GPS for Speedo

[Lou Girardin]

My Garmin works just fine. It's got a large display showing speed as the main readout. Refresh rate is better than a bike computer. I've never lost coverage while mobile. The real catch is that the odometer can be easily reset.

[Ixion]

Don't think there's any requirement to have an odometer for a WoF

[jonbuoy]

about 1 sec resolution aint they, How often do you check your speed?



Phitek doesn't he?

Depends on the model, some of them you can change speed filtering. They are fine for average speed or distance over a decent run. I guess for a warrant it would have to be within 10% accuracy??. Be annoying to get a ticket when you didn't think you were speeding.

[-df-]

GPS works differently depending on the brand, so you can't give it a blanket statement.

The systems I work on don't do it over an average distance so the speed is resonable reliable for what you are currently doing, this is up to the programmer though and how much filtering they wish to do with the NMEA data being recv'd.

New SurfStar3 chips are very good at getting fix's (can also update them over the net so that you able to get a fix within 5 secs average, and up to 1 sec for a good coverage area (I've even got a fix inside a building, with another floor above me...couldn't belieive it the first tiem I tried).

Biggest problem with them is the built in error that the US mill have put, that and reflections from hills/buildings etc (can really stuff it up). its only acurate 95% of the time (within 3-5m) but I've seen it be out by 50+ meters (that 5%), but good software would filter that.

[scumdog]

about 1 sec resolution aint they, How often do you check your speed?

Not all the time - but when you do you would want it to be giving an accurate reading RIGHT NOW -and not have to stare at it for a second or so to see if it is just about to change to reading the current speed.

[Fub@r]

Biggest problem with them is the built in error that the US mill have put, that and reflections from hills/buildings etc (can really stuff it up). its only acurate 95% of the time (within 3-5m) but I've seen it be out by 50+ meters (that 5%), but good software would filter that.

Thought the US military had turned off selective availability now? (intentional errors)

As for using GPS as a speedo would not be accurate enough. There are far too many variables and errors in GPS to give instant and accurate readings.

I base that upon the GPS equipment I use which cost $80,000 10 months ago. With this equipment I can get an accuracy of +/-10mm vertical and horozontal position, can rig it to a vehicle and plot a road at set intervals in distance etc. But to get this accuracy and reliability has a massive pricetag and even then still fails around trees, buildings, reflective surfaces, poor satelite constellation at any given time, atmospheric conditions. The list is endless.

Cant see how a typical vehicle GPS is going to do something that even an 80k set can't completely do

[Lou Girardin]

I've compared the readings on my bike speedo, bike computer and GPS against the cops speed trailers. The bikes speedo is 10% fast, the others are within 1 km/h.
That's accurate enough for me.

[jonbuoy]

They still spoof it but not as much. They will also under read when traveling up or down hills as most GPS units don't take into account altitude changes when working out Speed over ground. So if you are on a 10% hill your GPS will under read by about 10%. For every 10meters you move forward you moved 1 m up or down which the GPS didn't work out - it only worked out the 10 meters forward.

[Clive]

In a previous posts, there was a request for a view from somebody who does actually use a GPS for determining their speed. So, here's my tuppence worth:

I have used a Garmin eMap for a number of years now. It's pretty old hat, but does everything I need of it for biking purposes. I do use it to give me a far more accurate speed than the 8% optimistic Blackbird speedo, particularly when going through speed cameras, and in speed restricted roadworks. As just about every standard speedo fitted to bikes or cars is designed to be optimistic, to ensure that you don't exceed a specific limit when the instrument is reading that speed, then I always find myself doing a few mph more than everyone else around me.

The eMap doesn't give instantaneous readings, as there is a lag (as already mentioned) and some calculations for it to do. My unit's set to 1 sec updates, so it's doing the averaging over that period - often enough for a second's glance to give a pretty accurate indication of true speed, but if you're accelerating/decelerating, then the lag is very noticable (especially on a Blackbird!). But, most of the time, if not always, you're particularly concerned about an accurate speed when you're at a fairly constant pace - going through speed cameras, being followed by a patrol car, etc. When accelerating/decelerating there's usually far more going on of more immediate interest.

The unit presents its current accuracy on one of the data pages, and in the UK I generally get around 15ft accurate location. This may seem poor for determining speed, but in fact the spot accuracy is not the important thing. The relative accuracy is. It may be 15ft out in absolute terms, but it still knows far more accurately how far you've moved at each reading, as the variables that effect the location's accuracy don't change that quickly.

My unit is at least as consistent as any speedo I've matched it against, and the variance against any particular instrument never changes. If my 'Bird's saying I'm doing 30mph, then the eMap will always say I'm doing 27mph. Likewise, if my Beemer says I'm doing 30mph, then the unit will always agree.

In practice, I can attest that the GPS sped is invariably more 'true' than the bike's speedo, so much so that I very rarely use the speedo, prefering the GPS's reading.

Interestingly, the only vehicle I have ever come across that has a 'true' reading speedo is my '86 BMW K100RS. It's not only true within any specific range, but actually true over it's entire range! Typical BMW!

Hope that helps answer some of your queries.