Having just bought several GlobalSat BU-353 units with the expectation for the capability of the devices to compute an SBAS corrected 'differential' (DGPS) 3D position fix, it has been found that at no time do any of the units do so...
Each BU-353 unit only computes a 'standalone' 3D position fix despite using the 'factory default' settings etc...
$PSRFTXT,POS: 6378137 0 0*2A
$PSRFTXT,Baud rate: 4800*65
Sample NMEA data is:-
The results seen are that the while the units are computing a 3D fix:-
o NMEA GGA telegram message Mode Indicator field always equals ‘1’
o NMEA GGA telegram message differential correction age field always ‘null’
o NMEA GGA telegram message Reference Station ID field always equal to ‘0000’
o NMEA RMC telegram message Mode Indicator field always equals ‘A’
o NMEA GSV telegram messages never show tracking of any SBAS PRN IDs
So delving deeper into the BU-353 SiRFIII operation by using the 'SirfDemo' application, it was found that the SBAS PRN ID for North Americas (WAAS) was being tracked (Channel #12), however the device was not using the WAAS PRN for any differential correction 3D position fix solution, or reporting the WAAS PRN ID in the NMEA GSA or GSV telegram messages... The question is; WHY?
It appears that the GlobSat BU-353 device requires "a strong" SBAS PRN satellite tracking signal in order to compute a differential GPS 3D position fix as advertised (ie; WAAS enabled default unit)... Placement of the unit outside with a clear view of the sky is required. Placing near a window may not work very well to obtain a DGPS 3D fix solution.