS Code lengths CIA code is 1023 bits long P-code is 37 weeks long(2X1014 bits in code Only one P-code, satellites use different weeks from same code(P-code repeats each week) As far as we know Y-code never repeats(again classified) Data message: Implemented by changing sign of code at rate of 50 bits/second(low data rate) 02/27/02 12.540Lec07
02/27/02 12.540 Lec 07 4 Specifics of GPS • Code lengths: – C/A code is 1023 bits long – P-code is 37 weeks long (2x1014 bits in code) – Only one P-code, satellites use different weeks from same code (P-code repeats each week) – As far as we know Y-code never repeats (again classified) • Data message: Implemented by changing sign of code at rate of 50 bits/second (low data rate)
Specifics of GPS 10.23 MHZ is fundamental frequency in GPS All radiofrequencies and codes generated from the same 10.23MHz crystal whose long term stability is controlled by Cesium or Rubidium clock(older satellites) The following graphics show schematically the construction of the GPS signal 02/27/02 12.540Lec07
02/27/02 12.540 Lec 07 5 Specifics of GPS • 10.23 MHz is fundamental frequency in GPS • All radiofrequencies and codes generated from the same 10.23MHz crystal whose long term stability is controlled by Cesium or Rubidium clock (older satellites) • The following graphics show schematically the construction of the GPS signal