The L1C signal on GPS III

The recently-launched GPS III satellite, SVN 74, is transmitting on PRN 4, though not yet set healthy. But it’s a good opportunity to have a look at the L1C signal.

The pilot is TMBOC; here is its correlation peak:


It looks proper to me, though I haven’t compared it against the ideal waveform. The BOC(6,1) wiggles are clearly present, narrowing the central peak. Note the small amount of signal on the quadrature phase. This could have a number of sources: the passband of my receiver is not ideally flat in amplitude and group delay (I may add some equalization at some point), and the GPS L1 carrier is significantly offset from the center of the passband (by about 9 MHz) so as to accommodate GLONASS. The satellite signal itself could also be contributing. The recording was taken with a gain antenna (a 15-turn helical, about 11 dBi), and the integration time for the correlation was two seconds.

The data signal is pure BOC(1,1) with no BOC(6,1) content:


I’m not sure what tracking is most appropriate for GPS III. Of course the individual observables for C/A and L1C{p,d} are available, and, in the case of L1C, a combined mode with PLL tracking of the pilot and Costas-loop tracking of the data channel (or PLL tracking after data wipeoff), suitably weighted. But should a joint weighting of both C/A and L1C be considered, which would seem to include even more signal energy, yielding a higher-quality observable? There’s no RINEX name for this mode.