A GPS range question.
 

Just wondering....if my very basic GPS not even a map...just lat long....gives me a distance from a ground way-point of 5 miles and I'm flying at 10.000ft ....aprox 2 miles high over the sea,

That 5 miles from the way-point... is that ground based 5 miles or a slant angle 5 miles.

Nick.

Ground distance based.

So altitude is irrelevant....its a straight line on the ground/surface....have I got the right.

It is certainly not a slant range, but I've never seen it stated if it is the distance at your present altitude or the distance on the WGS84 surface (ellipsoid) - although in practice the difference will not be huge unless you are seven miles high and traveling half way around the world where there could be a 22 mile or so difference!

Noting that Jim59 did say it is not slant distance...

For my experience, when you enter, then navigate toward an intermediate waypoint on the GPS, you have not entered an altitude associated with that navigating waypoint. With accurate flying, you could arrive exactly on the waypoint - zero distance, without being told that you are above or below it.

Upon reflection it will give horizontal distance assuming that you are on the surface of the WGS84 ellipsoid - a mathematical approximation to the shape of the Earth used for aviation GPS purposes..

Mmmmm but isn't the GPS receiver measuring the distance from the transmitting satellite....not a ground position.

Nick.

In reality, and in simple terms, the GPS unit derives it's position by reference to several satellites. Simple speed, distance, time calculations knowing the speed and time elements of signals generated by the satellites. From that position translated to the surface it will work out the distance to the required waypoint.

The correct answer is "It entirely depends on the receiver, and possibly the settings".

If it's important, I suggest testing it by flying at 5,000 ft directly over the top of a ground way-point and look at the distance being displayed.

If you want to be really anal, remember also that "ground distance" is a very imperfect term. Does it mean "surface distance"? Highly unlikely, so it will almost certainly only be an approximation, as Jim59 stated, based on a strategy like WGS84 (but not necessarily that one), which means that the "ground" distance being calculated could be an imaginary surface several hundred feet above or below the surface you're flying over - although unless you're flying a long way, that won't make a massive difference.

It was only a hypothetical question really....just something that had crossed my mind.

Nick.

Indeed it does by triangulating ranges from the satellites, it produces a 3 dimensional position and by comparing that with the WGS 84 geoid it works out that you cannot be on the surface and thus gives the difference as your altitude. The range from your 3D position to the waypoint may continue to assume that you remain at a constant altitude and gives you a range to a point vertically above the waypoint. If you change altitude, it will readjust the range to measure the new altitude above the waypoint.

I will put it to the test next time I'm at the airfield....setting the airfield to a waypoint when on the ground.

Nick.

AFAIK GPS never gives you any kind of "slant" range.

If you set say two waypoints, which the GPS says are 10.000nm apart, that will be the great circle distance between them as measured along the surface of the ellipsoid which approximates the MSL surface.

One could argue that since an ellipsoid is not a sphere, then if you start on say a point on the equator and move to the north or to the south, you are actually going slightly downhill even if you remain at sea level the whole time. But this effect is miniscule.

The altitude portion of the GPS solution comes out of the same calculation as lat/long but is normally presented separately because that's the way we like it. The altitude reading does have a correction applied to it, to correct for the deviation between the ellipsoid and the true MSL surface, but this doesn't affect the horizontal distance.

The science behind GPS is extremely complicated but the end result is very very simple.

Aviation GPSs definitely measure range to a position vertically above a waypoint. They definitely show 0 distance when 10000 feet above a VORDME with the DME showing 2 miles.

Peter,

It entirely depends on the GPS device. 'GPS' doesn't have a protocol for distances between waypoints, so it's at the discretion of the developer of the device. But as aviation navigation devices are designed for navigation rather than true vector/distance finding, then you'll have likely been exposed to more examples where the device measures waypoints as you describe.

The OP didn't state what the GPS device was, and as it sounded like it was an incredibly simple device, it may not have been programmed as a navigation device in the way you're used to.

So back to my original response 'it entirely depends in the device / settings'.

Also, the principal of GPS is actually incredibly simple. The translation of GPS into a navigation tool is extremely complicated. And of course, the implementation of GPS is an astonishing accomplishment in itself!

SDB73, your correct its is a very basic non aviation hand held GPS solely used in a very basic flexwing microlight.

Nick.

I would be surprised if any GPS other than one designed specifically for surveying, ordnance targeting, or rock climbing were to measure a slant range from the current position directly to a way point.

Unless your GPS allows you to set target elevations for your waypoints, it seems pointless to have it do its navigation calculation from a point in space to another point in space (i.e. a 3d track - probably on a trajectory, as a straight line over a reasonable distance may well actually go under ground).

The basic position derived from the satellites is going to be a declination, 'right ascension' and range (i.e. a spherical coordinate system). Ignoring the range, you map this pretty straight into Lat/Lon on the ellipsoid. Then the standard range and direction formula give Great Circle distance along a sphere, which I imagine is what the cheap GPS use rather than the ellipsoid formula or some more complex logic of a 3d path through a specific frame of reference.

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The simple answer remains, it is virtually certain a 'normal' or 'cheap' GPS will do range and bearing calculations based on the lat/lon and ignore altitude (either yours or the waypoints) - hence not slant range.