Sat-nav accuracy.

[BUSHWHACKER]

Hello there, how accurate are sat-navs at measureing vehicle speed?
Does it vary between manufacturers, or is the signal received by them the same for all and is spot on?

[Apache]

Should all be the same, though display latency differs from one to the next. If you can drive in a straight line for 30 seconds or so, its should be as accurate as you'll get. The accuracy goes if you are driving through bends.

[sv1000spilot]

I have a garmin geko 201 gps & nokia 6110 with gps.

I have compared them and they vary by a couple of mph in a straight line, not that one in particular reads higher they just jump a bit.

You do need a longish straight though, would say 1/2 mile to be sure your speed reading is accurate

[Apache]

SV, its because the two have different sample rates / update speeds, but the receiver itself will be processing the speed accurately. Its just generally, for car navigation, speed doesn't need to be updated that often.

[BUSHWHACKER]

Should all be the same, though display latency differs from one to the next. If you can drive in a straight line for 30 seconds or so, its should be as accurate as you'll get. The accuracy goes if you are driving through bends.

Thats what I thought, driving along the M4 this morning, kept the speedo needle spot on 70mph for well over three minutes, but the sat-nav said 65mph.
Tried again at 60, sat-nav said 56.
This was in the Corolla on 17" alloys. Talking to the fella I was meeting, he said his speedo had the same inaccuracy (Standard '04 Toyota Yaris)

[darkdezigns]

id trust my sat nav goin thru a speed camera more than speedo on car

[stormforce]

with my speedo reading 70 and the sat nav will read 73 so i keep about 3-4 mph below what the speedo says now so i know i'm doing the speed limit

[Apache]

Thats what I thought, driving along the M4 this morning, kept the speedo needle spot on 70mph for well over three minutes, but the sat-nav said 65mph.
Tried again at 60, sat-nav said 56.
This was in the Corolla on 17" alloys. Talking to the fella I was meeting, he said his speedo had the same inaccuracy (Standard '04 Toyota Yaris)

Trust your sat-nav. Speedos usually over-read, except when you put very big wheels on, then you risk under reading speedos.

[BUSHWHACKER]

with my speedo reading 70 and the sat nav will read 73 so i keep about 3-4 mph below what the speedo says now so i know i'm doing the speed limit

So your speedo is under-reading. I thought Surfs were more accurate on 33"s.

[gwh200]

So your speedo is under-reading. I thought Surfs were more accurate on 33"s.

oh no sir..oh no.

[JUDWAK]

well there are 1099 satalites in orbit
so i think its safe to say that sat nav's dont all get a reading from the same place
i read once that the data dose not slow down but
when a satalite is beaming at a 22 degree angal or 44 cant remeber rightly
its at its full travel distance and at that piont the glare from the earth means the data gets rebouned horizontaly to a mast in stead of going directly there

that afecks a sat nav must do
it afects everything else thats gps

depends on what satalite its comeing from
the only way around that would be if they bounce it from 1 satalite to another to keep you in maximum range

or so i read

[Apache]

Mine was just about dead on on the 33 Mudzillas (though they were actually around 32") and slightly under-reads on the BFG ATs which do actually measure 33.

[Apache]

well there are 1099 satalites in orbit
so i think its safe to say that sat nav's dont all get a reading from the same place
i read once that the data dose not slow down but
when a satalite is beaming at a 22 degree angal or 44 cant remeber rightly
its at its full travel distance and at that piont the glare from the earth means the data gets rebouned horizontaly to a mast in stead of going directly there

that afecks a sat nav must do
it afects everything else thats gps

depends on what satalite its comeing from
the only way around that would be if they bounce it from 1 satalite to another to keep you in maximum range

or so i read

Satnav doesn't work like that. The satellites have an ID and the receiver (your satnav) receives the data from several of them (often 8 or more) and uses their reported position to triangulate its own position.

You're never out of range, and signals from them dont 'skip' as such (not like lower frequencies transmitted within the earths atmosphere). You can have difficulty receiving signals under tree canopies, when your cars windscreen has a heater in it etc.

[JUDWAK]

Satnav doesn't work like that. The satellites have an ID and the receiver (your satnav) receives the data from several of them (often 8 or more) and uses their reported position to triangulate its own position.

You're never out of range, and signals from them dont 'skip' as such (not like lower frequencies transmitted within the earths atmosphere). You can have difficulty receiving signals under tree canopies, when your cars windscreen has a heater in it etc.

iant that what i said?

[stormforce]

my 33's actually measure at 31.5" so that may be why the reading is out a bit more than it should be