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Date: Wed, 13 Nov 2002 14:31:41 +0000
From: Iwo Mergler
User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.1) Gecko/20020826
X-Accept-Language: en-us, en
Subject: Re: vechile speed meter
Mike Page wrote:
> James Meyer wrote:
>>On Mon, 11 Nov 2002 03:02:21 +0100, "abacus-ri"
>>>Thanks to all for fast reply.
>>>I didn't mean to make a unit without sensor; just if is possible without
>>>outside sensor. I was think about some kind of acceleration sensor or
>> You want a device completely contained and not connected directly to the
>>vehicle that is able to measure the vehicle's speed.
>> Acceleration is not a good solution. You have to mathematically
>>integrate the acceleration in order to get the speed. Then you have to
>>integrate the speed in order to get the position. The original acceleration
>>measurement is very hard (expensive) to obtain with no errors. Any error in the
>>original measurement will be magnified many times over when the measurement is
>> Using a GPS receiver will give you very accurate information about
>>position. From that, the speed can be easily and accurately calculated.
> As a total outsider to GPS, I was interested to read this. What kind of response
> time can be had from a GPS ? Would it notice a speed increase lasting say 10 seconds
> ? What about roundabouts ?
The GPS speed measurement is done independent of the position one.
Since the Doppler shifts of the signals are independent of atmospheric
conditions, the speed measurement is usually quite accurate.
A typical GPS unit is capable of outputting up to 2 speed measurements
per second. You would most definitely catch a 10 second speed increase,
given a good view of the sky.
There are a few potential problems here:
Many roundabouts have flyovers, resulting in GPS outages. Some GPS units
try to mask the outage, but this information is made up. That is, it
assumes either constant speed or constant g-force and predicts where
you ought to be.
Most handheld units have very sluggish tracking loops. This gives nice
smooth readings, as long as you don't subject the receiver to any
acceleration. Used in a car, the readings may be seriously wrong when
going around roundabouts.
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