# GPS - Speed over ground VTG

Hi, just trying to understand the full meaning of speed over ground. I do know that it is speed relative to the earths surface. But does this mean that it is based on the earth being perfectly spherical? If a car is driving up a steep hill at 60km/h (as displayed on the cars speedo), is the (SOG) speed over ground likely to be slower, lets say 58km/h (even though the car is on the "ground")?

Have I understood this correctly?

 jcaron2 Posts: 983, Reputation: 1034 Senior Member #2 Mar 21, 2011, 10:25 AM
I'm not an expert in this subject matter, but I think you're mostly correct. I don't think GPS models of the Earth's surface are based on a perfect sphere though; I think the equatorial bulge is taken into account. Nevertheless, in the spirit of your question, yes I think SOG is, by definition, only the horizontal component of velocity (as opposed to a car's speedometer which, as you pointed out, is really measuring speed in the true [not necessarily horizontal] direction of travel).
 Newton1Law Posts: 60, Reputation: 20 Junior Member #3 Mar 29, 2011, 11:52 AM
The GPS system makes its measurements from space based on time, altitude and location of your GPS receiver. Time and location measurements are corrected by ground stations used to corrected for realitivity affects, slight altitude differences, gravational differences, etc. Imposed on the satellites. Most aviation GPS units with differential capabilities do actually calculate vectors in three dimensional space. Newer ground based units also now have this ability and will give you the correct speed on even on a 45 degree hill.

Hope this helps,

Newton1Law
 jcaron2 Posts: 983, Reputation: 1034 Senior Member #4 Mar 29, 2011, 04:37 PM
Good answer. But isn't there a difference specifically between velocity (in three dimensions) versus SOG, which should by definition ignore the Z-component? I assume modern aviation GPS units could give you either.
 excon Posts: 21,005, Reputation: 15480 Expert #5 Mar 29, 2011, 04:45 PM
Quote:
 Originally Posted by rllycr If a car is driving up a steep hill at 60km/h (as displayed on the cars speedo), is the (SOG) speed over ground likely to be slower, lets say 58km/h (even though the car is on the "ground")? Have I understood this correctly?
Hello r:

Sure... It takes MORE pavement for the car going up the hill at 60 to travel x distance, than it would if there were no hill. That's why it's going, say 58 over the ground.

excon
 Newton1Law Posts: 60, Reputation: 20 Junior Member #6 Mar 31, 2011, 03:32 PM
Your question then is really asking if the GPS will show you the speed in the "X" or horizontal direction when in fact the car is moving in both the horizontal and vertical directions. The answer is that the GPS will give you the velocity you are traveling at and will not give you just the speed in the "X" or horizontal. The next time I fly I will check this to make sure.

As to your question, "But isn't there a difference specifically between velocity (in three dimensions) versus SOG, which should by definition ignore the Z-component?" I would say no because your speed over the ground is still 60 MPH even if you were on a 45 degree hill (assuming your car this kind of power). Consider that if your cars speed indication was 60 MPH, then you are moving at 60 MPH which is the speed over ground, specifically the road. As far as displacement in space is concerned, you are moving vertically at 42.43 MPH and horizontally at 42.43 MPH. However in an aircraft, your question is perhaps better understood in that I can climb at say 200 miles per hour at say 25 degrees and thus I am traveling at 181.26 horizontally and 84.52 MPH vertically.
 jcaron2 Posts: 983, Reputation: 1034 Senior Member #7 Mar 31, 2011, 09:16 PM
Thanks.

And just to make sure we're on the same page, I think the OP was asking if in that last example you gave, would the SOG be 200 mph or 181?

From your flying experience it sounds like it would be 200.

It's actually difficult to find a definition for SOG that ever even mentions vertical velocity. However, I think I found something relevant at this link, supposedly a glossary of terms for avionics systems. Notice that it makes a distinction between "Earthspeed" - the velocity in the direction parallel to the Earth's surface directly below (which is exactly what a car's speedometer is measuring) - and "groundspeed" which is explicitly the horizontal component of travel (i.e. The earthspeed vector projected onto a horizontal plane).

I realize that "groundspeed" may not by definition be the same as "speed over ground", as well as the fact that reading something on the internet doesn't necessarily make it true. However, it does lend credence to the OP's hunch that SOG really does just refer to the horizontal component of velocity.
 Newton1Law Posts: 60, Reputation: 20 Junior Member #8 Apr 1, 2011, 07:00 AM
Well I tried this out yesterday on my GPS system and it did indeed give ground speed. I was climbing out at about 2000 fps at 150 mph. GPS indicated ground speed at 148 while I was at about 10 degrees of pitch. I got this when I selected ground speed, but I understand not all GPS units have this as selectable and just give ground speed. Sorry for the confusion.
 steve149c Posts: 4, Reputation: 10 Junior Member #9 Apr 25, 2011, 08:20 AM
Ground speed - firstly there are a lot of variables to this - primarily due to Latitude.

Ground Speed, or Speed over the Ground is generally used for Aviation and Shipping. Imagine a plane, doing 150 Air Speed knots heading West, there is a Westerly wind (ie a head wind) of 50 knots. Not the Air Speed Indicator will show 150 knots, but your ground speed will be 100 knots. If it was a 50 knot tail wind, then your ground speed would be 200 knots.

It also calculates your set and drift due to the wind at angles. Whilst there are small errors for height and earth rotation, these are generally calculated out - as the GPS will know your height (assuming 5 satellites) - and the earths rotational difference from sea level to 40,000 feet is negligable.

Hope that this has helped

Steve

## Check out some similar questions!

Air speed versus ground speed [ 8 Answers ]

NT="Comic Sans MS"] What is ground speed versus air speed and why is the difference important? Howdo you compute one if you have the other?

What is the difference between gps and a-gps [ 1 Answers ]

what is the difference between gps and a-gps?

A car travels a certain distance with a speed of 40m/s and returns with a speed 'u'. If the average speed for the whole journey is 48m/sec what is the value of'u'?

How the ground speed is calculated in GPS [ 2 Answers ]

How the GPS calculates ground speed. How it is different from ground speed calculated by DME. Whether these two are same are there is a difference between two.