James86004
Expedition Leader
Does it take measured points and divide the distance between them by the elapsed time, or does it calculate it from the Doppler effect? Or does it depend on the unit?
How does the speedometer in a GPS work?
- Thread starter James86004
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It is (almost) constantly calculating your position. As you move it can calculate the speed by calculating the time it takes you to get between two positions. Like you said in the first part.
I'm sure that it averages it out on a 'sliding window' basis to give an average speed over the last few minutes.
I'm sure that it averages it out on a 'sliding window' basis to give an average speed over the last few minutes.
soonenough
Explorer
Velocity_avg = (change in distance) / (change in time)
GPS can tell your position pretty accurately, and it has an extremely accurate clock that syncs with the internal clock of the satellites, so it can calculate velocity very easily.
Keep in mind, the greater the distance traveled over some given interval of time, the more accurate this calculation will be. The reason is that an error of a few meters makes a big difference if you've only traveled 10 meters during that time interval, but if you've traveled a mile or more, it's fairly insignificant. For example, if you're using a GPS unit to calculate your average speed over land while hiking, there's probably going to be some significant errors, because in some given interval of time (say 30 seconds or a few minutes), you're not moving very far, so a position error of a few meters is a big chunk of the distance you've traveled. If, on the other hand, you're driving down the highway at 80mph for a few minutes, an error of a few meters is nothing.
EDIT: after writing this, I did a quick google search and found this, and apparently the GPS units do take into account the Doppler shift of the signal coming from the satellites. I wouldn't have guessed that...
GPS can tell your position pretty accurately, and it has an extremely accurate clock that syncs with the internal clock of the satellites, so it can calculate velocity very easily.
Keep in mind, the greater the distance traveled over some given interval of time, the more accurate this calculation will be. The reason is that an error of a few meters makes a big difference if you've only traveled 10 meters during that time interval, but if you've traveled a mile or more, it's fairly insignificant. For example, if you're using a GPS unit to calculate your average speed over land while hiking, there's probably going to be some significant errors, because in some given interval of time (say 30 seconds or a few minutes), you're not moving very far, so a position error of a few meters is a big chunk of the distance you've traveled. If, on the other hand, you're driving down the highway at 80mph for a few minutes, an error of a few meters is nothing.
EDIT: after writing this, I did a quick google search and found this, and apparently the GPS units do take into account the Doppler shift of the signal coming from the satellites. I wouldn't have guessed that...
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James86004
Expedition Leader
I got the EEs next door buzzing about this. Their conversation then drifted to cell phones. They are just amazed you can fit a GPS receiver, WiFi communications, Bluetooth, and a cell phone into a candy bar and have them all play nice together.
tdesanto
Expedition Leader
Agreed. It amazes me what can be reduced to a few ICs and software.
cnynrat
Expedition Leader
Remember, Doppler shift is the result of the relative motion of two bodies. The rotational motion of the earth is 400 m/sec (at the equator) and the satellites are moving at 7 km/sec. So the relative motion is actually quite fast.
tdesanto said:
Moore's Law at work!
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esh
Explorer
(tangent)
There was a neat Nova episode on fractal geometry this past year. The resulting antenna technology helped shrink package size and increased effectiveness. When GPS "double helix" antennas came out a few years ago, fractal-based design was the reason.
Story goes that a radio operator in a city apartment discovered it because his landlord was against having antennas poking up out of his balcony.
(/tangent)
There was a neat Nova episode on fractal geometry this past year. The resulting antenna technology helped shrink package size and increased effectiveness. When GPS "double helix" antennas came out a few years ago, fractal-based design was the reason.
Story goes that a radio operator in a city apartment discovered it because his landlord was against having antennas poking up out of his balcony.
(/tangent)
Actually, many phones do include real GPS receivers.
Pskhaat
2005 Expedition Trophy Champion
My understanding is that the cell towers act as differential GPS? No?
Sorry, I don't know which specific models do. Neither of mine do - blackberry and Windows mobile. One of the problems is that while the GPS may be there, various carriers (Verizon for example) have a habit of disabling it or making it not available to the user directly without a montly fee.
Wait, huh? You aren't asking if you can use the phone through the GPS are you? That would be, "no".
A little googling around makes me think that the 1st Gen iPhone did not have GPS but that newer ones do.