27 replies to this topic

[reallybig]

satellites pack up etc.

you mean cloud

[Ivor Badger]

The legal requirements for a speedo are.
1 Must be accurate to within 10%
2 Must be visible to the driver
3 Must be illuminated at night.

As most satnavs or Decca navigators in their day worked in giving you the speed over a previous set time. Ie, took the position change in the time period and then worked out the average speed. 4 mins in Deccas case. this once resulted in an average boat speed of 75kns, which was quite inpressive as we were drifting while waiting for the signal to correct.

Decca navigators ... LORAN... now those were the days

Unfortunately they turned off GEE very early and Oboe is not really suitable for cars.

Edited by Ivor Badger, 10 April 2012 - 11:56 AM.

[AntsClubby]

Made me laugh too! The coppers actual words were...you need a conventional speedo incase your unit fails or the satellites pack up!lol

[reallybig]

[reallybig]

doing geological mapping was a ******* trying to get gps reading in heavy cloud cover right next to tryfan....used to take a good 15 mins sometimes to get enough satellite locks and that was with a dedicated handheld gps

[JVA10L]

Have a look at this. It is legal on any car. Combines speedo, rev counter and temp gauge. I fitted one to a car recently and it works really well.

http://www.trailtech...y_of_Vapor.html

[AntsClubby]

That must have been a bad day!! When i climbed tryfan i only used my ip4 and viewranger worked a treat! )

[AntsClubby]

That vapor one looks good! Bit like this but cheaper!
http://www.digital-s...h-mini-23-c.asp

[reallybig]

That must have been a bad day!! When i climbed tryfan i only used my ip4 and viewranger worked a treat! )

usually it was fine but had to get a gps location for every single rock outcrop over 28 days

[AntsClubby]

Got any jobs going?? That sounds awesome! What do you do?...im guessing it wasnt just for fun?lol

[Dan]

Personally I disagree with that Police interpretation of the law, as there is a specific clause in the C&U regs that states it is accepted that a speedo may fail during a trip. Mechanical speedo cables can snap, there is no requirement to have a backup second unit. The liklihood that a GPS unit will fail is probably higher though, due to cloud and buildings and everything else that goes wrong with them. There is nothing specific to bar them, apart from the fact that any error that a GPS speedo might make in speed will be just as likely to be positive as negative and as said above the law is that the speedo may not under-read at all.

Domestic/ Civvy GPS units have the POTENTIAL to be vague, but this is controlled by the US military

Not what I'm referring to at all, the US military relaxed their general restriction many years ago following a civilian lawsuit and the system is now only restricted in certain countries. I'm talking about the assumptions built into the unit (and I'm referring to sat-nav type units all through this, not surveying equipment) that make it possible for the tiny electronics involved to perform the huge amount of number crunching required in the time permitted when travelling at 70 mph. The biggest of these being that you are on the surface of the planet. Then they assume you are on a road on their map, travelling along that road etc. etc. Road navigation units have another drawback over other units in that they have only a very basic concept of altitude, which adds enormous errors. Civilian units don't use the full ephemeris from every satellite in every calculation either, and they don't update the ephemeris data they have every time they receive it from each star (think how long it takes to find satellites when you switch it back on in a different location from where you switched it off, it's aquiring all new data for every satellite it needs). They make similar assumptions in their calculation of speed. Every time they perform a speed calculation, they assume your position is accurately known. They have to, they can't allow for the error. The speed is not calculated over the duration of the trip, it's simply calculated from translation and duration since the last known speed, so errors won't average out. Speed is only known because it thinks it knows your position on a grid, but even with a pretty accurate unit it can be 3m off. That means you can be anywhere inside a 3m circle around the position. That's quite a long way if it's checking the speed several times per second. Also as we all know, accuracy is dependant on how much of the constellation the unit is receiving from. As said above, it's safe to assume that it's way more accurate than an uncalibrated mechanical speedo, but probably no better than a recently calibrated one. In fact I think I'm right in saying that the way the errors work in both systems makes a mechanical speedo more accurate at lower speeds, and a GPS more accurate at higher speeds. The difference between the speed reported by a GPS speedo and a doppler radar is not insignificant.

[matt615]

think basically, it'll pass an MOT because speedo isn't tested

Not for long. The rules are changing, and soon the speedo will be tested as part of the MOT.

I'd imagine it will be tested by the MOT tester driving on the rollers (as used for the brake test), and seeing if the speedo registers a speed. There would be no way an MOT tester could test a GPS speedo with the car stationary in a garage, so I'd imagine this would fail the MOT.

[AntsClubby]

I can see that being abit of a headache...same model of car, different alloy sizes throughout the range/diff rolling radius etc.
What happens if your 3yr old car is 1mph out of the test criteria...back to manufacturer or garage for costly mods!
Sounds like another way of creaming a little more money out of me /us motorists i rekon!