SatNav as a Speedometer

A couple of members have hit on the idea of using a SatNav system for calibrating speedometers. This seems like a reasonable idea, after all, GPS positioning is very accurate, so the speedometer readout from the unit must also be very accurate.

Sorry to be a spoilsport, but there are a couple of snags which need to be considered. The speedo readout on a satnav works by knowing the change in location as a function of time. Move a tenth of a mile in half a minute equates to moving one mile in five minutes, or 12MPH. So, at first sight, using a satnav to show your speed seems a very good idea.

But it isn't quite that straight forward. Satnavs don't measure speed; they calculate it from the change of position in a given time. Same thing, you'd think, but the snag is in the way the satnav identifies position. Unless you are using an aircraft grade satnav with an altimeter input, they assume the earth is flat and identify the position as latitude / longitude. Perfectly adequate for finding your way around the roads or down the street.

Where it goes a bit sticky is when you try to use this information to calculate speed when the earth isn't flat. The satnav calculates distance as the difference between latitude / longitude positions, and divides by time to get indicated speed. This is perfectly ok on a straight, flat road, but goes awry when there are significant curves or worse still inclines. What this means is the secondary school trigonometry comes back to haunt you, with old man Pythagoras and all that jazz. On a really bendy road, the satnav will straighten out the corners and thus calculate the distance as a bit shorter than the route you have actually travelled. Not a big deal on most roads, but this error really builds up on a track or circuit.

More significantly for us Austineers, the flat earth society really screws up speed measurements. When you go downhill, (or uphill, but that is never very fast in a Seven), you travel between latitude / longitude positions but you also change altitude. The car will have travelled FURTHER than the simple GPS will calculate. If you drive a half mile down a one in ten (1:10) hill, you have actually travelled ten percent further than the satnav will measure. So, if you use the satnav to determine speed, you are travelling 10% FASTER than indicated. The bigger the incline, the bigger the error. If you know the slope as a percentage, (all those new road signs will help) then you need to up the satnav reading by that amount. On a 1:6 hill, you travel 16% faster. For example, if you try storming down a 1:5 hill with your engine screaming and your satnav reading 50mph, you are actually going 20% faster, or 60MPH. Maybe the difference between a speeding ticket or not!

Incidentally, for the cycle speedo fraternity: these don't suffer from this effect in a straight line. But if you are cornering, then when you turn towards the side with the speedo pickoff, it will read slow and when you turn away, it will read fast. The tighter the turn, the worse the error. The only cure for this is to put the pickoff on the propshaft which turns at the average speed of the two rear wheels irrespective of corners.

This article, written by Geoff Hardman, originally appeared in CA7C Seven Focus in May 2008 p26.

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