GPS question

[tigertim20]

probably wrong section, but fuckit...

I wasthinking about gps today, and in my hungover haze of thought, I wondered whether there was a weakness to GPS measured speed.

We know bike speedos are almost always out, and I know many people who use GPS to measure their speed.
Now my thinking was that, while you are on a flat piece of ground, the GPS would be pretty wellbang on accurate, but my question relates to hills.

I thought that if you were going up, or down a hill a GPS would read lower than your actual speed. My reason for thinking this is as follows.
As I understand it, GPS works by pinging off satellites, and judging how far you have travelled from point a to point b, however if you are on a steep hill, the 'as the crow flies' distance, and the actual distance will differ, like a triangle standing up, the distance up and over will be greater than the distance underneath.
So does GPS read low for this reason, or does the GPS measure your altitude in real time, and use this in its calculation of your actual speed?

sorry if mypost doesnt make sense, I blame bourbon

[Blackshear]

With absolute zero knowledge in the subject, it is my understanding that gps can measure height as well, so it seems only fair to assume the satellites can track a to b on a 3d plane.
Your argument makes sense, though.

[JustNick]

From memory ...

A decent GPS will normally track off a number of satellites for a triangulated positional reading and then work out your speed between position fixes. I'm pretty sure that tracking off a minimum of three satellites gives you a three dimensional position which negates the whole going up and down hills inaccuracies.

[bogan]

http://lmgtfy.com/?q=how+does+a+gps+...alculate+speed

Position is the main but not only aspect of GPS, Doppler effect from the satellites can be used for instantaneous speed.

[KoroJ]

Track log includes a profile of elevation and distance, so in tracking both, one would assume the speed is accurate.

[banditrider]

Track log includes a profile of elevation and distance, so in tracking both, one would assume the speed is accurate.

Not always. I'm pretty sure some of my "tracks" have missed the odd update when checking the recorded speeds for certain sections of a ride...

[Scuba_Steve]

Speaking of the GPS for speed, I borrowed one. This weekend some interesting numbers did result.
Seems some vehicles aint pulling what they claim

[nzspokes]

I use a bicycle computer that works of tyre size. You mark the tyre and ground, do one revolution and mark ground again. The distance between them you put in the computer and its ready to go. I get max speed, average speed, distance on trip etc.

I know nothing about GPS.

[tigertim20]

I use a bicycle computer that works of tyre size. You mark the tyre and ground, do one revolution and mark ground again. The distance between them you put in the computer and its ready to go. I get max speed, average speed, distance on trip etc.

I know nothing about GPS.

I found a free app that gives all of that, but also max speeed, time travelled, and about 20 other things as well, free to download and free to use, its great!

[FJRider]

Considering a number (10 or more) of satilites are feeding information to your unit ... any inaccuracy in reading due to altitude gain/loss would be measured in less than .oo1 (or there abouts... I stand to be corrected ) although as I understand ... gps units give average speed ... and as such ... a varying speed may give inaccurate readings ... but dam close ... by human standards ...

[Gremlin]

Usually the GPS is accurate to about 3m, and yes, it also plots your elevation as frequently as it updates your speed.

I reasonably often get the GPS being inaccurate through tight usually wooded areas, and when you're diving from corner to corner in tight surroundings, yes, the GPS can be out by 10-20kph.

Your speedo also becomes less accurate as you go around corners, especially if the speedo is front wheel driven, due to the change in tyre circumference due to lean angle.

[TOTO]

Usually the GPS is accurate to about 3m, and yes, it also plots your elevation as frequently as it updates your speed.

I reasonably often get the GPS being inaccurate through tight usually wooded areas, and when you're diving from corner to corner in tight surroundings, yes, the GPS can be out by 10-20kph.

Your speedo also becomes less accurate as you go around corners, especially if the speedo is front wheel driven, due to the change in tyre circumference due to lean angle.

building up on that - you are not supposed to look at the speedo when in the corners, so who cares. Glad I cleared up for you all.

[Blackbird]

I've never bothered to investigate it but the elevation shown between the principally hand-held use Garmin 76CX I use on the bike and the Garmin Nuvi 3760 we have in the car differs significantly. Our road is right by the water and the GPS on the bike shows about 1 metre elevation whereas the other one in the car can show as much as +7 metres, not that I really care.

[avgas]

Seriously I did and I wish I hadn't.

A good unit will set/calibrate using 4-5 satellites, then will operate on 3 satellites. Which means they are accurate to 1uSec or 2m through triangulation.

however MOST units will calibrate on 3 satellites, then track on 1+ satellites..... meaning their stats can be up 5 times that amount (using good software) or 10+ times that amount (using bad software). Which means you could be a couple of km's out if it really hates you.

To make things worse, the whole thing is sitting on a backbone developed for US military......which means at the flick of a switch you can be a factor of 10 times out. There was a hiccup around 2007 where the GPS seemed to only be accurate to 50m! Thankfully for most this meant a minor inconvenience. I shat bricks as all my GPS time sync clocks had the wrong time!!!!
Only lasted a few hours though.

[slofox]

For what it's worth:

I calibrated my speedo with a speedohealer to match my GPS unit (Zumo 220).

The only time the two ever differ is when the tyre wear is getting fairly advanced. At which point they differ by 1km/hr at 100kph. Which means I need new tyres!

I have never seen variance between the two on hills.