Re: Park Assist goes off in sunlight - Saab 9-5 Bulletin Board - Saabnet.com
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Re: Park Assist goes off in sunlight
Posted by Simon Watkins (more from Simon Watkins) on Fri, 23 May 2003 03:32:54
In Reply to: Park Assist goes off in sunlight, Chris M, Thu, 22 May 2003 10:04:35
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The ultrasonic reversing aid (Park Assist) works by emitting pulses of ultrasound, and measuring the time it takes for the pulse to travel out, and back from a reflecting object. The continuous tone you hear occurs when the pulse is reflected back in a time that is less than some threshold value.
Unfortunately, one of the problems at extremes of temperature, is that sound travels faster through air - so the pulse can travel further out and then back in the same duration. Heat haze also causes discontinuities in the air behind the sensor that can also skew results.
At room temperature, the speed of sound in atmospheric air is 340 m/s or 762 mph. If the air is heated to 100 ° C, the speed of sound rises to 386 m/s. Indeed, the speed of sound is proportional to the square root of temperature. At near ambient temperatures, the speed rises by 0.6 m/s per each 1 ° C increase, corresponding to an increase of 0.18%/ ° C.
I suspect what you are experiencing are extremes of temperature, possible exacerbated by black tarmac in the vicinity, heating the air to beyond normal operating capabilities of the Park Assist function. The air temps are such that the pulse is getting out there and back under the threshold for the continous tone, even if reflecting objects are further away. Bear in mind that a reflecting object can also be the ground itself, and strange refraction effects due to "heat haze" could be making the ground "appear" closer, when normally it would be beyond the timing threshold.
A theory at least.
Simon
posted by 80.1...
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