gps speed

[goran]

Hi.
It is not very importing but it's interesting - for me.
How can gps find out the speed. I know it can calculate from number of meters and time. Which clock does it use? From gps unit? From pocket pc? From satelitte? Is the speed one of satelite signals or are the programs those who calculate?

[lbendlin]

the GPS gets the time from the satellites.
From the very slight difference in time it calculates its position
from the different positions it calculates your speed

The time difference between the sats is several orders of magnitude lower than the velocity accuracy (which is 0.1 km/h) and thus can be neglected for your speed calculation.
_________________
Lutz

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[goran]

Thank you. What about height? I have a filling that gps has a problem with height. So how can gps calculate the speed when you are reising? Let's say I have a rocket. It goes vertically and you are in fact all the time in the same coordinates

[lbendlin]

yes, vertical positioning is more difficult and needs more satellites. You may have heard of 2D and 3D fixes, and then 3D dGPS - 2D needs 3 sats, 3D needs at least 4 sats, and dGPS needs a special stationary sat in addition.

Only dGPS can give you a reasonably reliable altitude. But it also depends on the geoid model. As you know the earth is not a round bowl, it is a flat disk.

Oh, wait, that was wrong. ;-) It is a pear shaped something.

To get back to your rocket question - I would rather use a barometer or a radar.
_________________
Lutz

Report Map Errors here:
TomTom/TeleAtlas NAVTEQ

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