GPS Question

[paulster2626]

Lets say you had a GPS in your hand, and jumped out of a plane/off a building/whatever.

So as your'e falling straight down, will the GPS say how fast you are going, or will it say 0 km/h?

[AsheanMT]

It would tell you how fast you are going.

[Fenris]

Depends on how it's set up, and how fast it can get a new 3D fix. My old Garmin Etrex and 8 will both show 0 knots for speed if going vertical for a second.

I'm suspecting the Etrex wasn't capable of getting a new 3D fix in that second or two that the boat was traveling vertically, but it was able to get a couple of 2D fixes, so it could tell us that we were not going 20 knots anymore.

The 8 may or may not have been able to get a 3D fix at that point, but it was set up for nautical use, so it was only displaying horizontal information.

GPSs are much less accurate in the vertical dimension so they are more liable to throw out that data, if something is changing quickly. A good GPS instead of just telling error, it will tell horizontal error and vertical error. Even a handheld professional GPS can only get down to about 70 cm of vertical error if you are lucky, while some can now have horizontal error of under 20 cm.

[highangle]

GPSs are much less accurate in the vertical dimension so they are more liable to throw out that data, if something is changing quickly. A good GPS instead of just telling error, it will tell horizontal error and vertical error. Even a handheld professional GPS can only get down to about 70 cm of vertical error if you are lucky, while some can now have horizontal error of under 20 cm.

GPSs are not as accurate on the Z component of the vector because of the incomensurability of the reference ellipsoid vs the geoid
the ellipsoid is based on a theoretical (ECEF) point that's assumed to be the center of the earth
the geoid is based on an an equipotential gravity surface, normalized to a theoretical sea level set by convention

problem is, gravity is no where near isotropic and the actual center of mass of the earth is constantly changing (geoidal undulation), so the "true" geoidal separation from the ellipsoid varies by place and time, as does the orthometric height from the geoid to the receiver

GPS verticals can be very good compared to spirit leveling, but you have to understand how it computes orthometric height and have a very good local geoid model
this and other factors make things difficult, so simplified calcs yield an error ellipsoid like an egg, where the vertical error is considered to be twice the horizontal





short answer to the OP is YES, GPS can measure a hypothetical vertical speed just fine if it has the SV geometry
(it does this with rocket launches correctly all the time and the accelerometers on the rockets and radar tracking confirm it)

to demonstrate this, think of the satellites moving from one horizon to another
if the horizon masks are disabled, a handheld unit will use signals (however corrupted by multipath, the Earth's gravity, curvature, water vapor, and other error sources) from SVs that are almost "level" with or "below" the horizon of the falling receiver

can an Etrex swing it? don't know, but i think so


 

[jondrums]

holy tech talk batman!

I think the GPS will do better at measuring speed in the vertical direction than it does at measuring absolute altitude while you're standing still. but I bet highangle will correct me if I'm wrong...

[highangle]

holy tech talk batman!

I think the GPS will do better at measuring speed in the vertical direction than it does at measuring absolute altitude while you're standing still. but I bet highangle will correct me if I'm wrong...

what is absolute altitude?

it's like asking "what is absolute sea level?" -- ain't no such thing (think of tides...)

it's fine for laymen to have a model of time in their heads and on their wrists that answers their lay questions about when to go to work, but open up a watch and it's considerably more complicated

and any time you start launching satellites into orbit, shit gets tech real quick...

[Schnappi]

Is the plane on a treadmill?

[TeCeBe]

I was pretty sure nobody else on this board knew what the geoid was besides me. I am wrong.

I would personally not rely on any consumer level GPS recievers output of speed in the vertical sense for any means. It may give you a number, but I would doubt it is correct.

[iscariot]

Is the plane on a treadmill?

beat me to it...

[mcsquared]

I was pretty sure nobody else on this board knew what the geoid was besides me. I am wrong.

You are. I thought I was the only one who liked saying "equipotential gravity surface" as well.

[iscariot]

GPSs are not as accurate on the Z component of the vector because of the incomensurability of the reference ellipsoid vs the geoid
the ellipsoid is based on a theoretical (ECEF) point that's assumed to be the center of the earth
the geoid is based on an an equipotential gravity surface, normalized to a theoretical sea level set by convention

problem is, gravity is no where near isotropic and the actual center of mass of the earth is constantly changing (geoidal undulation), so the "true" geoidal separation from the ellipsoid varies by place and time, as does the orthometric height from the geoid to the receiver

GPS verticals can be very good compared to spirit leveling, but you have to understand how it computes orthometric height and have a very good local geoid model
this and other factors make things difficult, so simplified calcs yield an error ellipsoid like an egg, where the vertical error is considered to be twice the horizontal 

Do we know each other in real life?

[highangle]

Do we know each other in real life?

do you run a big treadmill?