Nmea Introduction

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  1. 1. NMEA Introduction Tom Chen
  2. 2. Overview What is NMEA? NMEA Data Format Intro of Selected Datatype Decode of Selected Sentences References
  3. 3. What is NMEA? Specified by National Marine Electronics Association Define the interface between various marine electronic equipments (HW / SW) NMEA 0183 Serial commnunication (EIA-422, RS-232) 4800 b/s, 8 bits of data, no parity, one stop bit ASCII format Latest release is v2.3 NMEA 2000 Control Area Network (CAN) Designed for auto industry 250 kb/s, Binary format
  4. 4. NMEA Data Format Composed of sentences (messages) Format of sentence Start with '$', End with 'rn' (CR LF). Each data field is delimited by ',' and could be empty. First field indicate the datatype of sentence Checksum is appended at last field and prepended by a '*' Bitwise-XOR all characters between '$' and '*' Example: $GPRMC,134807,A,2409.9030,N,12030.7368, E,0.000,332.460,230212,0.0,W*62
  5. 5. Intro of Selected Datatype Messages applicable to GPS receiver (All messages start with "GP") GGA: Fix information GSA: Overall satellite data GSV: Detailed satellite data RMC: Recommended minimum data for GPS Proprietary message GGYRO: Gyro state ELIN: ???
  6. 6. GGA Essential fix data which provide 3D location and accuracy data. $GPGGA,123519,4807.038,N,01131.000,E,1,08,0.9,545.4,M,46 .9,M,,*47 123519: Fix taken at 12:35:19 UTC 4807.038,N: Latitude 48 deg 07.038' N 01131.000,E: Longitude 11 deg 31.000' E 1: Fix quality, GPS fix, 0 = fix not available, 2 = Differential GPS fix 08: Number of satellites being tracked 0.9: Horizontal dilution of position (HDOP) 545.4,M: Antenna Altitude (meters) above/below mean-sea-level (geoid) 46.9,M: Geoidal separation, the difference between the WGS-84 earth ellipsoid and mean-sea-level (geoid) (empty field): time in seconds since last DGPS update (empty field): DGPS station ID number *47: Checksum
  7. 7. GGA c.1
  8. 8. GSA GPS DOP and active satellite DOP (Dilution Of Precision) is an indication of the effect of satellite geometry on the accuracy of the fix For 3D fixes using 4 satellites a 1.0 would be considered to be a perfect number $GPGSA,A,3,04,05,,09,12,,,24,,,,,2.5,1.3,2.1*39 A: Auto selection of 2D or 3D fix (M = manual) 3: 3D fix, 1 = no fix, 2 = 2D fix 04,05... : PRNs of satellites used for fix (space for 12) May have different representation 2.5: Positional (3D) Dilution Of Precision (PDOP) 1.3: Horizontal dilution of precision (HDOP) 2.1: Vertical dilution of precision (VDOP)
  9. 9. GSA c.1
  10. 10. GSV Satellites in view One GSV sentence can provide data for 4 satellites and thus need 3 sentences for the full information $GPGSV,2,1,08,01,40,083,46,02,17,308,41,12,07,344,39,1 4,22,228,45*75 2: Number of sentences for full data 1: Sentence 1 of 2 08: Number of satellites in view 01: Satellite PRN number 40: Elevation, degrees 083: Azimuth, degrees true 46: SNR(Signal to Noise Ratio) - higher is better
  11. 11. GSV c.1 Elevation
  12. 12. RMC Recommended Minimum Specific Essential GPS pvt (position, velocity, time) data $GPRMC,123519,A,4807.038,N,01131.000,E,022.4,084.4, 230394,003.1,W*6A 123519: Fix taken at 12:35:19 UTC A: Status A=active or V=Void. 4807.038,N: Latitude 48 deg 07.038' N 01131.000,E: Longitude 11 deg 31.000' E 022.4: Speed over the ground in knots 084.4: Track angle in degrees true (Could also be magnetic north) 230394: Date - 23rd of March 1994 003.1,W: Magnetic Variation
  13. 13. Example
  14. 14. References http://www.gpsinformation.org/dale/nmea.htm http://en.wikipedia.org/wiki/NMEA_0183