1. ## GPS Speed

Hello,
It stands to reason that and Aircraft's GPS will give the Ground Speed, horizontal speed of the aircraft. This will not always match the True Airspeed, Indicated Airspeed, or Mach on the indication due to winds aloft, tempature ect.
However,
I have a hand held GPS reciever. This speed nearly always matches the speed on my car's speedometer, unless say I'm driving well into the 5th amendment on the highway.
However,
My GPS reciever computes position in 3 dimentions. I get Elevation, Lat, and Lon,
But, does the speed computed by my reciever only compute horzontal speed,
For example,
If Im driving down a steep hill my car's speedometer says 100 Mph (To make the math easyer)
But because of the angle of the hill, I would be moving vertically at 60mph while horzontally at 80mph, would my GPS reciever comput the vertical 60mph and horzontal 80mph, or would it mearly show that I was only going 80mph?
(This hill would make a standard 6-8-10 right triangle)
-Jonathan

2. Originally Posted by InsyleM
Hello,
If Im driving down a steep hill my car's speedometer says 100 Mph (To make the math easyer)
But because of the angle of the hill, I would be moving vertically at 60mph while horzontally at 80mph, would my GPS reciever comput the vertical 60mph and horzontal 80mph, or would it mearly show that I was only going 80mph?
(This hill would make a standard 6-8-10 right triangle)
-Jonathan
I am no expert but it make sense that the GPS calculate horizontal speed. Its kind of like flying a DME arc, if you are flying straight to the VOR then then ground speed is calculated, when flying a DME arc the ground speed will be zero!

A minimum of four satellites are needed. the first one will calculate your position on a sphere, with the satellite as the center point. No help there! with a second satellite you have two spheres intersecting, and now you narrowed you position to a circular plane. again not much help here.

with a third satellite, 3 satellites (spheres) intersecting you narrow your position to two point. one is your position and the other point can be in space far far way!

a fourth satellite, will get rid of that extra point and you have your position. as this point moves across the surface of the earth it makes sense that only the horizontal speed is calculated!

It is possible to calculated vertical speed though. but by default it will give ground speed (horizontal speed)

ME THINKS!

3. A RW aviation GPS (such as a Garmin 296 etc) can show vertical speed, altitude etc, and can calculate VNAV profiles for you, giving you top of descent points, rate of climb/descent, arrival point at target altitude etc. In fact the most basic RW handheld is much more powerful than the GPS built in to FS.

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Ludicrous speed.

5. Your automotive GPS calculates your "horizontal" speed (and measures distance) on the surface of its mathematical model of the earth, not across the actual surface of the earth. Close enough, at the level of precision it reports speed.

6. By the way, you won't be going 60 mph vertically when going 80 mph horizontally, unless you are doing the end shot for "Thelma and Louise."

The steepest normal grades on the highway system in the U.S. are 6%, which means about 4.8 mph vertically for 80 mph along the road (not horizontally). Extreme grades might be double that, but you would not likely be traveling so fast, more like 30 mph along the road, 4 mph vertically.

That vertical speed is the critical figure, as it defines the amount of power needed to lift your vehicle against gravity, or the rate at which of energy needs to be dissipated as heat to prevent it running away downhill.

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Originally Posted by BushPilot
I am no expert but it make sense that the GPS calculate horizontal speed. Its kind of like flying a DME arc, if you are flying straight to the VOR then then ground speed is calculated, when flying a DME arc the ground speed will be zero!
Ummmm.... no. Perhaps your "ground" speed will be zero in relation to the VOR but not the actual ground speed. (I'm sure that's what you meant, BTW)

But, this can ONLY happen IF you are flying a perfect DME Arc, which is practically impossible. When you fly a DME Arc, you don't fly it at a constant radius turn (You would have to be able to hold an impossibly shallow angle of bank!) To fly a DME Arc, you fly straight-line tangents of the arc for about 20 degrees, then make a correction to the arc.

Here is a link to a great explaination of how to fly a DME arc:

http://www.altairva-fs.com/training/...ifr_dmearc.htm

8. Originally Posted by Iceman07
Ummmm.... no. Perhaps your "ground" speed will be zero in relation to the VOR but not the actual ground speed. (I'm sure that's what you meant, BTW)

http://www.altairva-fs.com/training/...ifr_dmearc.htm
Yep, I was talking about the DME indicator! And as far as the GPS goes I was referring to a car GPS. I got a HUD GPS that displays the speed on the windshield its really cool. the speed is 5mph greater than the one on the speedometer. I now never look down and hate driving without it.

But if i am going down hill, what it the gps calculating. is it horizontal or the slope speed.

I all practically, the difference between horizontal and downhill slope speed is probably not much difference!

Thank for the DME ARC link...

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Originally Posted by Iceman07
But, this can ONLY happen IF you are flying a perfect DME Arc, which is practically impossible. [/url]

I fly perfect DME arcs.

10. Originally Posted by metro752
I fly perfect DME arcs.
agreed. It's not impossible. You can even fly a really awesome one with just DME and an GA style OBS. It just takes practice!!

If someone would've told me when I first started flying instruments that an ILS where the needles don't move is almost industry standard, I would've debated quitting.

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