Why are the two GPS Transmitter frequencies (1575.42 and 1227.6 MHz) coherently selected integer multiples of 10.23 MHz master clock?
Question posted on CANSPACE on October 30, 2006, by Sivaraman Ranganathan.
The document defining the GPS signal, IS-GPS-200, states that “The carrier frequencies for the L1 and L2 signals shall be coherently derived from a common frequency source within the SV.” This makes the L1 and L2 multiples of the common frequency source 10.23MHz. (Section 18.104.22.168). Why is this? I believe this is done for simplicity of system design and operation. All components of the signal (code, carrier, and navigation data) are derived from the atomic frequency standards on board the satellite. If this were not done and separate frequency sources were used, then biases between the different components would occur, which would have to be calculated and removed.
IS-GPS-200 furthermore states in Section 22.214.171.124 that the C/A and P(Y) digital codes are as well derived from the same frequency standard. “All transmitted signals for a particular SV shall be coherently derived from the same on-board frequency standard; all digital signals shall be clocked in coincidence with the PRN transitions for the P-signal and occur at the P-signal transition speed. On the L1 channel the data transitions of the two modulating signals (i.e., that containing the P(Y)-code and that containing the C/A-code), L1 P(Y) and L1 C/A, shall be such that the average time difference between the transitions does not exceed 10 nanoseconds (two sigma)”.
Despite the coherence of the two carriers, it is understood there is a difference between the radiated L1 and L2 signals due in part to the different paths the signals take within the on-board electronics. This is called the differential group delay and an estimate of this difference is broadcast to users in the navigation message. The difference between L1 P(Y) and L2 P(Y) is designated Tgd (reference paragraph 126.96.36.199.3.2). The difference between L1 P(Y) and L2C is called the Inter-Signal Correction (ISC) (reference paragraph 188.8.131.52.1.1).
For further technical discussion of this topic, see the book Global Positioning System, Signals Measurements, and Performance by Pratap Misra and Per Enge (section 2.3.1).
John Lavrakas, President
Advanced Research Corp.