GPS - Doubt regarding RMC speed value in NMEA files

In summary, this person is trying to do something with GPS data that comes from NMEA files, but they have two doubts. They also want to know more about how the speed value is calculated.
  • #1
Roughmar
6
0
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?
 
Astronomy news on Phys.org
  • #2
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
 
  • #3
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.
 
  • #4
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/
 
  • #5


I understand your concern about the accuracy and interpretation of the speed value in NMEA files. The speed indicated in the RMC sentence refers to the speed over ground, meaning the speed of the GPS device in relation to the Earth's surface. This is different from speed over water, which is commonly measured in knots and is used in marine navigation.

To answer your first doubt, the speed over ground in NMEA files is a 2D speed measurement, meaning it does not take into account changes in altitude. This may affect the accuracy of the measurement in certain situations, such as when traveling over hilly terrain or through tall buildings.

Regarding your second doubt, the accuracy of the speed value depends on several factors, including the quality of the GPS signal, the number of satellites being used for the calculation, and the processing algorithms used by the GPS device. Generally, GPS devices are accurate within a few meters, but this can vary depending on the circumstances.

If you require more precise speed measurements for your application, you may want to consider using other data sources, such as an accelerometer, to complement the GPS data. Additionally, it is always a good idea to perform validation and calibration tests to ensure the accuracy of your data.

In conclusion, while the speed value in NMEA files may not be as accurate as other measurement methods, it can still provide valuable information for your application. It is important to understand the limitations and potential sources of error when using this data, and to consider alternative methods for more precise measurements if needed.
 

1. What is the RMC speed value in NMEA files?

The RMC speed value in NMEA files refers to the speed over ground of the GPS receiver at the time the NMEA sentence was generated. It is typically measured in knots (nautical miles per hour) or meters per second.

2. How is the RMC speed value calculated?

The RMC speed value is calculated using the GPS receiver's position and time information, as well as the number of satellites in view and their signal strength. The receiver uses this data to determine the speed at which it is moving over the Earth's surface.

3. Why is there sometimes a discrepancy between the RMC speed value and the actual speed?

There can be several reasons for a discrepancy between the RMC speed value and the actual speed. This could be due to signal interference, changes in atmospheric conditions, or inaccuracies in the GPS receiver's hardware or software. It is important to note that the RMC speed value is just an estimate and may not always be 100% accurate.

4. Is the RMC speed value the same as the speedometer in my car?

No, the RMC speed value is not the same as the speedometer in your car. The RMC speed value is based on the speed over ground, while the speedometer in your car measures the speed relative to the wheels. This means that the two values may not always match, especially when driving on curved roads or in areas with poor GPS signal.

5. Can the RMC speed value be used for navigation purposes?

While the RMC speed value can provide an estimate of your speed over ground, it should not be solely relied upon for navigation purposes. It is always best to use a combination of GPS data, along with other navigation tools, to ensure accurate and safe navigation.

Similar threads

  • Aerospace Engineering
2
Replies
35
Views
3K
  • Programming and Computer Science
Replies
4
Views
1K
Replies
1
Views
2K
  • Mechanical Engineering
Replies
1
Views
3K
  • Programming and Computer Science
Replies
29
Views
2K
  • Aerospace Engineering
Replies
2
Views
4K
Replies
2
Views
4K
  • Mechanical Engineering
Replies
5
Views
19K
Back
Top