GPS - Doubt regarding RMC speed value in NMEA files

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Discussion Overview

The discussion revolves around the interpretation and accuracy of the speed value indicated in the RMC sentence of NMEA GPS data files. Participants explore the meaning of "speed over ground" and its calculation methods, as well as the availability of documentation for further reference.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions whether the speed value represents speed relative to the ground or a projection on the ground, indicating a need for clarification on its definition.
  • Another participant asserts that the speed is calculated from the Doppler shift of the incoming signal and is generally accurate to greater than 0.5%, while noting that cheaper receivers may use less accurate methods based on position change.
  • A participant expresses interest in finding a manual or datasheet that consolidates information about GPS data and its imprecision, indicating a desire for more accessible resources.
  • Another participant mentions the existence of various books on GPS and highlights that GPS chip datasheets can provide specific information, but notes that implementations may vary across devices.
  • It is mentioned that the NMEA standard is not freely available, but copies can be found, and an open-source library exists for reading different versions of NMEA data.

Areas of Agreement / Disagreement

Participants present differing views on the accuracy and calculation methods of the speed value in NMEA files, and there is no consensus on the best resources for obtaining comprehensive information.

Contextual Notes

The discussion highlights potential limitations in the accuracy of speed calculations based on the type of GPS receiver used, as well as the variability in NMEA data implementations across different devices.

Roughmar
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I am afraid that this is either too specific and\or that this is in the wrong forum (if the latter is the case, if moderators could change this, I'd appreciate :).

I'm trying to do an application that uses the GPS data that comes from NMEA files.

On a particular sentence (RMC) there is a value indicating "speed". After looking about it, I found a place that described this speed as in "Speed over ground, Knots".

I have two doubts regarding this value:

Exactly what does it mean? Is it the speed having the ground as a reference, or a projection on the ground, making it a 2D speed (not caring about differences in the altitude.)?

Also, how accurate is it? How is it calculated?
 
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It's just the normal speed, the over-ground bit is because it's often used by aircraft to distinguish it from airspeed (which depends on winds).

Most reasonable receivers calculate it directly from the incoming signal wavelength (essentially the doppler shift) and it should be accurate to > 0.5%
A cheap receiver might calculate it from change in position per second which is much less accurate.

I suppose it's essentially the straight line speed in 3D
 
Thank you for the speedy answer. =)

While I'm here I'll ask just one more thing: is there a manual or datasheet where this info is?
Sometimes I have doubts regarding the imprecision of certain aspects and find it cumbersome to google for each specific thing every time it happens. Having a .pdf with all the info would be great, but I don't know what to look for.
 
There are lots of books about GPS.
And the GPS chip will have a datasheet - if you know which one it is, the SiRFstar III is about the most common.

But the device it's in will have it's own computer which might add/remove features. Some makers also put extra data in their own proprietry format rather than NMEA.

The NMEA standard is pay-for information but you can find lots of copies of it, and anyway individual products implementation is a bit sketchy. There is an opensource library that will read almost all versions http://nmea.sourceforge.net/
 

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