This expression "du simple au double" is often used loosely to mean a very wide range. It is often used in reference to prices.
e.g. "Le prix des imprimantes jet d'encre bas de gamme vont du simple au double" meaning that they can vary from X to 2X.

In your case, I think it has the same meaning in that the estimated distances can vary from X to (roughly) 2X. However, as I said earlier, the term is used loosely and the higher estimate may not be strictly twice the lower estimate.

Hope this helps.

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Note added at 28 mins (2006-11-10 15:53:31 GMT)
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In view of Marc's comment regarding vagueness, it may be safer to say "varying by a factor of 2" or to expand it: "varying over a wide range with the highest roughly twice the lowest"... or words to that effect.

KIS - the "real" distance to be estimated is A but the differences put the system out to the degree that its distance estimate is doubled

As others have said, the two-fold increase/decrease may or may not be precise. Certainly the French expression need not be precise, though I think it is generally used with reasonable accuracy.

Measurement accuracy will all depend on 1) the variation of the angle of the car relative to the ground, 2) the distance of the object from the car and 2b) the range up to which the instrument will measure.

It surely all has to do with the length of the hypotenuse of a triangle relative to its base.

Over a long distance (long base), I don't see how it could be double, unless the obstacle is incredibly high and the angle of the car to the road is also very high (which equates to the the next scenario).

On a more realistic scale, if the distance to the object is short (short base) and the angle of the car quite steep (unrealistically steep?) then it might approach a reading twice the actual distance.

However, the above is very much a simplification based on what I understand from your short text. The system may well be far more complex than I imagine and I may be talking cobblers.

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Note added at 3 hrs (2006-11-10 18:46:04 GMT)
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CMJ - Indeed, the hypotenuse thing is not relevant to the actual calculation (or estimation as you put it; surely a device should measure a distance, not just give it a look and guess). I just introduced that to explain why the measurement could vary.

Take a level car on a level surface and place a wall in front of it. A measuring device (beam emitter?) placed parallel to the car axis (and therefore to the ground) will measure the distance to the wall, "A".

Now let down both back tyres. The car, and therefore the beam emitter, will no longer be level, but will point upwards. The line of the beam from the car to the wall will be the hypotenuse of the triangle whose other two sides are the ground and the wall. And the length of that hypotenuse will be greater than "A".

Now, writing this has made me think more about how a system might measure distance. The reversing radar on my car does this very approximately, in that it beeps faster and faster as I get closer to an object. Now, I assume that the beam transmitters transmit not a linear beam, but a fan-shaped or conical beam, and that a clever bit of circuitry averages out the distances of the most direct, "straight" beam and the beams on the outer fringes of the fan/cone which will be longer.

In my previous reasoning I imagined a straight beam. Point the beam up a couple of inches and it won't get much longer.

If the beam is fanshaped/conical, things change more radically. If the car is level, one side of the beam gives the same measurement as the other side (assuming the obstacle is vertical and the beam is symmetrical). If, however, you let down the rear tyres, the middle beam will get marginally longer, the bottom beam will get marginally shorter, and the top beam will, methinks, get considerably longer. Now THAT is where inaccuracy creeps in and you could soon be working with double distances, I suspect.