In doing some research, I'm trying to answer a 'Why' question. Here it is: Why are the fundamental GPS frequencies for L1 & L2 multiplied by the value of 154 and 120 respectively? I think I understand the concept of why the fundamental frequency is 10.23 Mhz, but can't seem to find a reason why the values of 154 and 120 were chosen to 'set' the L1 and L2 frequencies. I'm open to any suggestions and always welcome open dialogue. This is a great site for discussion and I hope that I'm able to contribute what little I can.
Cheers,
GG
shooting from the hip on this one; these were the two 'slots' available from already/nearby used frequencies at the time gps was introduced and when dual frequency was introduced.
the truth will involve research and reading. the above is simply my recollection of earlier reading
from gps world website...
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 3.3.1.1). 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 3.3.1.8 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 20.3.3.3.3.2). The difference between L1 P(Y) and L2C is called the Inter-Signal Correction (ISC) (reference paragraph 30.3.3.3.1.1).
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).
Transmission frequencies are regulated by international agreements. The L1, L2 and L5 frequencies are in bands designated for radio navigation.
See: http://www.navipedia.net/index.php/GNSS_signal
The choice of the multiplier puts them in these protected bands as well as provides sufficient separation to allow determination of ionospheric errors by frequency combinations.
HTH,
DMM
from gps world website...
Its taken me some time to get back, but I appreciate your response. It was very helpful and provided me the insight that I was failing to see! Things are coming together now!
Again, thanks for the response
Hi GeeOddMike! Thanks for the link! Definitely helped me get a better grasp of the design!