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
CA Code modulation CIA C/A code bits CIA bi-phase modulated signal 02/27/02 12.540Lec07
02/27/02 12.540 Lec 07 6 CA Code Modulation 0 Vo -Vo C/A carrier (a) 0 C/A code bits (b) 0 C/A bi-phase modulated signal (c)
P-Code generation P(Y carrier -√2V P(Y code bits P(Y bi-phase quadrature modulated signal (f) P-code rate should 10 times higher than CIA code 02/27/02 12.540Lec07
02/27/02 12.540 Lec 07 7 P-Code generation 0 P(Y) carrier √2 Vo -√2 Vo (d) 0 P(Y) code bits (e) 0 P(Y) bi-phase quadrature modulated signal (f) P-code rate should 10 times higher than C/A code
Composite: Sum of C/a and P code Composite C/A+P(Y) signals seconds ClA code bits 0 PY code bits () -20 seconds 02/27/02 12.540Lec07
02/27/02 12.540 Lec 07 8 Composite: Sum of C/A and P code 0 C/A code bits (h) 0 Composite C/A+P(Y) signals ~3 µseconds (g) 0 P(Y) code bits ~20 µseconds (i)
Composite GPS signal Last few slides show construction of composite signal There are sets of phase reversals on the L1 signal: C/A code at rate of 1.023 MHz and the P(Y code add g00 out of phase at a rate of 10.23 MHz How do you the gPs signal if you dont know both codes(since each reverses the sign and should average to zero)? Answer next slide 02/27/02 12.540Lec07
02/27/02 12.540 Lec 07 9 Composite GPS signal • Last few slides show construction of composite signal • There are sets of phase reversals on the L1 signal: C/A code at rate of 1.023 MHz and the P(Y) code add 90 o out of phase at a rate of 10.23 MHz • How do you the GPS signal if you don’t know both codes (since each reverses the sign and should average to zero)? Answer next slide
Tracking knowing only C/A code Tracking is possible for two reasons P-code is written with 50% power P-code is written out of phase Phasor diagram shows how this works Assume that CIA code known, so that sign reversals from c/a code are removed Consider tracking in -0.1 usec(1/10.23 MHZ segments of data( no range change and frequencies match exactly) 02/27/02 12.540Lec07
02/27/02 12.540 Lec 07 10 Tracking knowing only C/A code • Tracking is possible for two reasons: – P-code is written with 50% power – P-code is written out of phase • Phasor diagram shows how this works • Assume that C/A code known, so that sign reversals from C/A code are removed • Consider tracking in ~0.1 µsec (1/10.23 MHz) segments of data (no range change and frequencies match exactly)
