[FOSS-GPS] Does anyone know that why RTKLIB does not support NMEA input?

[John Morris]

"Raw" measurements include the distance and speed of each satellite from the
receiver. For each satellite, you typically get a "pseudo-range" and a
"Doppler shift" measurement. The GPS receiver knows the speed and location
of each satellite, so it uses the raw measurements to calculate the speed
and location of the GPS receiver.  The resulting values of lat, lon,
altitude, direction and speed are part of the NMEA messages, but NMEA does
not include the original raw measurements. 

RTKLIB uses a technique called "double differencing" which absolutely
requires "raw" measurements.  The NMEA protocol simply doesn't include
pseudo-ranges or Doppler shift data, so it is useless for RTKLIB.

support raw protocol input 
> instead of NMEA which is popularly used in common GPS receivers.
> Is there a reason for this?