wrong instant speed estimator

CAA_1130221413 · Lifetime Member Joined: Nov 30, 2011 Posts: 5

The algorithm of instant speed estimator implemented in CamerAlert seems to use an integrator on the known speeds during the last 5-10 seconds. I can understand if it were computing an average speed of the last 1-2 seconds, but using such a broad time interval for estimating the instant speed will lead to delayed over speed detection (if car accelerates) or it will continue to issue over speed warnings although the car speed was brought under the legal limit (if car decelerates). Furthermore, I suspect it will also have an impact on the computed average speed accuracy.

IMO "GPS Test" application does a better job in estimating the instant speed.

MaFt · Posted: Thu Feb 09, 2012 3:51 pm Post subject:

Hi,

What device are you using? We've had a handful of reports about strange speed readouts but it's not affecting all devices...

For example on my Motorola Atrix it is fine...

MaFt

DennisN · Posted: Thu Feb 09, 2012 6:03 pm Post subject:

Hmm, interesting. I have always understood that during acceleration and deceleration, the current speed will not be accurate - I often get an overspeed indication as I enter a speed limit, even though I know for certain I'm under the limit by the time I enter it. But I have to say my van won't do 0-60 (or 60-0) in the sort of time that might cause me concern.
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lbendlin · Posted: Fri Feb 10, 2012 1:32 am Post subject:

"Instant speed estimator" - good one, I like it.

We're not estimating anything, we simply take what the GPS chip gives us. Crappy GPS=crappy data. If anything we may be up to one cycle behind in displaying the speed (ie we display the speed that the GPS had half a second before) - maybe that's what you are observing.

In the grand scheme of things this doesn't seem to impact the intended functionality.
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M8TJT · Posted: Fri Feb 10, 2012 9:27 am Post subject:

I know that the SiRF Sar lll chipset has an 'averaging' mode (can't remember the real term) that I tried in my Mio C710. That introduced quite a lag on speed and position

CAA_1130221413 · Lifetime Member Joined: Nov 30, 2011 Posts: 5

Duddy · Posted: Fri Feb 10, 2012 3:25 pm Post subject:

A speed calculation, based on GPS positioning, will always give an average speed.

Say the position is calculated every 1 second, the GPS will give the linear distance between the 2 GPS readings and carry out a distance over time calculation i.e if your are driving around a curve the calculation will be based on a straight line between the 2 points and slightly over estimate the speed.

When looking at the speed, shown at a GPS point, it will be the average speed over the previous second (look at Google earth .kml files as a good example).

The above shows that what you are reading as your current speed is actually your average speed for the previous second.

The speed readings become even more "average" if your GPS reads locations at greater than 1 second and even more delayed if you use a BT GPS receiver as the BT transmission takes an appreciable time to reach the speed processor.

If you look closely at a live mapping system on a laptop in a vehicle using a BT GPS you will see your location, on the screen, lags slightly behind your actual position.

GPS is great but you need to know it's limitations
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M8TJT · Posted: Fri Feb 10, 2012 3:39 pm Post subject:

Speed from the GPS chipset is not normally averaged between two position readings. It's a direct reading from thge satellite data and is called velocity.

Duddy · Posted: Fri Feb 10, 2012 3:56 pm Post subject:

M8TJT · Posted: Fri Feb 10, 2012 5:21 pm Post subject:

OK already. The signals transmitted by the GPS satelite are far more complex that you indicate, but the 'averageing' between two position readings a second apart does not give the averaging effect that the OP indicates.

As I said before, my GPS device (SiRF Star ll) has an averaging (smoothing)mode, which averages the speed and position over several 1 second readings. This gives the effect that the OP indicates i.e. speed and positional lag considerably more that the 'instantaneous' lag of an unsmoothed (usual type) output.

lbendlin · Posted: Sun Feb 12, 2012 3:01 pm Post subject:

253 · Posted: Mon Feb 13, 2012 1:19 am Post subject:

A little addition, and mainly because of what CAA_1130221413 posted because it may / may not be relevant.

The HTC Desire on 2.2.2 is very low on internal memory (I have one) and no longer use it for navigation / CA etc for that reason. I have a Sensation instead.
That may be part of the problem, I don't know, just saying.

Bob
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CAA_1130221413 · Lifetime Member Joined: Nov 30, 2011 Posts: 5

Darren · Posted: Mon Feb 13, 2012 9:39 am Post subject:

Two issues there, firstly, this issue doesn't affect the majority, and secondly, it doesn't affect the accuracy of the speed measurement when moving.

As Lutz has said, it's purely cosmetic and even as you have described it, this has no affect on the speed calculation when you are moving at more than a few miles or kilometres per hour.
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M8TJT · Posted: Mon Feb 13, 2012 9:53 am Post subject:

Then do like TomTom do when they have a minor problem (or not) with a feature. Remove the facility all together Very Happy

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