Speed Calculator Help

[mattadata]

Originally Posted by yurtle

Is a GPS app smart enough to account for hills? Up and down run up miles, while not moving horizontally. Just curious.

Yes, it is smart enough to figure that out. I have launched numerous high altitude weather balloons which carry payloads of cameras to about 100,000 feet. After the balloon bursts, it falls straight down and reaches 100+ MPH without much movement in longitude or latitude. I am not exactly sure how it does it, but I believe it uses altitude and/or Doppler shift to calculate speed.

Here is one of my videos seconds before the balloon burst and falls back to earth. https://www.youtube.com/watch?v=lxZeYfLcqcE

[JohnnieB]

Originally Posted by yurtle

Is a GPS app smart enough to account for hills? Up and down run up miles, while not moving horizontally. Just curious.

Not quite sure what you are asking.

Is it something like: If you moved 4 miles horizontally while climbing 3 miles vertically, what would a GPS receiver report as the distance traveled?

(Climbing a 30° incline is unlikely, but using a 3-4-5 triangle for the example makes the math easy. )

An odometer that counted tire revolutions would say 5 miles, while many GPS receivers would say 4 miles, a few highly sophisticated ones would say 5 miles and the rest would say something in between. Since the speed reported is the distance between two Lon/Lat points vs time, most consumer grade GPS receivers will report a speed slower than what a conventional speedometer does while going up or down hills.

Basically, if dropped straight down, most GPS receivers will report a speed of 0.0 MPH.

I'm guessing, but a GPS App on a smart phone is probable the equivalent of a base model dedicated GPS receiver, so changes in elevation are not likely to be included in speed or distance calculations.

However, even with a 30° incline, the error is only 20%, so the error on the milder hills we usually encounter will be less and at speeds under 30 MPH, it won't be much.

[JohnnieB]

Originally Posted by mattadata

Yes, it is smart enough to figure that out. I have launched numerous high altitude weather balloons which carry payloads of cameras to about 100,000 feet. After the balloon bursts, it falls straight down and reaches 100+ MPH without much movement in longitude or latitude. I am not exactly sure how it does it, but I believe it uses altitude and/or Doppler shift to calculate speed.

Here is one of my videos seconds before the balloon burst and falls back to earth. https://www.youtube.com/watch?v=lxZeYfLcqcE

Are the GPS devices consumer grade, scientific or military?

Is the GPS altimeter data supplemented with barometric or radar altimeter data?

I haven't looked into GPS systems in several years, so the current crop of consumer grade receivers may be more sophisticated than what I based my previous post on.

[mattadata]

They certainly were not military grade. I used a Trimble Lassen IQ. No additional barometer or radar on board. I assume it is consumer grade from a few years ago.

I also sent up a cell phone GPS, but that stopped recording the altitude limit around 20,000 feet if I remember right.

Lots of GPS devices don't work at high altitude and/or high speeds. There are some laws out there (COCOM limits) to stop bad guys from using them as navigation systems on missiles.

[JohnnieB]

Originally Posted by mattadata

They certainly were not military grade. I used a Trimble Lassen IQ. No additional barometer or radar on board. I assume it is consumer grade from a few years ago.

I also sent up a cell phone GPS, but that stopped recording the altitude limit around 20,000 feet if I remember right.

Lots of GPS devices don't work at high altitude and/or high speeds. There are some laws out there (COCOM limits) to stop bad guys from using them as navigation systems on missiles.

I looked up the specs on that postage stamp sized module. Wow!!!

[yurtle]

I know we used "controlled" GPS in the early 90's for xyz surveying to centimeter accuracy. May only be a rumor, but only certain entities could use this technology. Shortly after this, we saw the first Gulf War, where this accuracy was proven. We had software where we could see where the satellites were. They mysteriously all moved away during the war.

[JohnnieB]

Originally Posted by yurtle

I know we used "controlled" GPS in the early 90's for xyz surveying to centimeter accuracy. May only be a rumor, but only certain entities could use this technology. Shortly after this, we saw the first Gulf War, where this accuracy was proven. We had software where we could see where the satellites were. They mysteriously all moved away during the war.

Back in the SA (Selected Availability) era prior to May-2000, unless you had a GPS receiver that had the right circuitry and the operator had the right decoding key, the error deliberately induced into the system, limited the fix accuracy to about 100 meters. The attached chart illustrates how much the Instantaneous Error was reduced when the SA was turned off.

[yurtle]

I worked for a local government that had "permission".