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)