![]() ![]() Typical GPS real-time metadata used in understanding the quality of the GPS coordinate include: PDOP, # of satellites tracked, correction method, and horizontal/vertical standard deviation values. The reason is that GPS metadata is very important for the high-precision user as a method of assisting in determining the quality of a particular GPS coordinate. In fact, I would say it’s a requirement for high-precision users. The above screenshot examples are useful for the general GPS user. For high-precision GPS mapping and surveying, the $GPGST message is particularly useful. Rarely does the $GPGGA message have enough information by itself. For example, the following screen requires: $GPGGA, $GPGSA, $GPGSV. $GPGST – Estimated horizontal and vertical precision. $GPVTG – Speed over ground and tracking offset. $GPGSV – Detailed GPS satellite information such as azimuth and elevation of each satellite being tracked. $GPGSA – Detailed GPS DOP and detailed satellite tracking information (eg. In addition to NMEA messages that contain a GPS coordinate, several companion NMEA messages offer additional information besides the GPS coordinate. Here are a couple of popular NMEA messages similar to the $GPGGA message with GPS coordinates in them (these can possibly be used as an alternative to the $GPGGA message): There are alternative and companion NMEA messages that provide similar or additional information. M denotes the units used by the geoidal separation.ġ.0 denotes the age of the correction (if any).Ġ000 denotes the correction station ID (if any). Meters or Feet)Ģ9.200 denotes the geoidal separation (subtract this from the altitude of the antenna to arrive at the Height Above Ellipsoid (HAE). Decimal places are variable.Ģ = Differentially correct coordinate (e.g., WAAS, DGPS)Ĥ = RTK Fix coordinate (centimeter precision)ġ3 denotes number of satellites used in the coordinate.ġ.0 denotes the HDOP (horizontal dilution of precision). Decimal places are variable.Ġ7044.3966270 is the longitude in the DDDMM.MMMMM format. GP represent that it is a GPS position (GL would denote GLONASS).ġ81908.00 is the time stamp: UTC time in hours, minutes and seconds.ģ404.7041778 is the latitude in the DDMM.MMMMM format. This particular message was output from an RTK GPS receiver: To understand the NMEA message structure, let’s examine the popular $GPGGA message. Furthermore, NMEA data can be transmitted via different types of communications interfaces such as RS-232, USB, Bluetooth, Wi-Fi, UHF, and many others. So, just like there are all kinds of GPS receivers with different capabilities, there are many different types of NMEA messages with different capabilities. What makes NMEA a bit confusing is that there are quite a few “NMEA” messages, not just one. Without a standard such as NMEA, it would be time-consuming and expensive to write and maintain such software. For example, VisualGPS software (free), accepts NMEA-formatted data from any GPS receiver and graphically displays it. NMEA-formatted GPS data also makes life easier for software developers to write software for a wide variety of GPS receivers instead of having to write a custom interface for each GPS receiver. The purpose of NMEA is to give equipment users the ability to mix and match hardware and software. Today in the world of GPS, NMEA is a standard data format supported by all GPS manufacturers, much like ASCII is the standard for digital computer characters in the computer world. According to the NMEA website, the association was formed in 1957 by a group of electronic dealers to create better communications with manufacturers. NMEA existed well before GPS was invented. ![]() ![]() NMEA is an acronym for the National Marine Electronics Association. You may have heard about “NMEA data” with respect to GPS. ![]()
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