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Calculating and validating nmea checksums

calculating and validating nmea checksums-44

The MTK NMEA checksum can be calculated by using the following procedure: The NMEA Checksum is calculated on a character by character basis using an XOR operator.Each character is fed into an XOR with the current checksum value. The characters that exist between the "$" and "*" are the only characters to be used in the calculation.

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Full NMEA sentence -------------------------------------------------------------- $GPGGA,205118.000,2206.2392, N,03356.2124, W,1,8,0.95,223.8, M,-27.1, M,,*G,00,, Lattitude, Longitude ------------------------------------------------------------ 22.103989,-33.936867 Full NMEA sentence -------------------------------------------------------------- $GPGGA,205215.000,2206.2368, N,03356.2130, W,1,8,0.95,223.5, M,1 Lattitude, Longitude ------------------------------------------------------------ 22.103947,-33.936882 Full NMEA sentence -------------------------------------------------------------- $GPGGA,205319.000,2206.2346, N,03356.2131, W,1,7,1.13,223.3, M,-27.1, M,,*P26859G010724 Lattitude, Longitude ------------------------------------------------------------ 22.103909,-33.936882 Full NMEA sentence -------------------------------------------------------------- $GPGGA,205419.000,2206.2350, N,03356.2118, W,1,8,0.96,223.0, M,-27.1, M,,*6,36,,*S2,3,,1G,1,724 Lattitude, Longitude ------------------------------------------------------------ 22.103920,-33.936867 Full NMEA sentence -------------------------------------------------------------- 33,,,,7,04,,, Lattitude, Longitude ------------------------------------------------------------ 22.103909,-33.936813 Full NMEA sentence -------------------------------------------------------------- ,13*M069,11 Lattitude, Longitude ------------------------------------------------------------ 22.103882,-33.936813 Full NMEA sentence -------------------------------------------------------------- $.04 Lattitude, Longitude ------------------------------------------------------------ 22.103897,-33.936813 I modified the library Asset Tracker.h by adding in a println(nmea) into the Adafruit_GPS::parse function.Each NMEA report is a text packet, or sentence, that begins with a dollar sign and ends with a carriage-return and line feed.The data elements in in NMEA sentences are just text fields separated by commas, like this: This means that log files of collected NMEA sentences are easy to read and edit.The text I added always comes across the same, which is good, but the full NMEA sentence seems irregular.Does the routine which prints the last NMEA sentence have a problem?My Asset Shield and Electron seems to be working fine as far as publishing GPS coordinates.

But when I made a serial connection to COM3: (9600, N,8,1) the output seemed erratic like bits and pieces were missing.

What I saw is that malform lines were being fed into the parse function.

Worse, it looks like if the "*" isn't in the right place, it doesn't attempt to confirm the line's checksum, and it goes forward with processing the line.

I’m the lead of the GPSD project, a service daemon that monitors GPS receivers on serial or USB ports and provides TPV (time-position-velocity) reports in a simple format on on a well-known Internet port. But it’s not — oh, it’s decidedly not — and thereby hangs an entertaining tale of hacker ingenuity versus multiple layers of suck.

Away back in the dark and backward abysm of time when GPS technology was first being made generally available (1993), only military-grade receivers were sensitive enough to use it where there were things like buildings and trees partly blocking the sky view.

It is used to get informations about current speed, GPS validation, ground coarse and date Distance and bearing informations are very useful when working with autopilot or autonomous devices. Both can be calculated with single function GPS_Distance Between.