Ultrasonic speed sensors?

[Orbit`]

Has anyone had any experience with them?
Thoughts and comments.

Here is a link.
http://www.airmartechnology.com/airm...All&PageNo=131

[WATERWINGS]

Looks like it a hole in the boat that you don't need.....and drag?

I am interested in how well they work, and how much they cost.

[afr]

looks cool
did i read it right it only has a speed range up to 46 mph

[Orbit`]

I've got a through hull paddle wheel as it is, so all it would do is remove moving parts and drag while adding to accuracy, rather then adding holes, but they also do a transom mounted version.

The physics say it should work very well and accuratley, but i just wonder what peoples thoughts are.

Different models have different speed ranges.

The transducer is about 500usd, plus gauges IIRC.

[afr]

cool at that price it will have to wait for me to have one
but i would love to compare it to the gps
i dont feel the gps is as acurate as everyone thinks

[bigboy]

Problem it that this things measures the speed over the wather (I guess?) So when there is a little current on the river it will not be accurate.

[Orbit`]

cool at that price it will have to wait for me to have one
but i would love to compare it to the gps
i dont feel the gps is as acurate as everyone thinks

The accuracy of GPS units depends on alot of factors, from the number of satalites, accuracy of the units internal timepiece, refresh/sampling rate, wether it is tri or biaxial, (x,y or x,y,z).

I've experienced the situation in planes level flight maintaing the same path in "formation" but with the equilivent 5mph difference on identical GPS units. i've also been in a situation with 5 of 6 gps modules reading the same and one reading high or low in terms of speed in the one craft.

There are several good articles that discuss this. Below are a few:
http://www.ncbi.nlm.nih.gov/pubmed/15519597

It's also worth noting that the GPS civilian grid alters in it's accuracy from time to time depending on defence department access and needs. it is also limited to readings below 999mph and 60,000ft for the civilian market.
GPS satalites also can vary the rate at witch they transmit data, which will influence the accuracy of the GPS as well.
Most manufactures claim an accuracy of .1-.5mph in "ideal" conditions. and typically as you increase speed the error percentage should reduce all other factors remaining equal. They are the best available navigation and speed equiptment readily available, but everything has it's limitations.

[afr]

i t tottaly agree
cause i know how it feels and no way is that thing right
but it is what it is so we are stuck with it for now

[Orbit`]

Problem it that this things measures the speed over the wather (I guess?) So when there is a little current on the river it will not be accurate.

This is true (it measures speed of the water that passes the sample area) though the faster you go the lower the error. However if you are truely looking for acurate speed measurement radar traps are where it's at.
It would be intresting to drift in a current and test the accuracy of the unit, i.e it should read close to zero.

As the unit works on doppler and the focus is quite a limited size unless you were in quite rapid water the current would induce a maximium of ~ 3mph error. Which is fine for my application.

[Orbit`]

AFR what altitude do you boat at? if your unit is only biaxial GPS data, it will assume sea level reciept and calculates accordingly. resultingin some big errors.

[afr]

florida so i would say sea level
it has to be the lowest i would think

[Orbit`]

ah...well that rules that out.

[afr]

i still dont belive its right
i got a garmin 182c mounted on board now
i did a few handhelds a few times