I've done NMEA parsers; although the protocol looks straightforward, there
is still a lot of subtleties and a vendor/device dependent things.
It is not very simple to do a universal parser which decodes correctly all
possible strings in all possible variants.
Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
Yes, I have a specific questions relating to a NMEA 0183 $GPGGA Sentence
on a SiRF StarIII GPS Chipset.
(see GGA extract below)
Questions
======
a) What does [ (6) GPS Quality Indicator = 1] really indicate? More than 4
channel locked and Accuracy is within spec?
b) Is [ 8) Horizontal Dilution of precision ] really indicate metres or a
relative error of measurement ?
c) How do I determine when the NMEA 0183 $GPGGA Sentence will give me
accurate position?
Thanks in advance Joe
Extract from
http://gpsd.berlios.de/NMEA.txt
GA - Global Positioning System Fix Data
Time, Position and fix related data for a GPS receiver.
1 2 3 4 5 6 7 8 9 10 | 12 13 14 15
| | | | | | | | | | | | | | |
$--GGA,hhmmss.ss,llll.ll,a,yyyyy.yy,a,x,xx,x.x,x.x,M,x.x,M,x.x,xxxx*hh<CR><LF> Field Number: 1) Universal Time Coordinated (UTC) 2) Latitude 3) N or S (North or South) 4) Longitude 5) E or W (East or West) 6) GPS Quality Indicator, 0 - fix not available, 1 - GPS fix, 2 - Differential GPS fix (values above 2 are 2.3 features) 3 = PPS fix 4 = Real Time Kinematic 5 = Float RTK 6 = estimated (dead reckoning) 7 = Manual input mode 8 = Simulation mode 7) Number of satellites in view, 00 - 12 8) Horizontal Dilution of precision (meters) 9) Antenna Altitude above/below mean-sea-level (geoid) (in meters) 10) Units of antenna altitude, meters 11) Geoidal separation, the difference between the WGS-84 earth ellipsoid and mean-sea-level (geoid), "-" means mean-sea-level below ellipsoid 12) Units of geoidal separation, meters 13) Age of differential GPS data, time in seconds since last SC104 type 1 or 9 update, null field when DGPS is not used 14) Differential reference station ID, 0000-1023 15) Checksum